Chapter12 Flavor Formation

7212019 Chapter12 Flavor Formation

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277

Chapter 12

Flavor Formation

Barbara drsquoAcampora Zellner Paola DugoGiovanni Dugo and Luigi Mondello

CONTENTS

121 Brief Introduction to Dairy Flavor Formation 277

122 Flavor Formation in Milk 2791221 Milk Flavors Developed During Heat reatment and Storage 2791222 Flavors Developed from the Autoxidation of Milk Lipids 2801223 Light-Induced Flavor Formation 2811224 Flavor Compounds Derived from Package Component Migration 281

123 Flavor Formation in Fermented Dairy Products Cheese 2821231 Fermentation of Lactose and Conversion of Citrate 2821232 Proteolysis and Catabolism of Amino Acids 284

12321 Metabolism of Branched-Chain Amino Acids 28512322 Te Catabolism of Aromatic Amino Acids 285

12323 Methionine and Its Catabolic Pathways 2861233 Lipolysis and Catabolism of Fatty Acids 286

124 Concluding Remarks 288References 288

121 Brief Introduction to Dairy Flavor Formation

It is widely acknowledged that milk and its derived products are characterized by a heterogeneousnature comprising signi1047297cant levels of lipids proteins and carbohydrates In this respect the

classi1047297cation of 1047298avor compounds based on their polarity or volatility turns out to be a diffi culttask In addition as is well known the 1047298avor composition varies according to the state of the dairy



278 Handbook of Dairy Foods Analysis

sample so that raw milk presents a distinct 1047298avor when compared to that of heated or processedmilk with different classes of compounds responsible for the characteristic 1047298avor of distinct sam-ples Generally esters are responsible for the 1047298avor of raw milk samples while lactones and het-

erocyclic compounds for that of heat-treated and pasteurized milk On the other hand fermenteddairy products such as cheese and yogurt are characterized by the presence of fatty acids [1]It is mandatory to highlight that fermentation has a major impact on the organoleptic quality

of milk-derived products raditionally fermentation was primarily applied to improve and pro-long shelf-life since the growth of fermentative microorganisms typically results in the productionof acids or alcohols that inhibits the growth of food pathogens or spoilage organisms preservingthe food Tough the chemical transformation of components in dairy food raw materials andthe release of microbial metabolites contribute to the 1047298avor appearance and texture In dairyproducts lactose citrate milk fat (lipids) and proteins are converted into a broad range of volatileand nonvolatile 1047298avor compounds

Tree main pathways for the 1047298avor formation of dairy products can be distinguished ieglycolysis proteolysis and lipolysis [23] (see Figure 121) Tese processes may already generate1047297nal 1047298avor compounds though further reactions can occur yielding an even greater number oflow-molecular weight components that essential to the overall odor of the product

In general 1047298avors derived from lactose or citrate (glycolysis) such as lactic acid acetaldehydeand diacethyl (23-butanedione) are produced by the lactic 1047298ora Lipids are also another sourceof a wide range of dairy 1047298avors and by means of lipolysis and oxidation are prone to cause hydro-lytic and oxidative rancidity respectively In this manner small amounts of γ - and δ-hydroxyacids present in milk lipids can be readily converted to γ - and δ-lactones unsaturated fatty acidsmay hydrate to hydroxy acids and then via oxidation produce γ - and δ-lactones β-oxidationfollowed by decarboxylation produces methyl ketones and oxidation at double bonds can gener-ate straight-chain aldehydes and ketones which under reducing conditions may be converted tothe corresponding alcohols Moreover protein-derived 1047298avors also comprise a large amount ofpathways Strecker aldehydes such as 2-methylpropanal 2-methylbutanal and 3-methylbutanalcan be synthesized via amino acids and in turn may produce their corresponding alcohols andsulfur-containing compounds such as hydrogen sul1047297de methanethiol and dimethyl disul1047297de(DMDS) can be derived from cysteine and methionine In addition other 1047298avor origins have

Triglycerides

Free fatty acids

Milk

Amino acids

Dairy flavor compounds

Peptides

CaseinLactosecitrate

Pyruvateα-acetolactate

Figure 121 Dairy 1047298avor formation pathways in fermented products



Flavor Formation 279

been reported such as compounds derived from lactose without any interaction of the lactic 1047298orathat are developed during heat treatment or that result from alkaline degradation of lactose or itscomponents glucose and galactose eg maltol [4] and 2-acetylfuran [5]

Te 1047298avor of cheeses is composed of a blend of compounds generated by all three pathwaysthe same is true for key-odor compounds Cheddar [6] and Camembert [7] cheeses for instancehave half of their potent odorants derived from lactose fermentation or citrate degradation a verysmall amount produced by lipolysis and the other half resulting from amino acid degradation(leucine and methionine)

aking into consideration that 1047298avor is the primary index of dairy food acceptance and is strictlyrelated to product development and improvement this chapter provides an overview of the dairy 1047298a-vor formation pathways Numerous dairy food 1047298avors have been thoroughly studied though dem-onstrating that even widely investigated metabolic pathways may continuously reveal new facets Inthis chapter 1047298avor formation processes in milk and cheese will be dealt with more extensively

122 Flavor Formation in Milk

Fresh good quality milk is characterized by three features a characteristic subtle and delicate1047298avor a pleasant mouth-feel since it is an emulsion of fat globules dispersed in a colloidal aqueoussolution and a slightly sweet and salty taste due to the presence of lactose and salts When com-pared to other dairy products such as butter or cheese the concentration of 1047298avor compounds infresh milk is very low

It is widely accepted that the 1047298avoring compounds in milk are products of the animalsrsquo metab-olism which is also in1047298uenced by the feed As previously cited another aspect to be considered

is the samplersquos state so that the 1047298avor of raw milk is not identical to that of heated or processedmilk Esters are considered as responsible for the 1047298avor of raw milk samples while lactones andheterocyclic compounds for that of heat-treated and pasteurized milk [1]

Furthermore almost all milk types and milk-derived products are submitted to heat treatmentsand storage over a period of time inducing 1047298avor development In addition autoxidation of lipidslight exposure and also packaging effects may yield milk 1047298avors Moreover considering that milkis composed of a multitude of compounds belonging to distinct chemical classes several of thesecompounds can be precursors of 1047298avor and off-1047298avor chemicals Te latter may be de1047297ned as anunpleasant odor or taste imparted to a food sample though internal deteriorative change [8]

1221 Milk Flavors Developed During Heat Treatment and Storage

Flavor most frequently off-1047298avor development during heat processes has been already thoroughlyresearched and the formation of ketones lactones aldehydes furans alcohols acids and sulfur-containing compounds is widely accepted

Ketones such as diketones cyclic ketones and methyl ketones formed in heat-processed milksare considered as the major contributors to that of matrices cooked and stale 1047298avors [9] whilediketone compounds diacethyl and 23-pentanedione were described as signi1047297cant contributorsto the heated burnt and fermented notes elicited by heated milk [10] With regards to diacethylits production is considered to be started by methyl glyoxal generated by heating lactose which

then reacts with glycin or its derivatives forming diacethyl Cyclic ketones eg cyclopentanoneand 2-methyl-tetrahydrofuran-3-one are also considered as contributors to the heated burnt andfermented notes in heated milk [10]




Milk has been studied in depth by Moio et al [11ndash13] who analyzed the matrix under distinctconditions Analyses of raw bovine ovine caprine and water buffalo milk samples demonstratedthat dimethylsulfone ethyl butanoate ethyl hexanoate heptanal indole nonanal and 1-octen-3-ol

are odor-active compounds common to all samples Even though similarities between the distinctsamples were found the overall odor of each milk variety presented a different 1047298avor pro1047297le withethyl butanoate and ethyl hexanoate as the most potent 1047298avorants in cow sheep and goat milk

while water buffalo milk 1047298avor was better characterized by nonanal and 1-octen-3-ol [11] On theother hand while investigating high-temperatureshort-time (HS) and ultrahigh temperature(UH) pasteurized cow milk dimethylsulfone and 2-heptanone were found to be the most potentodor-active compounds instead of ethyl butanoate and ethyl hexanoate observed in raw cow milkBoth odorant couples ethyl butanoate and ethyl hexanoate and dimethylsulfone and 2-heptanonecan be used as aroma quality indicators for raw and heated milk respectively [12]

Further compounds that are not detected in raw milk are furans particularly furfural andhydroxymethylfurfural (HMF) the former eliciting an almond-like sweet note while the latter afatty musty and waxy odor Both are formed when milk is heated above 80degC by the browning ofa lactosendashcasein mixture [14] ie the casein catalyzed degradation of lactose induced by heat

Te formation of aldehydes is also noteworthy during prolonged storage of casein these com-pounds particularly alkanal compounds and benzaldehyde are responsible for an unpleasant staleoff-1047298avor [15] Te production of alkanal compounds can be attributed to the oxidation of lipidsbound by casein while benzaldehyde can be formed by reactions occurring between phenylala-nine and lactose [1617] Another class of compounds where formation of milk is induced by heattreatments is alcohol most commonly represented by acetol and acetoin (3-hydroxy-2-butanone)Te former is the product of carbohydrate degradation during nonenzymatic browning reactions[10] while the latter may be formed by the reduction of diacetyl [9] Both the compounds are alsodetected in raw milk and in increasing concentrations when heated above 90degC [10] Heat treat-ments can also in1047298uence the pro1047297le of acids in milk at temperatures superior to 100degC an incre-ment may be observed in the concentrations of acetic butyric hexanoic octanoic and decanoicacids [10] contributing to the chemical and rancid 1047298avor of heated milk

Freshly secreted milk fat does not contain free lactones but their precursors are present inbound form [18] Indeed small amounts of δ-hydroxy alkanoic acids esteri1047297ed to glycerol can bedetected which upon hydrolysis during heating or storage form a homologous series of saturatedaliphatic δ-lactones Te latter contribute positively to milk 1047298avor eliciting a milky fatty andcoconut-like note Moreover the cooked 1047298avor commonly observed in milk and derivative prod-ucts is caused by hydrogen sul1047297de Tis compound is a product of the oxidation of free sulfhydrylgroups formed in the denaturation of β-lactoglobulin the major whey protein of cowrsquos milk [19]

Urbach investigated the effects of cold-storage on raw cow milk samples [20] reporting thatcarbonyls are reduced to the corresponding alcohols among the most abundant ones were ethanol2-propanol and 3-methylbutan-1-ol After prolonged storage these alcohols were partially esteri-1047297ed with volatile acids

1222 Flavors Developed from the Autoxidation of Milk Lipids

Milk 1047298avors can also be developed as products of the autoxidation of lipids rendering a foodunpalatable due to a rancid or oxidized odor In short the nonenzymatic autocatalytic oxidation

reaction of unsaturated fatty acids most commonly polyunsaturated (oleic linoleic linolenicand arachidonic acids) producing hydroperoxides which often dismutate to secondary oxidationproducts such as aldehydes ketones alcohols acids hydrocarbons lactones furans and esters




Aliphatic aldehydes are considered as the most important breakdown products of hydroperoxidesdue to their major contribution to unpleasant 1047298avors in food products Moreover aldehydes result-ing from autoxidation may undergo other reactions contributing further to the 1047298avor of dairy

products

1223 Light-Induced Flavor Formation

Light is known to induce metabolic reactions in dairy products triggering the degradation of pro-teins or the oxidation of lipids resulting in the formation of off-1047298avors Furthermore it is generallyaccepted that ribo1047298avin plays a major role as photosensitizer in milk and milk-derived products[21] Photosensitized oxidations produce hydroperoxides which are prone to decompose formingfree radicals and these are responsible for autoxidation reactions and off-1047298avor developmentsRibo1047298avin acts by means of two reaction modes ype I occurs when a triplet ribo1047298avin (electronic

excited state with two unpaired electrons) is directly deactivated by the abstraction of an electronfrom a substrate which is then oxidized Subsequently ribo1047298avin can abstract a further electronforming a reduced ribo1047298avin or react with ground-state oxygen to generate a superoxide radicalype I reaction may be exempli1047297ed by the photoxidation of lipids yielding alkanals and 2-enals

which are not present in autoxidized milk samples while 24-dienals are only detected in autoxi-dized samples According to Wishner [22] the oxidizing substrates in light-exposed milk are themonoene fatty acids of the triglycerides while in spontaneously oxidized milk polyene fatty acidsof the phospholipids are involved

On the other hand in type II reactions a triplet ribo1047298avin is deactivated physically by oxy-gen and a singlet-oxygen is formed Te latter reacts with the amino acid methionine which

then undergoes Strecker degradation forming aldehydes (eg methional) and sulfur-containingcompounds (eg dimethyl sul1047297de) It is noteworthy that the broth and potato-like 1047298avors pro-duced when milk is exposed to light are mainly related to the presence of methional though theunderstanding of the role of this compound in light-induced 1047298avor is made diffi cult by its easyconversion to other sulfur compounds [23] Samuelsson [24] reported that methionine in milk isconverted to mercaptan sul1047297des and disul1047297des at pH 68 in the presence of light and oxygen

1224 Flavor Compounds Derived from Package Component Migration

Flavor formation may be triggered by substances transferred from the package into a dairy matrix(migrants) often monomers (eg ethylene glycol) residual reactants (eg catalyst residues) ordegradation products (eg acetaldehyde) Te migration of compounds may in1047298uence the sensoryquality and acceptability of a food product A commonly detected migrant is acetaldehyde alsoknown as a degradation product formed in the melting process of poly(ethylene terephthalate)(PE) which is a material that is increasingly used in milk packagings Acetaldehyde has a pun-gent ethereal odor and is fruity and pleasant when diluted it strongly decreases the acceptabilityof milk while acting as the most important volatile 1047298avor compound in plain yogurt [25]

Even acetaldehyde concentrations of 10 mgL exceed the human threshold values of 082 mgLin milk decreasing strongly the acceptability of milk Freshly pasteurized milk contains about10 ppb of acetaldehyde [3] which are most often detected in fermented products eg yogurt at

concentrations ranging from 5 to 40 mgL [26] Moreover according to sensorial tests (triangle testperformed by 25 panelists) no signi1047297cant difference in the acetaldehyde threshold in milk of vari-ous fat contents could be observed thresholds of 3939 4020 and 4040 ppb were determined for




nonfat low-fat and whole milks respectively while chocolate-1047298avored milk showed a thresholdof 10048 ppb [27] Tis information assisted in the prediction of the in1047298uence of acetaldehydemigration on the 1047298avor of milk products

123 Flavor Formation in Fermented Dairy Products Cheese

Te compounds forming the subtle and delicate milk 1047298avor blend are transformed by meansof fermentation into other 1047298avor compounds derived either from lactose fermentation and cit-rate conversion protein degradation or amino acid catabolism or lipid degradation As a conse-quence fermented milk-based products present complex variety and type-speci1047297c 1047298avor pro1047297lespresenting compounds that belong to several chemical classes such as alcohols aldehydes ketonesesters lactones furans nitrogen-containing compounds as also pyrazines and sulfur-containingcompounds terpenes and their derivatives aromatic compounds and free fatty acids (FFAs)

One of the most widely studied fermented products is cheese which has more than 500 varieties[28] Cheese manufacture consists on the addition of salt an enzyme coagulant and a culture of astarter organism to milk the latter is mainly represented by prokaryotes and is the principal sourceof enzymes involved in the biochemical processes [29] Predominantly lactic acid bacteria (LAB)are used as starters comprising the homolactic (lactococci streptococci lactobacilli group I) andthe heterofermentative genera (leuconostocci lactobacilli group III) Within LAB a large strain-to-strain diversity can be observed in 1047298avor formation thousands of strains are known in theenvironment both in public and in private culture collections representing an important reservoirof natural diversity of phenotypes

Additional cultures are also utilized eg Propionibacterium in the manufacture of Swiss-

cheeses and frequently aerobic cultures in the production of surface-ripened cheeses suchas bacteria (Brevibacterium Arthrobacter and Staphylococcus ) fungi (Penicillium) and yeasts(Debaromyces ) [30] Apart from these starter organisms also lactobacilli originating from the milkenvironment might grow in dairy products turning to be a source of enzymes involved in theformation of 1047298avors Nonstarter organisms (molds and surface bacterial 1047298ora) are also reported tobe of relevance for 1047298avor formation [31]

Furthermore it is worthwhile to emphasize that even closely related bacterial strains may exhibitvery different 1047298avor-forming activities due to the fact that (1) the genes encoding key enzymes for1047298avor formation is not produced by all the strains of a species (2) different variants of a 1047298avor-form-ing enzyme among strains may occur leading to altered catalytic activities and (3) the expression

of many of the enzymes involved in 1047298avor production is strongly subjected to regulation

1231 Fermentation of Lactose and Conversion of Citrate

Flavor formation during the manufacture of cheeses is initiated immediately after the addition ofstarter cultures to milk also known as starter organisms Te widely utilized LAB are able to fer-ment lactose to lactic acid that on its own is the main 1047298avor of several fresh cheeses At 1047297rst lactoseis hydrolyzed by starter cultures producing glucose and galactose Te former is then oxidized topyruvate by the Emden-Meyerhof pathway of glycolysis while the latter is converted by galactose-positive bacteria and leuconostocs through the Leloir pathway to glucose-6-phosphate and to

glyceraldehyde-6-phosphate by lactococci through the tagatose pathway (refer to Figure 122)Te intermediate pyruvate can also be converted to various short-chain 1047298avor compounds suchas diacetyl acetoin acetaldehyde ethanol and acetate Diacethyl imparts a buttery nutty 1047298avor




and is considered to be signi1047297cant for the 1047298avor of Camembert [732] Cheddar [3334] Emmental[35] and cottage cheeses [36] while acetoin was considered to contribute to the 1047298avors of Cheddar[34] and Mozzarella cheeses [11] with a sour milk-resembling odor Moreover acetaldehyde wasdescribed to impart a green note to Camembert [732] and Gruyegravere [37]

In addition other starter organisms such as heterofermentative starters present the abilityto ferment other noncarbohydrate substrates such as citrate producing acetaldehyde ethanolbutan-12-diol and diacetyl yielding a more buttery 1047298avor [36] In citrate-utilizing LABs thissubstrate is initially cleaved to oxaloacetate and acetate by a citrate lyase Moreover in culturesof Lactococcus and Leuconostoc spp oxaloacetate is decarboxylated to pyruvate wo molecules ofthe latter are then condensed by an α-acetolactate synthase forming α-acetolactate and carbondioxide Te former is unstable and its decarboxylation may be oxidative yielding diacetyl or non-oxidative forming acetoin while the latter is responsible for the eye formation in some semihardcheeses [38] It should be noted that the initial breakdown of citrate and the further conversionof the intermediate pyruvate into speci1047297c fermentation products can be regulated at differentlevels according to the microorganism As aforementioned heterofermentative starters producecarbon dioxide that promotes an open texture Te use of Propionibacterium freudenreichii subspshermanii which ferments lactose producing propionic acid and carbon dioxide is also notewor-thy the former contributes to the characteristic 1047298avor of Swiss-type cheese while the latter is asmentioned earlier responsible for the eye formation in the cheese [39]

Nonstarter organisms may also ferment lactose producing a variety of compounds the coli-form fermentation generates formic acid carbon dioxide and hydrogen the butyric fermentationleads to butan-1-ol butyric acid acetone propan-2-ol and carbon dioxide formation and theethanolic fermentation pathway generates ethanol and carbon dioxide When the lactose supply is

depleted and pH water acidity and reducing potential are not optimal surviving homofermenta-tive starters may widen their range of fermentative activity Moreover the lactose depletion ratevaries according to the cheese type from 24 h in Swiss-type to 20 days in Cheddar cheese [40]

Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P

DiacetylAcetaldehyde

Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P

Figure 122 Biochemical pathways of lactose catabolism in milk




1232 Proteolysis and Catabolism of Amino Acids

Te important raw materials for cheese 1047298avor generation include not only low-molecular weightcompounds as citrate and lactose but also high-molecular weight proteins from which many of the

most important 1047298avor compounds are derived especially in hard-type and semihard-type cheesesTe proteolytic process is initiated by the conversion of casein to large peptides by proteases

Commonly LAB strains produce a series of peptidases which are able to further degrade largepeptides to smaller oligopeptides and amino acids Tese on their own contribute to the 1047298avorpromoting a sweet or brothy note On the other hand peptides with an undesired bitter taste mayalso accumulate to concentrations higher than their threshold which may be removed by the addi-tion of adjuncts able to reduce bitterness the so-called debittering peptidases [41] Furthermoreamino acid convertases trigger the formation of 1047298avor compounds from amino acids yieldingvarious alcohols aldehydes acids esters and sulfur compounds

In the past proteolysis was considered as a rate-limiting process in the maturation of many

cheeses especially in semihard cheeses However it was demonstrated that enhancing the freeamino acid release by intensifying peptidolysis by LAB [42] or adding free amino acids did notaffect aroma formation in cheese [43] suggesting that the rate-limiting factor was not the releaseof free amino acids but their conversion to 1047298avor compounds

In general proteolysis proceeds at a slower rate especially in cheeses that are low in moisture orhave a rich salt content both these characteristics are considered to control the maturation rate Inaddition the concentration of free amino acids was found to be closely linked to the maturity ofcheeses and was recognized as cheese 1047298avor precursors Amino acids are substrates for transami-nation dehydrogenation decarboxylation and reduction producing a wide variety of 1047298avor com-pounds or further precursors such as amines and Strecker aldehydes (see Figure 123) Te ability

Peptides

Carboxylic acids

Milk

CoA-esterHydroxy acids

α-Keto acids

Aldehydes

Thio-estersAlcohols(Thiols) Acyltransferases

esterases

Alcohol dehydrogenase Acyltransferases

esterases

Phosphotransferasekinase Aldehyde

dehydrogenase

Aminotransferase

Hydroxyacid

dehydrogenaseα-keto acid

dehydrogenase

α-keto acid decarboxylase

Deiminasedecarboxylase

Aldolases lyases

Protease

Peptidase

lyases

Amines Aldehydes Thiols Sulfur-compounds(methanethiol)

PhenolIndole

Amino acids

Casein

Figure 123 Overview of protein metabolic pathways involved in the 1047298avor formation of fer-mented dairy products




of microorganisms to generate 1047298avor compounds from amino acids in cheese is considered tobe highly dependent on strain among the most common strains are LAB coryneform bacteriayeasts and Geotrichum candidum among others [44]

Te study of the compounds responsible for the characteristic 1047298avor of various cheeses dem-onstrated that amino acid degradation is a major process for 1047298avor formation in cheese Moreoverbranched-chain amino acids (leucine isoleucine and valine) aromatic amino acids (phenylalaninetyrosine and tryptophan) and methionine are the major precursors of these aroma compounds[44] In this respect monitoring amino acid degradation during cheese ripening may be a reason-able approach to control 1047298avor formation

12321 Metabolism of Branched-Chain Amino Acids

Te catabolism of branched-chain amino acids is initiated by a transamination reaction catalyzed

by amino acid aminotransferases [44] Tis conversion pathway of amino acids is responsiblefor the transamination of leucine isoleucine and valine forming α-keto acids intermediatesα-ketoisocaproate α-keto-β-methyl valerate and α-ketoisovalerate respectively Te conversionpathways of the branched-chain α-keto acids on the other hand may involve three biochemicalreactions the oxidative decarboxylation to carboxylic acids the decarboxylation to aldehydes andthe reduction to hydroxyacids With the exception of hydroxyacids all of these products presentstrong 1047298avors It has to be highlighted that the speci1047297city of the aminotransferases may differaccording to the microorganism

Te intermediate α-ketoisocaproate for instance may be decarboxylated to 3-methylbutanal which presents a fruity pleasant odor at low concentrations and green malty acrid pungent odor

at higher concentrations 3-methylbutanal may undergo oxidation catalyzed by aldehyde dehy-drogenases to isovaleric acid which elicits rancid cheesy sweaty and putrid notes that probablycontribute to ripe cheese aroma this is more prevalent in Camembert than in Cheddar and is alsodetected in Swiss Gruyegravere cheese [45]

Similar reactions may also be observed for isoleucine and valine both are decarboxylated pro-ducing the malty 1047298avors 2-methylbutanal and 2-methylpropanal respectively which thereafterare oxidized to 2-methylbutyric acid and isobutyric acid In general fatty acids originated frombranched-chain amino acids yield sweaty rancid fecal putrid and ester-resembling 1047298avors [44]the in1047298uence of these 1047298avors on a cheese is based on the ratio of its concentration in that product toits threshold concentration in that same matrix In addition the conversion of the aldehydes pro-

duces the corresponding alcohols 3-methyl butanol 2-methyl butanol and 2-methyl propanolresponsible for alcoholic and fruity 1047298avors [46]On the other hand in Emmental cheese 3-methyl butanol is able to suppress the sweaty odor

of butyric acid originated from lipolysis [47] and Cheddar cheese presents improved character-istics when 1047298avor formation from branched-chain and aromatic amino acids is enhanced [48]Besides 3-methyl butanol also confers a pleasant odor to fresh cheese [49]

12322 The Catabolism of Aromatic Amino Acids

Te degradation of aromatic amino acids is initiated with a transamination step catalyzed by ami-

notransferases producing indole pyruvate phenyl pyruvate and p-hydroxy-phenyl pyruvate fromtryptophan phenylalanine and tyrosine respectively

Elimination reactions catalyzed by amino acid lyases may also be observed when yeast micro-cocci and Brevibacterium linens are used Tese lyases are able to cleave the side chain of tyrosine




and tryptophan yielding in a single-step phenol and indole respectively Moreover this pathwayhas not been detected in any LAB [50] though several of them are known to yield tyramine andtryptamine by means of enzymatic decarboxylation of tyrosine and tryptophan

Generally compounds derived from aromatic amino acids are responsible for the impact 1047298avorof diverse cheeses common 1047298avor compounds are benzaldehyde (bitter almond resembling odor)phenyl acetaldehyde (honey-like 1047298oral rosy violet-like note) phenyl ethanol (1047298oral rosy violet-like odor) phenyl acetate (honey-like note) and phenyl propanoate (1047298oral) while less pleasantnotes are indole and skatole both eliciting fecal putrid musty odor at higher dilution and p-cresol(phenolic medicinal 1047298avor) Phenyl acetaldehyde for instance is of importance for the impact 1047298a-vor of Gruyegravere [3751] and bovine Mozzarella cheeses [49] while phenyl ethanol apart from beingone of the major volatile compounds identi1047297ed in Camembert [52] is also commonly detected insoft smear cheeses [53] and was already identi1047297ed in Gruyegravere cheese [37]

12323 Methionine and Its Catabolic Pathways

Te methionine conversion to 1047298avor compounds can proceed in different pathways Eliminationreactions (demethiolation) catalyzed by amino acid lyases yielding methanethiol can further beoxidized to DMDS and dimethyl trisul1047297de (DMS) Te latter is the major pathway for methi-onine degradation in some organisms eg B linens while Lactobacillus lactis is capable of cleav-ing the side chain of methionine producing directly methanethiol Te enzymes involved in bothelimination reactions are different in B linens it is the γ -liase and in L lactis the enzymes arenot speci1047297c to methionine A further catabolism pathway consists of the transamination catalyzedby aminotransferases and the resulting α-keto acids are degraded to methanethiol via one or two

(methional forming) additional reaction stepsMethanethiol is considered as one of the responsible compounds for the characteristic 1047298avor

of Camembert [732] and Cheddar [33] and is also of relevance for the 1047298avor of Gruyegravere cheeses[3751] Moreover DMDS and DMS were also found to contribute signi1047297cantly to the 1047298avor ofthe latter cheese [37] providing sulfurous and cabbage-like sulfurous notes respectively

1233 Lipolysis and Catabolism of Fatty Acids

Lipid degradation is an important biochemical process that occurs during cheese ripening It is trig-gered by lipolytic enzymes which are hydrolases able to cleave the ester linkage between a fatty

acid and the glycerol core of the triacylglyceride (AG) producing FFA and monoacylglyceridesand diacylglycerides Tese enzymes may be classi1047297ed as esterases or lipases which are distinguishedaccording to the length of the hydrolyzed acyl ester chain the physicochemical nature of the substrateand enzymatic kinetics FFAs are not only 1047298avor compounds by themselves but also precursors ofcatabolic reactions producing further 1047298avor compounds such as methyl ketones esters and thioesterslactones aldehydes and secondary alcohols as outlined in Figure 124 It is worth noting that lipolysisis particularly important in soft cheeses such as Camembert [54] and Blue cheeses [30] and especiallyshort- and medium-chain FFAs present strong 1047298avors that are generally considered as undesirable butin some cheeses contribute directly to their 1047298avors as can be observed for Cheddar cheese [55]

In general LAB contribute relatively little to lipolysis but additional cultures such as molds in

surface-ripened cheeses often exert high lipolytic activity [30] generating FFAs with 4ndash20 carbonatoms Te important products of the catabolism of FFAs are the methyl ketones cited earlier(alkan-2-ones) which are formed due to the action of mold lipases Te degradation of lipids byPenicillium spp for instance is started by the release of FFAs promoted by lipases these FFA are




then oxidized to α-ketoacids which are decarboxylated to alkan-2-ones and the latter undergoesreduction yielding alkan-2-ol Furthermore mold cultures can also produce methyl ketones usingketoacids naturally occurring in milk at low concentrations as substrate or by the oxidation ofmonounsaturated fatty acids

Methyl ketones are important 1047298avor compounds contributing particularly to the 1047298avor ofBlue cheese [56] with their concentration increasing until the 70th day of ripening then decreas-ing substantially similar was observed for Emmental cheese [57] Studies performed on Cam-embert cheese led to the identi1047297cation of 11 methyl ketones out of these especially 2-nonanone(fatty green note) 2-heptanone (fruity fatty 1047298avor) and 2-undecanone (herbaceous fresh note)

increased in concentration throughout the ripening process [52] Te ketone 2-heptanone has alsobeen considered of importance for the 1047298avor of Parmigiano Reggiano cheese when present inconcentrations above its sensory threshold [58] as also for Emmental [35] and natural and creamyGorgonzola cheeses [59] Another predominant methyl ketone in natural Gorgonzola [59] is also2-nonanone Besides in full-fat Cheddar cheese the concentrations of 2-heptanone 2-nonanoneand 2-undecanone increased until about 35 months of ripening and then decreased while in low-fat Cheddar types the concentration of methyl ketones are drastically lower [60]

Esters are further products of the fatty acid catabolism and are formed by reactions betweenshort- and medium-chain fatty acids with alcohols derived from lactose fermentation or fromamino acid catabolism A great variety of esters imparting fruity 1047298avors were identi1047297ed in diverse

cheese types Imhof and Bosset [31] investigated the 1047298avor of Emmental cheese identifying 14esters and Meinhart and Schreier [61] detected 38 esters in Parmigiano Reggiano cheese withethyl ethanoate ethyl octanoate ethyl decanoate and methyl hexanoate as the most abundantones Tioesters formed by the reaction of esters of short-chain fatty acids with methional imparted

Free fatty acids

β-Oxidationβ-Oxidation

β-Ketoacyldecarboxylase

β-Keto acids

Methyl ketones

Secondary alcohols

Redutase

4-or 5-Hydroxyacids Unsaturated fatty acids

Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

Figure 124 Lipid catabolism pathways of some microorganisms utilized in the 1047298avor formationof fermented dairy products




the characteristic cheese-like aroma to Cheddar cheese [34] while S -methyl thioesters contribute with a characteristic strong 1047298avor to various smear-ripened soft cheeses such as Limburger andHavarti [62]

Lactones occur naturally in milk and also in cheeses but are not considered to be of great rel-evance to the 1047298avor of cheeses In Parmigiano Reggiano cheese for instance diverse lactones weredetected δ-octalactone being the most signi1047297cant molecule [61] while Camembert 1047298avor com-prised γ -decalactone δ-decalactone γ -dodecalactone and δ-dodecalactone [63] Moreover thecommonly detected δ-decalactone is one of the most important lactones not only for the 1047298avor ofCamembert as also for the one of Emmental [47] and Blue cheeses [63] On the other hand Brit-ish farmhouse Cheddar cheese 1047298avor is characterized by the presence of δ-dodecalactone [64]

Aldehydes may also be detected in several cheese types such as straight-chain aldehydesspecially n-nonanal characterized by a green grassy odor in water buffalo Mozzarella cheese [49]Most commonly aldehydes are transitory compounds which rapidly reduce to primary alcohols oroxidize into the corresponding acids In addition the enzymatic reduction of methyl ketones mayyield secondary alcohols such as 2-propanol and 2-butanol in Cheddar cheese due to reductionof acetone and butanone respectively [65] or 2-heptanol and 2-nonanol in mold-ripened cheeseseg Camembert [52] Brie [66] and Blue cheeses [63]

124 Concluding Remarks

It is widely agreed that a single volatile compound does not elicit the characteristic 1047298avor of a dairyproduct Furthermore it is important to outline that synergistic or suppressive effects of different1047298avors present in a dairy food matrix should be taken into consideration Te characteristic 1047298avor

of a cheese is de1047297ned by the so-called component balance theory [28] which is ruled by a widerange of parameters such as cheese age micro1047298ora and biochemistry

Dairy product development is nowadays directed toward new items with distinct 1047298avors Inmany cases this requires the development of special starter cultures delivering speci1047297c 1047298avor com-pounds in a selected product It is worthwhile to highlight that the presence of starter cultures isnot suffi cient to explain 1047298avor formation in raw milk cheeses and their selection has never beenmade with respect to the 1047298avor criterion Starter cultures metabolic pathways have been exploitedto increase knowledge on dairy food 1047298avor formation and at present diverse metabolic engineer-ing strategies are reported providing solutions such as metabolic interventions in Lactococcus lactis and other LAB improving the 1047298avor of fermented foods Several authors make reference to the use

of tailor-made mixed cultures of known species of bacteria to provide speci1047297c dairy 1047298avorsMoreover the further growing demand for new 1047298avor compounds triggers the development

and application of analytical methodologies which are exhaustively described in Chapter 38 As is well known the complexity of the samples that are analyzed typically mandates that some type ofseparation be achieved before the component analytes can be measured and characterized in thisrespect the utilization of gas chromatography and mass spectrometry is widely diffused

References 1 Friedrich J E and Acree E Gas chromatography olfactometry (GCO) of dairy products Int

Dairy J 8 235 1998 2 Smit G Smit B A and Engels W J M Flavour formation by lactic acid bacteria and biochemical

1047298avour pro1047297ling of cheese products FEMS Microbiol Rev 29 591 2005




3 Marilley L and Casey M G Flavours of cheese products Metabolic pathways analytical tools andidenti1047297cation of producing strains Int J Food Microbiol 90 139 2004

4 Patton S Te isolation of Maltol from heated skim milk J Dairy Sci 33 102 1950 5 Patton S Studies of heated milk III Mode of formation of certain furan compounds J Dairy Sci

33 904 1950 6 Christensen K R and Reineccius G A Aroma extract dilution analysis of aged Cheddar cheese

J Food Sci 60 218 1995 7 Kubiacutečkovaacute J and Grosch W Evaluation of potent odorants of Camembert cheese by dilution and

concentration techniques Int Dairy J 7 65 1997 8 Baigrie B Introduction in aints and Off-Flavours in Food 1st edn Baigrie B Ed Woodhead

Publishing Ltd Cambridge MA 2003 Chapter 1 9 Adda J Flavour of dairy products in Developments in Food Flavours 1st edn Birch G G and

Lindley M G Eds Elsevier Applied Science London 1987 pp 151ndash172 10 Shibamoto Flavor volatiles formed by heated milk in Te Analysis and Control of Less Desirable

Flavors in Foods and Beverages 1st edn Charalambous G Ed Academic Press New York 1980

pp 241ndash265 11 Moio L et al Powerful odorants in bovine ovine caprine and water buffalo milk determined by

means of gas chromatographyndasholfactometry J Dairy Res 60 215 1993 12 Moio L et al Detection of powerful odorants in heated milk by use of extract dilution sniffi ng

analysis J Dairy Res 61 385 1994 13 Moio L et al Odorous constituents of ovine milk in relationship to diet J Dairy Sci 79 1322

1996 14 Ferretti A and Flanagan V P Te lactosendashcasein (Maillard) browning system Volatile compo-

nents J Agric Food Chem 19 245 1971 15 Parks O W and Patton S Volatile carbonyl compounds in stored dry whole milk J Dairy Sci 44

1 1961

16 Ramshaw E H and Dunstone E A Volatile compounds associated with the off-1047298avour in storedcasein J Dairy Sci 36 215 1969

17 Rijnen L et al Lactococcal aminotransferases Ara and Bca are key enzymes for the formation ofaroma compounds from amino acids in cheese Int Dairy J 13 805 2003

18 Dimick P S Walker N J and Patton S Occurrence and biochemical origin of aliphatic lactonesin milk fatmdashA review J Agric Food Chem 17 649 1969

19 Hutton J and Patton S Te origin of sulfhydryl groups in milk proteins and their contributionto ldquocookedrdquo 1047298avor J Dairy Sci 35 699 1952

20 Urbach G Headspace volatiles from cold-stored raw milk Aust J Dairy echnol 45 80 1990 21 Bekboumllet M Light effects on food J Food Prot 53 430 1990 22 Wishner L A Light-induced oxidations in milk J Dairy Sci 47 216 1964 23 Allen C and Parks O W Evidence for methional in skim milk exposed to sunlight J Dairy Sci

58 1609 1975 24 Samuelsson E G Model experiments on sunlight 1047298avor in milk di- and tripeptides and methionine

Int Dairy Cong A 152 1962 25 Bills D D et al Effect of sucrose on the production of acetaldehyde and acids by yogurt culture

bacteria J Dairy Sci 55 1570 1972 26 Bodyfelt F W obias J and rout G M Te Sensory Evaluation of Dairy Products Van Nostrand

Reinhold New York 1988 27 van Aardt M et al Flavor threshold for acetaldehyde in milk chocolate milk and spring water using

solid phase microextraction gas chromatography for quanti1047297cation J Agric Food Chem 49 13772001

28 Singh K Drake M A and Cadwallader K R Flavor of Cheddar cheese A chemical and sen-sory perspective Comp Rev Food Sci Food Saf 2 165 2003

29 Hammes W P Bacterial starter cultures in food production Food Biotechnol 4 383 1990




30 Molimard P and Spinnler H E Compounds involved in the 1047298avour of surface mold-ripenedcheeses Origins and properties J Dairy Sci 79 169 1996

31 Imhof B and Bosset J O Relationship between microorganisms and formation of aroma com-pounds in fermented dairy products Z Lebensm Unters Forsch 198 267 1994

32 Kubiacutečkovaacute J and Grosch W Evaluation of 1047298avour compounds of Camembert cheese Int Dairy J 8 11 1998

33 Milo C and Reineccius A Identi1047297cation and quanti1047297cation of potent odorants in regular-fat andlow-fat mild Cheddar cheese J Agric Food Chem 45 3590 1997

34 Arora G Cormier F and Lee B Analysis of odor-active volatiles in Cheddar cheese headspace bymultidimensional GCMSsniffi ng J Agric Food Chem 43 748 1995

35 Preininger M Rychlik M and Grosch W Potent odorants of the neutral volatile fraction of Swisscheese (Emmentaler) in Trends in Flavour Research 1st edn Maarse H and van der Heij D GEds Elsevier Amsterdam 1994 pp 267ndash270

36 Hugenholtz J Citrate metabolism in lactic acid bacteria FEMS Microbiol Rev 12 165 1993 37 Rychlik M and Bosset J O Flavour and off-1047298avour compounds of Swiss Gruyegravere cheese Evalua-

tion of potent odorants Int Dairy J 11 895 2001 38 Boumerdassi H et al Effect of citrate on production of diacetyl and acetoin by Lactococcus lactis ssp

lactis CNRZ 483 cultivated in the presence of oxygen J Dairy Sci 80 634 1997 39 Vangtal A and Hammond G Correlation of the 1047298avour characteristics of Swiss-type cheeses with

chemical parameters J Dairy Sci 69 2982 1986 40 Laht et al Role of arginine in the development of secondary micro1047298ora in Swiss-type cheese Int

Dairy J 12 831 2002 41 Sridhar V R et al Identi1047297cation of endopeptidase genes from the genomic sequence of Lactobacillus

helveticus CNRZ32 and the role of these genes in hydrolysis of model bitter peptides Appl Environ Microbiol 71 3205 2005

42 Christensen J E Johnson M E and Steele J L Production of Cheddar cheese using a Lactococcus

lactis ssp cremoris SK11 derivative with enhanced aminopeptidase activity Int Dairy J 5 367 1995 43 Wallace J M and Fox P F Effect of adding free amino acids to Cheddar cheese curd on proteolysis

1047298avour and texture development Int Dairy J 7 157 1997 44 Yvon M and Rijnen L Cheese 1047298avour formation by amino acid catabolism Int Dairy J 11 185

2001 45 Bosset J O Collomb M and Sieber R Te aroma composition of Swiss Gruyegravere cheese IV

Te acidic volatile components and their changes in content during ripening Lebensm WissenschaftTechnol 26 581 1993

46 Tierry A and Maillard M B Production of cheese 1047298avour compounds derived from amino acidcatabolism by Propionibacterium freudenreichii Lait 82 17 2002

47 Preininger M Warmke R and Grosch W Identi1047297cation of the character impact 1047298avour com-pounds of Swiss cheese by sensory studies of models Lebensm Unters Forsch 202 30 1996

48 Banks J M et al Enhancement of amino acid catabolism in Cheddar cheese usingα-ketoglutarate Amino acid degradation in relation to volatile compounds and aroma character Int Dairy J 11 2352001

49 Moio L et al Volatile 1047298avour compounds of water buffalo Mozzarella cheese Ital J Food Sci 5 571993

50 Gummala S and Broadbent J R ryptophan catabolism by Lactobacillus casei and Lactobacillushelveticus cheese 1047298avor adjuncts J Dairy Sci 82 2070 1999

51 Rychlik M and Bosset J O Flavour and off-1047298avour compounds of Swiss Gruyegravere cheese Identi1047297ca-tion of key odorants by quantitative instrumental and sensory studies Int Dairy J 11 903 2001

52 Dumont J P et al Etude des composeacutes neutres volatils pregravesents dans le Camembert Lait 538 5011974

53 Sablegrave S and Cottenceau G Current knowledge of soft cheeses 1047298avor and related compounds J Agric Food Chem 47 4825 1999




54 Gripon J C Mould-ripened cheeses in Cheese Chemistry Physics and Microbiology 2 Major CheeseGroups 2nd edn Fox P F Ed Elsevier London 1993 pp 111ndash136

55 Bills D D and Day E A Determination of the major free fatty acids of Cheddar cheese J DairySci 47 733 1964

56 Dartley C K and Kinsella J E Rate of formation of methyl ketones during Blue cheese ripening J Agric Food Chem 19 771 1971

57 Tierry A Maillard M B and Le Quere J L Dynamic headspace analysis of Emmental aqueousphase as a method to quantify changes in volatile 1047298avour compounds during ripening Int Dairy J 9 453 1999

58 Qian M and Reineccius G Potent aroma compounds in Parmigiano Reggiano cheese studied usinga dynamic headspace (purge-trap) method Flavour Fragr J 18 252 2003

59 Moio L Piombino P and Addeo F Odour-impact compounds in Gorgonzola cheese J DairyRes 67 273 2000

60 Dimos A Urbach G E and Miller A J Changes in 1047298avour and volatiles of full-fat and reduced-fat Cheddar cheeses during maturation Int Dairy J 6 981 1996

61 Meinhart E and Schreier P Study of 1047298avour compounds from Parmigiano Reggiano cheese Milchwissenschaft 41 689 1986

62 Lamberet G Auberger B and Bergere J L Aptitude of cheese bacteria for volatile S-methylthioester synthesis II Comparison of coryneform bacteria Micrococcaceae and some lactic acidbacteria starters Appl Microbiol Biotechnol 48 393 1997

63 Gallois A and Langlois D New results in the volatile odorous compounds of French cheese Le Lait 70 89 1990

64 Suriyaphan O et al Characteristic aroma components of British Farmhouse Cheddar cheese J Agric Food Chem 49 1382 2001

65 Urbach G Relations between cheese 1047298avour and chemical composition Int Dairy J 3 389 1993 66 Karahadian C Josephson D B and Lindsay R C Contribution of Penicillium sp to the 1047298avors

of Brie and Camembert cheese J Dairy Sci 68 1865 1985




sample so that raw milk presents a distinct 1047298avor when compared to that of heated or processedmilk with different classes of compounds responsible for the characteristic 1047298avor of distinct sam-ples Generally esters are responsible for the 1047298avor of raw milk samples while lactones and het-

erocyclic compounds for that of heat-treated and pasteurized milk On the other hand fermenteddairy products such as cheese and yogurt are characterized by the presence of fatty acids [1]It is mandatory to highlight that fermentation has a major impact on the organoleptic quality

of milk-derived products raditionally fermentation was primarily applied to improve and pro-long shelf-life since the growth of fermentative microorganisms typically results in the productionof acids or alcohols that inhibits the growth of food pathogens or spoilage organisms preservingthe food Tough the chemical transformation of components in dairy food raw materials andthe release of microbial metabolites contribute to the 1047298avor appearance and texture In dairyproducts lactose citrate milk fat (lipids) and proteins are converted into a broad range of volatileand nonvolatile 1047298avor compounds

Tree main pathways for the 1047298avor formation of dairy products can be distinguished ieglycolysis proteolysis and lipolysis [23] (see Figure 121) Tese processes may already generate1047297nal 1047298avor compounds though further reactions can occur yielding an even greater number oflow-molecular weight components that essential to the overall odor of the product

In general 1047298avors derived from lactose or citrate (glycolysis) such as lactic acid acetaldehydeand diacethyl (23-butanedione) are produced by the lactic 1047298ora Lipids are also another sourceof a wide range of dairy 1047298avors and by means of lipolysis and oxidation are prone to cause hydro-lytic and oxidative rancidity respectively In this manner small amounts of γ - and δ-hydroxyacids present in milk lipids can be readily converted to γ - and δ-lactones unsaturated fatty acidsmay hydrate to hydroxy acids and then via oxidation produce γ - and δ-lactones β-oxidationfollowed by decarboxylation produces methyl ketones and oxidation at double bonds can gener-ate straight-chain aldehydes and ketones which under reducing conditions may be converted tothe corresponding alcohols Moreover protein-derived 1047298avors also comprise a large amount ofpathways Strecker aldehydes such as 2-methylpropanal 2-methylbutanal and 3-methylbutanalcan be synthesized via amino acids and in turn may produce their corresponding alcohols andsulfur-containing compounds such as hydrogen sul1047297de methanethiol and dimethyl disul1047297de(DMDS) can be derived from cysteine and methionine In addition other 1047298avor origins have

Triglycerides

Free fatty acids

Milk

Amino acids

Dairy flavor compounds

Peptides

CaseinLactosecitrate

Pyruvateα-acetolactate

Figure 121 Dairy 1047298avor formation pathways in fermented products

































products
































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961

























































































products
































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961










































































products
































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961





























































products
































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961














































































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961
































































Oxaloacetate

Citrate

Acetoin

Milk

Dihydroxyacetone-P


Ethanol

Acetate

Pyruvate

Glyceraldehyde-3-P

Tagatose-6-P

GalactoseGlucose

Lactose

Glucose-6-P












Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961



































































Peptides

Carboxylic acids

Milk


α-Keto acids

Aldehydes


esterases


esterases


dehydrogenase

Aminotransferase

Hydroxyacid


dehydrogenase



Aldolases lyases

Protease

Peptidase

lyases


PhenolIndole

Amino acids

Casein









































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961
































































































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961















































































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961

































































Free fatty acids



β-Keto acids

Methyl ketones

Secondary alcohols

Redutase


Hydroperoxides

Aldehydes

Acids Alcohols

Lactoperoxidase

Hydroperoxide lyase

γ-or δ-Lactone

Lipase

Milk

Triglycerides

































1 1961
























































































1 1961









































































1 1961





















































































































Documents

Chapter12 Flavor Formation