lournal Club

Thermal gelation of globular proteins: weight-average molecular weight depen- dence of gel strength C-H. Wang and S. Damudaran

’ 1. Agric. Food fhcz. 38, 1157-l 164

Although empirical rules can be used to predict the physical appearance cf heat-induced gels of glohrutar proteins, there is little information regarding the molecular properties that determine the rl-eological properties of such esls. The

strengths of gels of bovine serum albu- mm, egg wi!i:e. soy protein isolate, soy 75 globulin and soy 11s globulin were de&mined after thermal Se!z;L~n irr the presence and abxnce of cysteine and N-e?,rimaleimide. For boyine serum albumin and egg white, the gel strength was of the order:

no additive > protein + cysteine > protein + r’+thy:~aleimide.

in contrast, tire tirder of ~rl strength for soy 11 S globulin was: ! 1 S globulin > 11s globulin * Wzthyt- mateimide > 1lS globulin + cybteine.

For all the protein: ~~unred, the square ror? 25 gel hardness wai linearly r&ted to the weight-average mo!ec:l!ar wcrght of polypeptides in the gti net- :vo:k. By extrapolating this relationship to zt;? gel hardness, it ‘*as e&mated that the minimum Iteight-average molecular weight for the formation of a get netwrk from globular pmte:ns was -23 IrOn. The aurhorr. ioncl&d irom their data that the ~izr and shape ui n?!ypeptides in a get retwork are mere intportant determinants of se! hardnccs than the chemical properties of the pelv~p:ioes.

Survival of pathogenic microorganisms in an egg-nag-like product containing 7% ethanol S. Notermans. P.S.S. Soentoro and E.H.M. Delfgou-Van Arch, I&. 1. Food Microbial. 10, 209-218

Recent trends have led to the devel- opment of eggnog-like products with low atcohol contents (e.g. 7% w!v; versus -17% wiv, in conventional

nxipes). To assess rhe microbiologicat safety of ruch productj, liqueurs con- sisting ef whole egg, sugar {25X, w/v) and varying proportions of brandy* to give a range of final ethanol contents, were artiiicially contaminated with a variety of common food pathogens.

Ethanol concentratons greater than 3% @v/v) were effective in preventing the growth of Salmonella spp., and con- centrations of 51’, C\r:/v! or *ore pre- vented the growth of Staphylococcus aureus. Alcohol at a concentration of

7% fw&l was more effective at reduc- inp pathogen counts at 22’C than et 4’C. For example, ;*:er three xeeks of storage at 22X, 1. ? wmhr_ of Sahwne/la spp._ Staph, aweus &xi Lisreria momcyfogenes decreased by more :han 3 log!,? c)cles. in contrast, a similar reduction in pathogenic microorganisms required seven weeks of incubation at 4’C. Uniortunatelv.. however, while pathogen COWlS

decreased at IX. total colony caunt~ increased. limiting the shelf life at this temperature.

Approaches to protein hydration and water activity in food protGns by nuclear magnetic re!axz?tIon and vapor pressure equilibration techniques P.P.. Myers-Betts and I.C. Baianu, 1. Agric. Food Chem. 38, 1171-1177

The relationships between protein- protein interactions and water activity in corn zein and lysozyme solutions were studied using I70 and ‘H NMR re!axation measurements. Values for protein activity (a$ were calculated based on a virial ewpanrion. Theoretical

water activity (a,) values v:e;e derhed from the ap data. However, when the NMR-derived a, values were compared to nrub!ished vapour sorption isotherm data, there were significant differences between the values at protein concentrations greater than 0.2 t$ml. The au!hsrs proposed that differences be&Wen the hvo sets oi water activity data were due to the fact that moisture sorption equi!ibratis:: depends on the refatively slow translational diffusion oi water molecules, whereas NMR relaxation values depend primariiy on the &I niotions oi water molecutes. Differences between the a, values derived by ‘;O NMR and ‘H NMR were attributed to the different mechanisms contributing to the :\&:I? relaxation of the two nuclei.

The Journal Club section comprises a selection of brief reports on recently published research papers of interest to food scientists. Though not exhaustive in its scope, the section is designed to keep readers informed of developments in areas other than their own.

Trends in Food Science & Technulogy.September 1990 ., i

Modeling &mica! and biochemical changes under sinusoida!! tern-perature fluctuations

Since chemical and biochemical re- x!:ons depend strong’? on Lempera- ture, the timctemperature historv ai stored food produG: is an important factor in determining the o~rall extent of changes during stordgc. !+xk

sror4 in war~heusr~~; or m fifi%cars undergo cyclic diurnal-nocturnal temperature fluctuations that often iX0w a sine function. C:asrically. the .4rrheniwr equation has been used to model the effect oi temperature on reactmn rate. Howe,;=r, there is no anah?ical solution for the Arrhenius modeI under sinusoidal temperature iluctrrations. and m&rods hare been jsed that apprnuimate to the intetxal of the Arrhenrus equation. A new mudel. based on a modiiied Bessel iurictiun, W’s.

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