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126 AMERICAN POTATO JOURNAL [Vol. 51 ACIDIFICATION OF "DRY" CAUSTIC PEELING WASTE BY LACTIC ACID FERMENTATION M. GEE, C. C. HUXSOLL AND R. P. GRAHAM 1 ABSTRACT The WURPEEL process used by the potato industry produces a peel waste of high solids. This material is easily handled and collected, but due to the caustic used for peel release it has high residual alkalinity. This waste when acidified to neutralize the excess alkali yields a useful feedstuff for cattle. A nlethod for neutralization of the excess alkali by lactic acid fermentation is described. INTRODUCTION Recent developments in pollution abatement at the Western Regional Research Laboratory resulted in the "dry" caustic peeling process called "WURPEEL," Graham, et al. (5). This method has been quickly adopted for commercial installations by the fruit and vegetable processing indus- tries (1, 6, 8). The success of this innovation has resulted in water con- servation, reduced pollution, and lower processing costs (8). As the result of the low water usage, the peel waste was produced at a relatively high solids content that would be suitable directly for animal feed except for the high alkalinity (5). Previous studies have shown that potatoes ir~ various forms, including potato silage, make excellent feed for livestock (7). These studies were made to find a relatively low cost method to convert the high-solids alkaline waste to an animal feed ingredient. Bacterial lactic acid fermentation, similar to that involved in the ensiling process, was considered a non-toxic, preservative acid system suitable to neutralize the sodium hydroxide used in the peeling process. METHODS AND MATERIALS The initial study on fermentation of "dry" caustic peeling waste was on material from the experimental mechanical peeling device which had a solids content of 22-25% and a pH of 10.5 (5). Peel wastes from com- mercially designed equipment had a solids content of 15-16% and a starting pH of 11.4-12.9 (2). Fermentation was initiated bv placing open dishes of the peel waste in the laboratory and allowing the ambient air to inoculate them with a bacterium suitable for culture. A desirable change was detected by a pH drop after overnight incubation. Once the desired lactic acid fer- mentation, was started in the waste material, this mash. was used to inocu- late further batches for our studies. To simulate possible commercial situations, a smalt test was made in which peel waste from a commercial dry caustic peeling operation was added incrementally to a pool of previously fermented material. Initially a slurry was made using 7 lb (3.18 kg) of ground barley and 9 lb (4.18 kg) of distilled water. The slurry was contained in a XWestern Regional Research Laboratory, Agricultural Research Service, U. S. De- partment of Agriculture, Berkeley, California 947,10. Received for publication August 3, 1973.

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Page 1: Acidification of “dry” caustic peeling waste by lactic acid fermentation

126 AMERICAN POTATO JOURNAL [Vol. 51

A C I D I F I C A T I O N OF "DRY" CAUSTIC P E E L I N G W A S T E BY LACTIC ACID F E R M E N T A T I O N

M. GEE, C. C. HUXSOLL AND R. P. GRAHAM 1

ABSTRACT

The WURPEEL process used by the potato industry produces a peel waste of high solids. This material is easily handled and collected, but due to the caustic used for peel release it has high residual alkalinity. This waste when acidified to neutralize the excess alkali yields a useful feedstuff for cattle. A nlethod for neutralization of the excess alkali by lactic acid fermentation is described.

INTRODUCTION

Recent developments in pollution abatement at the Western Regional Research Laboratory resulted in the "dry" caustic peeling process called "WURPEEL," Graham, et al. (5) . This method has been quickly adopted for commercial installations by the fruit and vegetable processing indus- tries (1, 6, 8). The success of this innovation has resulted in water con- servation, reduced pollution, and lower processing costs (8). As the result of the low water usage, the peel waste was produced at a relatively high solids content that would be suitable directly for animal feed except for the high alkalinity (5).

Previous studies have shown that potatoes ir~ various forms, including potato silage, make excellent feed for livestock (7) . These studies were made to find a relatively low cost method to convert the high-solids alkaline waste to an animal feed ingredient. Bacterial lactic acid fermentation, similar to that involved in the ensiling process, was considered a non-toxic, preservative acid system suitable to neutralize the sodium hydroxide used in the peeling process.

METHODS AND MATERIALS

The initial study on fermentation of "dry" caustic peeling waste was on material from the experimental mechanical peeling device which had a solids content of 22-25% and a pH of 10.5 (5). Peel wastes from com- mercially designed equipment had a solids content of 15-16% and a starting pH of 11.4-12.9 (2).

Fermentation was initiated bv placing open dishes of the peel waste in the laboratory and allowing the ambient air to inoculate them with a bacterium suitable for culture. A desirable change was detected by a pH drop after overnight incubation. Once the desired lactic acid fer- mentation, was started in the waste material, this mash. was used to inocu- late further batches for our studies.

To simulate possible commercial situations, a smalt test was made in which peel waste from a commercial dry caustic peeling operation was added incrementally to a pool of previously fermented material.

Initially a slurry was made using 7 lb (3.18 kg) of ground barley and 9 lb (4.18 kg) of distilled water. The slurry was contained in a

XWestern Regional Research Laboratory, Agricultural Research Service, U. S. De- partment of Agriculture, Berkeley, California 947,10. Received for publication August 3, 1973.

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1974] GEE, et a / : FERMENTATION OF POTATO WASTE 127

covered desiccator and placed in a 90 F (32.2 C) room for 3 days, during which time the pH dropped to 4.1. The fermented mash had a distinct "cheese-type" odor indicative of lactic fermentation. No butyric or putrid odors were noticeacble.

Alkaline peel waste, pH 12.3, with 20% added water, to make it less viscous, was added to the fermented barley to adjust the pH at 6.0, and the material was transferred to a stainless steel tank. A lid covered the tank, but the system was not anaerobic, the tank was kept at ambient temperature, approximately 75 F (23.9 C).

Over a period of 9 days, five additions of undiluted peel waste were made to the tank. Although the material was thoroughly stirred after each addition it was not homogeneous because of the high viscosity of the alkaline peel waste.

Measurements of pH were made before and after each addition of peel waste. After the last addition of peel waste on the ninth day, the material was held for an additional 5 days to determine if any significant changes occurred.

A proximate analysis of the peel waste is given in Table 1. Gas chromatography (3, 4) was used to follow the involvement of

amino and non-amino acids in the fermentation. Paper chromatography was used to follow the presence of lnono and disaccharides in the acidi- fication of the waste.

Microbial identifications were made as follows: A small weighed portion of the sample was aseptically placed in a dilution bottle containing 100 cc of diluent (0.5% NaC1, 0.1% Bacto tryptose) and shaken on a reciprocal shaker for about 5 minutes. This was diluted, placed on Bacto plate count agar and incubated at 90 F (32.2 C) for 2 days. The plates were counted and individual colonies streaked out on plate count agar. Cultures of the organisms were maintained on BBL Cystine trypticase agar to which an additional 0.2% agar had been added.

Enzymatic hydrolysis of the gelatinized starch in potato peel waste was accomplished by using barley grain and amylolytic enzymes, i(hozyme H39 and Rhozyme S. A combination of the two commercial enzymes at the 0.03% level was used.

TABLE 1.--Proximate analysis of alkaline potato peel waste.

As received basis (%) Dry basis (%) Solids 16.4

N 0.22 1.35 F 0.05 0.31

Fiber 0.58 3.61 Ash 2.23 13.9 Na ........ 4.34 P203 . . . . . . . . . . . . . . . .

Hydrolyzable carbohydrate in trichloroacetic acid 90 min I 120 C 63.3

Alcohol soluble sugar 0.45

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128 AMERICAN POTATO JOURNAL [Vol. 51

RESULTS AND DISCUSSION'

Air-borne bacteria were trapped in open dishes of potato peel waste at pH 10.8 and pH 12. Cultures of these bacteria produced an acid fermentation that lowered the pH from 10.8 to 6.3 in 3 days at 75 F (23.9 C). The bacteria in the pH 12 waste took 2 weeks or longer to reach pH 8.6. The strain of bacteria trapped, in the pH 10.8 dish showed the desired production of lactic acid and at a rate within reason but a starting pH of 12 was too alkaline to encourage an active lactic acid fermentation.

Gas chromatographic examination of the fermentation product showed that the increased acidity was from lactic acid production without any change in the malic and citric acids naturally occurring in potato material. The slow rate of reaction at pH 12 was considered undesirable as other fermentations could be introduced to contaminate the food value of the pcel wastes before a preservative lactic acid concentration could be pro- duced.

Attempts to accelerate the fermentation rate by increasing the incu- bation temperature to 85-90 F resulted in an acid fermentation with putre- fication after 24 hours. The initial free sugars apparently were fermented rapidly at 85-90 F (29.4-32.2 C) in a few hours and the lactic acid pro- duction ceased.

Lactic acid fermentation depends on the presence of mono and di- saccharides, Starch per se will not produce lactic acid .and must be hydrolyzed to mono and disaccharides. Analyses of various peel wastes from commercial plant operations showed a simple sugar content of 0.07 to 0.29% and a hydrolyzable carbohydrate content of 7.5-10% as received (2). The low simple-sugar content was not sufficient to produce a desirable level of lactic acid and either time must bc allowed for amylolysis of the starch by bacterial enzymes or enzymes must be introduced if a faster rate of lactic acid fermentation is desired from the hydrolyzable carbohydrates. During 3 days' fermentation at 75 F (23.9 C), solne starch had been hydrolyzed and the lactic acid production proceeded at a slower rate to reach a pH of 5.3 after 7 days. When barley was used in the fermentation to introduce amylolytic reaction a pI-I of 4.6 was reached as shown in Fig. 1. Using amylolytic enzymes Rhozyme H39 and Rhozyme S to hydrolyze the gclatinized starch to dextrins and then glucose and maltose, an enzyme level of 0.03% of each would drop the pH of the waste from 8 to 4 in 6 days. As this occurred the consistency of the waste changed to a watery thin gruel. Maximum lactic acid production was accelerated by adding barley or commercial enzymes for amylolysis.

At 90 F (32.2 C) fermentation of potato waste tended to produce butyric and caproic acids; 75 F (23.9 C) lactic and succinic acids were the principal products. Control of temperature or accelerated starch hydrolysis might be used to control the formation of putrid products.

Along with the increase in non-amino acids during fermentation, an increase in free amino acids by proteolytic reaction was observed by gas chromatographic examination.

The use of a barley mash to initiate and accelerate starch hydrolysis to maintain a steady rate of lactic acid production is shown in Fig. 1. These data resulted from the small scale fermentation tests in which alkaline peel

Page 4: Acidification of “dry” caustic peeling waste by lactic acid fermentation

1974] GEE, et d : FERMENrI'ATION OF POTATO WASTE 129

,s I . . o r.o

6.0 J J ~ - 70

l i l t ~ "., .,- I I~ l i

nl Io. J \

4.0 30

0 2 20 4 6 8 10 12 14

TIME-DAYS

FIG. 1.--Rate of lactic acid fermentation of potato peel waste. Fermentation initiated in barley mash with increments of alkaline peel wastes added.

A - - pH after fermentation. �9 - - pH after addition of alkaline peel waste. (D - - Weight of fermentation mash.

waste was added to a slurry of previously fermented material. Because of problems in thoroughly mixing the undiluted peel waste with the fermented material the pH values after a new addition were probably lower than they would have been with complete mixing. The sample was thoroughly stirred prior to each pH reading, and the values taken prior to addition of more peel waste should be representative of the entire batch of material.

There appeared to be no problem maintaining a good lactic fermenta- tion under these conditions. Lactic acid from previously fermented waste neutralized incoming waste so it too could be fermented. Within 24 hours after any single addition was made the entire sample had dropped to less than p H 5 (Fig. 1).

The largest percentage addition was made on the fourth day when the added alkaline peel waste was equivalent to 34% of the fermented material. On the seventh and ninth days additions of 27% and 25%, re- spectively, were made. During the 5-day period when no additions were made the sample appeared to remain stable indicating the preservative effect of the lactic fermentation. No doubt secondary fermentations would have occurred if the material had been held for a significantly longer time without the addition of more peel waste, but such long periods do not appear to be of practical interest.

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130 AMERICAN POTATO JOURNAL [Vol. 51

Several physical changes occurred as a result of fermentation. As the pH was reduced the color of the material changed from the "dark-ma- hogany" color of the alkaline peel waste to a light gray. The viscosity also decreased significantly during fermentation.

The alkaline peel waste had a distinct ammoniacal odor that disap- peared during fermentation, and the fermented material had the cheese- type odor normally associated with a lactic fermentation.

Although the conditions of the experiment did not allow an accurate mass balance to be made, only about 2% of the material added to the sample was unaccounted for at the termination of the experiment. This small loss included any spillage that may have occurred during pH sampling and peel waste addition. Apparently losses due to respiration during fermentation were quite low.

The species captured in the open dish produced lactic acid and were shown by culture identification to be Micrococcus and Bacterhtm species. Organisms found were as follows:

A. Small, bright yellow, shiny, round, raised colony; rods occurring singly or in pairs; gram positive, catalase positive, no evident spores when stained with malachite green. Approximate concentration 10~/gram sample.

B. Cream colored, shiny, irregular edge, raised colony; cocci occur- ring singly or in pairs; gram positive, catalase positive. Approximate con- centration 107/gram sample.

C. Orange, irregular edge, shiny raised colony; rods occurring mostly in pairs; gram positive, catalase positive, no evident spores when stained with malachite green. Apppeared to be similar to A except in colony color. Approximate concentration 10V/gram sample.

D. Cream colored, shiny, round colony; small rods occurring in chains of 2 or 3; gram positive, catalase positive, no evident spores when stained with malachite green. Approxilnate concentration 5 )< 106/gram sample.

DISCUSSION AND CONCLUSION

Many fermentations of sugars are known to produce lactic acid by bacterial action in neutral or acidic media. Generally sugar concentrations of 5-20% are used to maintain an active fermentation and temperatures closer to 90-100 F are used. However, potatoes are a suitable medium to support lactic acid fermentation as the necessary nutritional needs of lactic-acid producing bacteria are inherent to the potato tuber.

The WURPEEL waste represents a large gelatinized starch pool which has been essentially sterilized by the high alkalinity and heat treatment. The low initial free sugar content was enriched by starch hydrolysis which was slowly initiated by bacterial enzymes or under controlled con- ditions by the addition of barley grains or commercial amylolytic enzymes.

The rate and amount of lactic acid produced were sufficient to neutralize a residual peel waste alkalinity of 2-3% free caustic and to sustain a fermentation to a pH of 4.

Use of the WURPEEL process by the food industry for other foodstuffs, produces other high-solids, alkaline wastes. These too might be fermented in a similar manner if a large carbohydrate source is available to produce the sugars necessary for lactic acid production.

Page 6: Acidification of “dry” caustic peeling waste by lactic acid fermentation

1974] GEE, el al: FERMENFFATION OF POTATO WASTE 131

Thus a conversion of a food industry waste was developed to produce a useful animal feed which is currently being used.

ACKNOWLEDGMENTS

The authors wish to thank Mrs. Carol Nelson for her assistance in microbial identification.

Reference to a company or product name does not imply approval or recommendation of the product by the U. S. Department of Agriculture to the cxclusion of others that may be suitable.

LITERATURE CITED

1. Anon. 1970. Commercial infrared peeling process. Food Processing 31(1) : 23-29. 2. Block, F., G. E. Brown and D. F. Farkas. 1973. Utilization of alkaline potato

peel waste by fermentation. Amer. Potato J. 50: 357-354. 3. Gee, Mildred. 1955. Methyl esterification of nonvolatile plant acids for gas

chromatographic analysis. Anal. Chem. 37: 925-928. 4. Gee, Mildred. 1957. Preparation of methyl esters of amino acids for gas

chromatography using dimethyl dodcconedioate as an Internal Standard. Anal. Chem. 39: 1577-1579.

5. Graham, R. P., C. C. Huxsoll, M. R. Hart, M. L. Weaver and A. I. Morgan, Jr. 1959. Dry caustic peeling of potatoes. Food Tcch. 23: 195-201.

6. Graham, R. P., C. C. Huxsoll, M. R. Hart, M. L. Weaver and A. I. Morgan, Jr. 1959. Prevents potato peel pollution. Food Eng. 41(5) : 91-93.

7. Highland, M. E. 1957. Potatoes for livestock feed. In "Potato Processing," Talburt, W. F. and O. Smith, eds., Avi Publishing Co.

8. Willard, Miles J. 1971. Infrared peeling: Reduces pollution, cuts costs. Food Tech. 25 : 125-,129.