[Advances in Food Research] Advances in Food Research Volume 16 Volume 16 || Food Quality as Determined by Metabolic by-Products of Microorganisms
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FOOD QUALITY AS DETERMINED
OF MICROORGANISMS BY METABOLIC BY-PRODUCTS
BY M. L. FIELDS AND BONNIE S. RICHMOND Department of Food Science and Nutrition University of Missouri, Columbia Missouri
RUTH E. BALDWIN
Department of Food Science and Nutrition, and School of Home Economics University of Missouri, Columbia, Missouri
I. Introduction ........................ ............................... II. Definition and Criteria for Chemical Indicators ...........................
III. Chemical Indicators of Quality for Foods with High Protein Content .... A. Background .............................................................. B. Dominant Spoilage Flora ............................................... C. Chemical Indicators of Microbial Spoilage .............................
IV. Chemical Indicators of Quality for Foods with High Fat Content ........ A. Background ........................... ........................ B. Dominant Spoilage Flora ............................................... C. Chemical Indicators of Microbial Spoilage .............................
Content ...................................................................... A. Background ................................................. B. Dominant Spoilage Flora ..................................
V. Chemical Indicators of Quality for Foods with High Carbohydrate
C. Chemical Indicators of Microbial Spoilage ............................. VI. Research Needs .................................... .....................
161 162 163 163 167 168 198 198 198 199
203 203 204 206 218 2 19
Food quality is largely the sum of the characteristics which register favorably or adversely on an individual's senses. These characteristics include freshness, flavor, odor, texture, tenderness, consistency, color, size and shape, degree of ripeness, and presence or absence of defects. Nutritive value, chemical residues, and disease-producing organisms are also a part of food quality, although these are not measured by the senses of smell, taste, sight, or touch.
' Formerly with Dept. of Horticulture, University of Missouri. 161
162 FIELDS, RICHMOND AND BALDWIN
When one visualizes a spectrum ranging from good to poor quality, the extremes are easily differentiated. This is not true for the central part of the spectrum, however, especially for products in which the manufacturing process tends t o mask differences, as with comminuted foods. When foods become more abundant, standards of quality are likely to be higher and more clearly defined. Quality which is acceptable in one society may not be in another.
The consumer and many manufacturers depend upon the senses of smell, taste, sight, and touch for evaluating food quality. These organoleptic methods are qualitative and vary from individual to individual. The need for more adequate evaluation has focused atten- tion on chemical compounds which can be used for differentiation. Certain compounds arising from the metabolism of the dominant spoilage organisms answer this need and can be classified as chemical indicators of food quality.
Chemical indicators may be the only means of evaluating quality in some foods, since processing methods, such as filtration, preclude the use of conventional methods such as plate counts. In some foods, indicator compounds may supplement microbiological methods of analyzing food quality, including plate counts, mold counts, rot fragment counts, and direct microscopic counts. The use of chemical indicators will assist the manufacturer in producing and maintaining high-quality foods.
II. DEFINITION AND CRITERIA FOR CHEMICAL INDICATORS
A chemical compound which indicates deterioration due to micro- organisms may be defined as a metabolic by-product which is produced by the dominant spoilage organisms as a result of their growth in the food. The dominant spoilage flora is that group of microorganisms which persists and brings about deterioration in the quality of a food under the usual handling and storage conditions. Since a specific spoilage situation may involve a mixed culture, measurement of multiple compounds may be superior to the use of a single metabolic by-product as an indicator of quality.
Fields (1964b) proposed the following criteria for a chemical indicator of quality of foods: (1) The compound must be present a t low levels or absent in sound foods. (2) With increased spoilage, there must be an increase in the amount of the indicator. (3) The compound should make it possible to differentiate low-quality raw materials from
QUALITY EFFECTS OF MICROORGANISM BY-PRODUCTS 163
poor processing conditions. (4) The indicator should be produced by the dominant spoilage flora. Farber (1952) stated that a spoilage index must be as reliable as organoleptic criteria and should indicate stages of spoilage which cannot be established definitely by organoleptic testing. To be useful as an index of quality for seafood and ground beef, the test for the compound must be rapid and the analysis must be simple (Novak et al., 1956; Rogers and McCleskey, 1961). Patterson (1945) emphasized that the compound should never yield a false positive test, and for this reason a companion test is desirable. It is true that some metabolic by-products of the dominant spoilage flora also might arise by autolysis, but the amount of the compound would be markedly lower than the levels associated with spoilage due to microorganisms.
Table I summarizes the characteristics of potential and suggested chemical indicators of food quality. The suitability of the potential indicator may be determined, in part, by its physical properties. Compounds with low boiling points would be unsatisfactry where the use of heat in manufacturing or processing would volatilize them. Solubility of the compounds would affect the method of analyses. For example, acetic and formic acids can be removed from foods by steam distillation, while lactic acid must be extracted with ether or other solvents.
Some of the chemical indexes may be very acceptable in certain foods but not in others. Acetylmethylcarbinol is a valuable constitutent of the flavor components in butter. In apple juice, however, the presence of large quantities indicates the use of unfit raw materials and/or poor sanitary conditions in the processing plant (Fields, 1962a). If the chemical compound has a useful function as a component in the food, it cannot be used as a chemical indicator of the quality of the food. One may compare the presence of a chemical indicator to the presence of some microorganisms in foods. Penicillia are very essential in producing certain cheeses but are detrimental to quality when they grow in citrus fruits and produce decay.
111. CHEMICAL INDICATORS OF QUALITY FOR FOODS WITH HIGH PROTEIN CONTENT
Research on the quality of high-protein foods as indicated by meta- bolic by-products of microorganisms .has centered on seafoods, pro- bably because they are so highly perishable. Studies on the decomposi- tion of fish have been conducted by processors and by governmental
TABLE I SUh4MARY OF CHARACTERISTICS OF CHEMICAL hDICATORS OF MICROBIOLOGICAL QUALITY
Solubility ( s ) Boiling point Cold Alcohol
Indicator Description ("C) water acid, etc.
Acetylmethylcarbinol Liquid above 15OC with a pleasant
Clear, colorless acid, liquid C,H,O, Pungent odor
(AMC, acetoin, 3-
N H 3 Butyric acid
odor, a product of fermentation hydroxy-2-
Colorless gas, pungent odor. Lower
Colorless, limpid liquid. Rancid odor
Yellowish green liquid. Quinone odor
limit for human perception, 53 ppm
C*H,O, 2, 3-butanedione)
Dimethylamine Gas, strong arnmoniacal odor C,H,N
Ethyl alcohol (ethanol) C,H,OH
Formic acid Colorless, fuming liquid. Pungent CHZOZ penetrating odor. Dangerously
Clear, colorless, flammable liquid
needles Galacturonic acid White powder, forms monohydrate
s alcohol; ether
s alcohol; ether
s alcohol; ether
s alcohol; ether
slightly s hot alcohol; insoluble ether
Hydrogen sulfide H,S
Lactic acid C*H,N
c I sH34Oz C,tlH,,OZ
C 3 H P
Valeric acid C5HlOO2
Yellow crystals (needles) from water, has medical uses, occurs as a result of putrefaction
Colorless gas, flanlmable, offensive odor, sweetish taste, dangerously poisonous
intense fecal odor Colorless to yellowish scales,
Colorless to yellowish syrupy liquid
Colorless or nearly so, odorless
White, crystalline scales liquid
Clear, colorless liquid. Pungent odor
Colorless crystals. Odorless. Acid
Gas, pungent, fishy ammoniacal
Essential amino acid for rats. Found
in casein and other proteins Used as nutrient for humans White crystals
Disagreeable odor and taste Clear, colorless liquid
slightly s alcohol
very s alcohol; ether s benzene
s alcohol; ether glycerin; insoluble chlorine, petroleum ether
slightly s in cold; very s in hot
s alcohol; ether
slightly s s
S sp. s alcohol; ether
slightly s s hot alcohol
very s alcohol; slightly ether, benzene
slightly s alcohol; ether
166 FIELDS, RICHMOND AND BALDWIN
TABLE II BACTERIA AND FLOIENTOUS FUNGI CAUSING SPOILAGE OF EGGS AND SEAFOOD
Product Spoilage organism Reference
Eggs Pseudomonas fluorescens Pseudomonas sp., Achromabacter sp.
Mucor, Thamnidium, Botrytis, Alternaria, Cladosporium, Penicilliwn, Sporotrichwn
Alcaligenes. Flauobacterium, Paracolobactrum
Fish, salt Serratia, Micrococcus, Bacillus, Achromobacter, Pseudomoms
Flavobacterium, Achromobacter, Escherichia, Bacillus, Serratia
Achromobacter , Micrococcus , Kurthia , Pseudomonns, Flavobacterium, Proteus
Frazier, 1958 Frazier, 1958; Florian and
Frazier, 1958; Florian and
Florian and Trussell, 1957.
Snow and Beard, 1939.
Shellfish Flauobacterium, Pseudomonas, Bacillus, Frazier, 1958. Proteus, Achromobacter.
agencies charged by law with regulation of the purity of our food supply.
Because of the extreme perishability of some products, decomposed foods get into market channels occasionally. For example, the U. S. Department of Health, Education, and Welfare (HEW) (1966) publish- ed Notices of Judgment under the Federal Food, Drug, and Cosmetic Act which include seizures due to decomposed material in frozen eggs (FNJ 30337, 30338, 30340, 30341, 30342, 30343, 30345), frozen red snapper (FNJ 30351), frozen flounder fillets (FNJ 30354), and fro- zen salmon (FNJ 30356).
The chemical composition of the food and the metabolic activities of the organisms growing in the food determine the compounds which can be used as indicators. Although the high-protein foods (8.4- 25.2y0) included in this discussion contain 0.8-14.5% fat and 0.0-1.770 carbohydrate, our concern here is with compounds derived from the protein. It must not be overlooked, however, that substances arising from components other than protein may have a potential as indexes of decomposition. Used to detect spoilage in the past have been ammonia nitrogen, reducing substances such as dextrose, acidity of the fat, and bacteriological examination (Macomber, 1927).
167 B. DOMINANT SPOILAGE FLORA
The dominant spoilage floras for eggs and sea foods are listed in Table II. Molds cause spoilage in eggs, but to a lesser extent than do bacteria. Many of the bacteria which cause decomposition in fish may also be responsible for loss of quality in eggs. The dominant flora of fish is composed of species which grow in the sea. The organisms are mainly gram-negative rods present in the slime covering the surface of the fish. Since these bacteria are psychrophilic, spoilage may pro- ceed even under conditions of refrigeration or icing.
According to Zobell (1946), marine bacteria are actively proteolytic, and they rapidly attack most kinds of proteinaceous materials. Nearly all of them liberate ammonia from peptones, but only a few produce indole from tryptophane. Zobell also stated that marine bacteria are weakly saccharolytic.
QUALITY EFFECTS OF MICROORGANISM BY-PRODUCTS
NINE GENERA"^ PRODUCTION OF SELECTED METABOLIC B Y PRODUCTS BY 143 SPECIES OF
Number of species
Indicator Positive Negative
Ammonia 60 0 Indole 2 58 Hydrogen sulfide 31 29 Hydrolyzed fat 13 47
Achromobaeter. Flouobaeterium, Pseudomonos. Serratia, Microeoecw, Actinomyces, Vibrio, Bacterium,
*Compiled from Zobell and Upham. 1944. and Bacillus.
TABLE IV PRODUCTION OF SELECTED METABOLIC BY PRODUCTS BY 143 SPECIES OF
EIGHT GENERA'.^ Number of species
Positive Negative not given
Ammonia Indole Trimethylamine Hydrogen sulfide
29 14 6
7 107 101 28
8 129 34 95
aA chmmobacter . Flouobacterium , R e d o m o n a s , Sorcina , Serratia , A lcaligenes , Proteus, Kurthia , and bCompiled from Bergeys Manual of Determinative Bacteriology (Breed et al., 1957).
168 FIELDS, RICHMOND AND BALDWIN
If the breakdown is aerobic, the process is called decay, whereas a breakdown that is anaerobic is called putrefaction. When putrefaction occurs, various foul-smelling compounds are produced. These arise as a result of bacterial action on amino acids and include mercaptans, indole, hydrogen sulfide, ammonia, amines, and organic acids. If the protein is well aerated, digestion occurs and no ill-smelling compounds are formed.
The enzyme systems of microorganisms causing spoilage of protein include proteinases, peptidases, deaminases, and decarboxylases. The amino acids freed by the action of peptidases and the breakdown pro- ducts of amino acids resulting from deaminase or decarboxylase activity have been suggested as chemical indicators for the quality of protein foods. Other suggested compounds include succinic, formic and acetic acids, ammonia, indole, and hydrogen sulfide. Tables 111 and IV list genera of microorganisms and the number of species of these genera which are known to produce ammonia, indole, trimethylamine, or hydrogen sulfide.
C. CHEMICAL INDICATORS OF MICROBLAL SPOILAGE
a. Biosynthesis of Ammonia. Ammonia is a metabolic by-product of several bacteria which hydrolyze proteins. In fish, one of the principal sources of ammonia is urea (Elliott, 1952). As described by Salle (1961), ammonia is produced in conjunction with other products of enzyme action:
1. Production of a fatty acid and ammonia by deamination, de- car...