Flavor Defects

Consumer acceptance of milk is strongly determined by its sensory characteristics. The development of off-flavor in milk as a result of lipolysis can reduce the quality of milk. The enzymatic release, by milk lipase, of free fatty acids (FFA) from triglycerides causes a flavor defect in milk described as rancid . Triglycerides in milk contain both long chain and short chain fatty acids, which are released at random by milk lipase. The short chains FFA, like butyric acid, are responsible for the off-flavor. 

Puspitasari, N. L., Lee, K., and Greger, J. L. (1991). Calcium fortification of cottage cheese with hydrocolloid control of bitter flavor defects. /. Dairy Sci. 74,1-7. 

At the end of the fermentation the wine will be turbid and muddy from the suspended yeast cells and the debris from the fruit. Most of this material will settle quickly, forming a more or less thick layer in the bottom of the bottle. The new wine should be syphoned away from the deposited sediment promptly to avoid off flavors from autolysis of the yeast in the sediment. Also at this time wine acquires the defect of H2S which is produced by reduction of the elemental sulfur dust applied to the grapes as fungicide by the decomposing yeast cells in the thick layer of sediment. The first transfer of the new wine from the sediment should occur very soon after active fermentation, the second about two weeks later, and the third and possibly a fourth two to six months later. These rackings may be conducted under a blanket of nitrogen or carbon dioxide if the particular wine oxidizes easily. 

Dried whole milk is prepared by conventional spray or roller drying, with some modifications of the preheat treatment of the milk. The product is usually stored under nitrogen to delay lipid oxidation and off-flavor development. In spite of the processing changes, flavor defects and short storage life have limited the markets for dried whole milk most of it is used in the confectionery and baking industries. Federal and FAO standards require a minimum of 20% but less than 40% milk fat and a maximum of 5% moisture. 

The flavor defect commonly referred to as rancidity or, more specifically, as hydrolytic rancidity is caused primarily by the presence in milk of a single enzyme which was proposed to be designated as... 

Heat-Resistant Lipases. The heat-resistant lipases and proteinases and their effects on the quality of dairy products have been reviewed (Cogan 1977, 1980). Several reports have linked the lipases from bacteria with the off-flavor development of market milk (Richter 1981 Shipe et al. 1980A Barnard 1979B). The microflora developing in holding tanks at 4°C [and presumably in market milk stored at 40°F (Richter 1981)] may produce exocellular lipases and proteases that may survive ordinary pasteurization and sterilization temperatures. Rancidity of the cheese and gelation of UHT milk appear to be the major defects caused by the heat-resistant enzymes. 

Although rancidity is a serious defect in market milk, it has also been utilized profitably. Whole milk powder made from lipase-modified milk has generally been accepted by chocolate manufacturers. It is used as a partial replacement for whole milk because it imparts a rich, distinctive flavor to milk chocolate, other chocolate products like fudge, and compound coatings, caramels, toffees, and butter creams (Ziemba 1969). 

Fluid milks have been classified by Thurston (1937) into three categories based on their ability to undergo oxidative deterioration (1) spontaneous, for those milks that spontaneously develop off-flavor within 48 hr after milking (2) susceptible, for those milks that develop off-flavor within 48 hr after contamination with cupric ion and (3) resistant, for those milks that exhibit no flavor defect, even after contamination with copper and storage for 48 hr. A similar classification has been employed by Dunkley and Franke (1967). 

Day, E. A., Forss, D. A. and Patton, S. 1957. Flavor and odor defects of gamma-irradiated skim milk. I. Preliminary observations and the role of volatile carbonyl compounds. J. Dairy Sci. 40, 922-931. 

Fouts, E. L. 1940. Relationship of acid number variations to the qualities and flavor defects of commercial butter. J. Dairy Sci. 23, 173-179. 

Excessive or insufficient acid development during manufacture can produce variability in the moisture content of cheese and defects in flavor, body, texture, color, and finish (Van Slyke and Price 1952). The rate of lactose fermentation varies with the type of cheese, but the conversion to lactic acid is virtually complete during the first weeks of aging (Van Slyke and Price 1952 Turner and Thomas 1980). Very small amounts of lactose and galactose may be found in cheese months after manufacture. (Huffman and Kristoffersen 1984 Turner and Thomas 1980 Harvey et al. 1981 Thomas and Pearce 1981). Turner and Thomas (1980) showed that the fermentation of residual lactose in Cheddar cheese is affected by the storage temperature, the salt level in the cheese and the salt tolerance of the starter used. 

Hydrolytic rancidity flavor defects in Swiss, brick, and Cheddar cheeses have been linked to high concentrations of individual short chain free fatty acids (Woo et al 1984). Lipases from psychrotrophic bacteria have been implicated in causing rancidity in cheese (Cousin 1982 Kuzdzal-Savoie 1980), although most starter streptococci and lactobacilli isolated from cheese are also capable of hydrolyzing milk fat (Paulsen et al. 1980 Umemoto and Sato 1975). Growth of Clostridium tyrobutyricum in Swiss cheese causes the release of butyric acid and subsequent rancid-off flavors (Langsrud and Reinbold 1974). The endogenous lipoprotein lipase is also responsible for hydrolytic rancidity in nonpasteurized milk. 

Langsrud, T. and Reinbold, G. W. 1974. Flavor development and microbiology of Swiss cheese—a review. IV. Defects. J. Milk Food TechnoL 37, 26-41. 

Fruity flavor in Cheddar cheese is also associated with high levels of ethyl butyrate and ethyl hexanoate (Bills et al. 1965). However, this defect is usually caused by esterase activity from lactic acid bacteria, especially S. lactis and S. lactis subsp. diacetylactis (Vedamuthu et al. 1966). Fruity-flavored cheeses tend to have abnormally high levels of ethanol, which is available for esterification (Bills et al. 1965). Streptococcal esterase activity in cheese is affected by the level of glutathione, which suggests a dependence on free sulfhydral groups for activity (Harper et al. 1980). 

Musty or potato-like flavor and aroma have been observed as a defect in milk (Hammer and Babel 1957) and Gruyere de Comte cheese (Dumont et al. 1975). This off-flavor results from the production of nitrogenous cyclic compounds by Pseudomonas taetrolens and P. perolens (Morgan 1976). Musty-flavored compounds produced by these organisms include 2,5-dimethylpyrazine and 2-methoxy-3-isopropyl-pyrazine. The Gruyere de Comte with potato off-flavor contained 3-methoxy-2-propyl pyridine, as well as alkyl pyrazine compounds (Dumont et al. 1975). Murray and Whitfield (1975) postulated that alkyl pyrazines are formed in vegetables by condensation of amino acids such as valine, isoleucine, and leucine with a 2-carbon compound. Details of the synthetic mechanism in pseudomonads are unknown. 

The flavor and body of cultured milks are distinguished by a delicate balance between components of the cultured product. Unless conditions of culture are carefully controlled, this balance may not be achieved even when pure cultures are employed. Empirical formulations relating to proper cultural and environmental conditions constitute the art of fermentations. Apart from problems arising from the use of improper conditions of culture, other defects may occur in the milk products because of contamination of a milk supply by unwanted organisms and because some organisms required in fermentations can produce antibiotics (bacteriocius). 

Morgan, M. E. 1976. The chemistry of some microbially induced flavor defects in milk and dairy foods. Biotechnol. Bioeng. 18, 953-965. 

Sandine, W. E., Dedy, C., Elliker, P. R. and Vedamuthu, E. R. 1972. Causes and control of culture-related flavor defects in cultured dairy products. J. Dairy Sci. 55, 1031-1039. 

Prolonged storage of ice cream and exposure to severe temperature fluctuation commonly causes shrinkage, which is a defect due to partial thawing and loss of moisture and air. An additional defect common to ice cream after prolonged storage is oxidized flavor, which is caused by autoxidation of milk fat. This defect is especially important in ice cream products that contain frozen or dried milk ingredients. 

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