Milk products, rancidity

The restricted shelf life of liquid milk continues to be a problem that is often more influenced by the type of milk being sold rather than the pasteurisation technique. The shelf life of processed milk is determined primarily by the quality of the raw milk from the dairy herd. Increasing cell counts in the milk and a higher concentration of free fatty acids, contribute to rancidity in both liquid milk and milk products. Janzen (1972) reported that the 0-14 day shelf life of pasteurised milk is influenced by the somatic cell concentration in the raw milk and found that after 14 days any observed changes in the flavour and stability of the milk were attributable to microbial activity during storage. 

SOD is more heat stable in milk than in purified preparations in milk it is stable at 71°C for 30 min but loses activity rapidly at even slightly higher temperatures. Slight variations in pasteurization temperature are therefore critical to the survival of SOD in heated milk products and may contribute to variations in the stability of milk to oxidative rancidity. 

Deeth, H.C. and Fitz-Gerald, C.H. (1995) Lipolytic enzymes and hydrolytic rancidity in milk and milk products, in Advanced Dairy Chemistry, Vol. 2 Lipids, 2nd edn (ed. P.F. Fox), Chapman Hall, London, pp. 247-308. 

To increase the stability of milk products. Lipoprotein lipase is probably the most important in this regard as its activity leads to hydrolytic rancidity. It is extensively inactivated by HTST pasteurization but heating at 78°C x 10 s is required to prevent lipolysis. Plasmin activity is actually increased by HTST pasteurization due to inactivation of inhibitors of plasmin and/or of plasminogen activators. 

Hydrolytic rancidity in milk and milk products has been a concern to the dairy industry of most countries (Downey, 1975). Although it is not... 

Hydrolytic rancidity results from the hydrolytic degradation of milk lipids. The hydrolysis is catalyzed by lipases and produces free fatty acids (FFAs), some of which have a low flavor threshold and can cause unpleasant flavors in milk and milk products. These flavors are variously described as rancid, butyric, bitter, unclean, soapy or astringent. The lipases involved are of two types indigenous milk enzyme(s) and enzymes of microbial origin. 

Causes of Hydrolytic Rancidity in Milk and Milk Products... 

Mastitis and microbial contamination can also contribute to hydrolytic rancidity. In general, lipolysis caused by indigenous milk lipase accounts for most of the rancidity in raw milk and cream microbial lipolysis is of minor practical importance as little if any lipolysis occurs before the bacterial population reaches 106—107 cfu/ml (Suhren and Reichmuth, 1990). However, in stored milk products, lipolysis by microbial lipases is of greatest significance. Short shelf-life products such as pasteurized milks may be affected by pre-pasteurization lipolysis caused by milk lipase but may be affected by bacterial lipolysis at the end of their shelf-life (Deeth et al., 2002). 

Sufficient knowledge exists to enable dairy industry personnel to take precautions to ensure a low incidence of hydrolytic rancidity and associated problems in milk and milk products. Recommendations for prevention have been published (Deeth and Fitz-Gerald, 1976 IDF, 1991 O Brien et al., 1996). A summary of these is given below. 

An increasing problem is lipolysis in butter fat after manufacturing, which is caused by thermoresistant hpase enzymes that are created in the milk or cream by psycho-trophic bacteria or by residual native lipases that sirrvive pasteurization. Based on a determination of the lipase activity in cream, the keeping quality of manufactured butter in regard to hpolysis can be predicted with reasonable accirracy. A similar prediction for sweet cream butter can be based on lipase activity in the serum phase (71). The characteristic lipolytic flavors that can develop in milk products are primarily associated with the short- and medium-chain fatty acids that are relatively abundant in milkfat they have lower flavor threshold values than the long-chain fatty acids. As a result of improvements in the quality of raw milk and the standards of processing, lipolytic rancidity is seldom present in the fat source before its use in recombination (72). 

Electronic nose technology and analysis of volatiles has long been apphed in the food industry to control the quahty of food products and to determine shelf hves. For example, sensor arrays based on different Sn02 gas sensors can be used to distingiush milk products of different rancidity levels [41]. Standard microbial test prediction of shelf hfe of milk products has a low level of reliability due to relatively poor correlation between microbial counts and actual shelf hfe. Several alternative methods have therefore been developed. One method is based on dynamic headspace capillary gas chromatography analyses of volatiles in mUk followed by MDA analyses. [42]. Principals of this method were later used for development of a faster and simpler test, where the extraction was performed by the SPME technique, the extracts... 

The inhibition of the enzyme tyrosinase may very well be a key to the control of melanoma, and some of the known inhibitors include eommon substances. Thus, vitamin C, among other common and uncommon substances, has been listed as an enzyme inhibitor for tyrosinase in M.K. Jain s Handbook of Enzyme Inhibitors, 1965-1977 (1982). In addition to ascorbic acid (vitamin C), these other substances include the following halide ion (e.g., from the chloride of common salt, or from iodides and fluorides) butyric acid (from rancid butter) lactic acid (the end product of cancer cell metabolism, found naturally in sour milk products) oxalic acid (ordinarily considered toxic, although it occurs naturally in rhubarb and wood sorrel, etc.) formic acid (a component of ant stings) tyrosine itself and deadly cyanide (which is a chemically bound component of laetrile), as found in almonds (notably bitter almonds), in apricot seeds, and in certain legumes such as beans, etc., although the heat from cooking may drive off the cyanide content. 

With regard to melanoma, which involves the enzyme tyrosinase, there are a number of inhibitors listed in the handbooks for this particular enzyme. Among them, interestingly, is ascorbic acid, or vitamin C, as well as some other commonly encountered substances such as lactic acid (from sour milk products) and butyric acid (from rancid butter). The aforecited alpine sunflower/yucca extract developed by Owen Asplund of the University of Wyonming may very well act as an enzyme inhibitor for tyrosinase. 

Lipolysis is important from the standpoint of the qnality of milk and dairy products. The natural lipoprotein lipase in milk is the main agent in the lipolysis of raw milk. In dairy products, hydrolytic rancidity is primarily cansed by heat-stable lipases prodnced by psychrotrophic bacteria or by residnal native lipase that survived heat treatment. The milk lipase and most bacterial lipases preferentially attack the FA residnes in the terminal positions. Becanse milk fat contains relatively high proportions of short-chain FA, which are esterified in the in-3-position, lipolysis increases the level of these FA in free form. The short-chain FA are solnble in water and they may be present in dissociated form (salts) or associated form (acid form), depending on the pH of the aqueous phase of the milk product. The characteristic lipolytic flavors occur when the acids are present in the acid form. Some FA have low flavor thresholds, and even very small amonnts canse an unpleasant, rancid flavor of milk and dairy products. Only in some types of cheese (e.g., mold cheeses) is moderate lipolysis desirable for specific flavor. 

Oxidation of milk lipids is an important cause of flavor deterioration of dairy products that is often referred to as oxidative rancidity, to distinguish it from hydrolytic rancidity resulting from lipolysis. Milk products have complex compositions, physico-chemical properties, and contain natural prooxidants... 

The volatile carbonyl compounds developed from milk fat oxidation are detectable at extremely low levels (parts per bilhon). Flavor compounds are less volatile and mainly retained by the fat component. These compounds are thus much more easily perceived in milk, where they have much lower threshold values (0.004-0.1) than in oil systems (0.1-2.5). Because flavor compounds are very easily perceived in milk products, they can be detected at extremely low levels of oxidation. Fluid milk becomes rancid at peroxide values less than one. 

Lactic acid is also a natural product. As its name implies, it is in milk that, in 1780, Carl Wilhem Scheele found an acid, which he separated by crystallizing a calcium salt. Scheele had discovered the "milk acid" but thought it to be a milk component and not a fermentation product of rancid milk. Lactic acid has thus been used for centuries as a natural preservative in many food products. Lactic acid is the smallest chemical molecule with an asymmetric carbon and therefore exists with two optical-isomers the L (+) and the D (-). The L form is the natural one, and is naturally present in animal and human tissues as well as in numerous food products (meat, milk products, pickles, beer, etc.). Today, lactic acid, its salts and esters are extensively used in food, cosmetic and pharmaceutical industries. These industries show preference for the L-lactic acid and its related compounds because the D form cannot be metabolized by the human body. 

Regarding the compositional differences shown for animal products, whether they are important for health depends, as for the plants, on the overall composition of the diet of the people who eat them. The vitamin E content in milk is far too small for relevant differences to affect health (Nielsen ef al. 2004), and too little is known about the dose-response relations of the impact of conjugated linoleic acids on health. However, the increased vitamin E level may still be important to prevent oxidation of the fat, a problem that can be exacerbated by increased levels of polyunsaturated fatty acids (Dhiman ef al. 1999, Nielsen ef al. 2004). However, while oxidised milk is clearly not good for health, its rancid taste and smell allows detection and rejection before consumption and thus prevents harm to health, similar to plant toxicants. Also in line with the plants, while the use of roughage is clearly more extensive in organic farming, some conventional farmers use almost identical feed compositions and are therefore likely to produce the same quality of products in this respect. 

These acids are found ready-made in nature in great numbers. Some of them occur as free acids (citric acid, tannic acid, malic acid), others as esters (products of acids and alcohols, such as fats and oils and the flavors of many fruits and the odors of many flowers). Still other of these organic acids are produced by the action of bacteria (acetic acid from wine or cider, lactic acid when milk turns sour, butyric acid in rancid hutter). 

Market milk and some products manufactured from milk sometimes possess a flavor described as rancid . This term, as used in the dairy industry, denotes implicitly the flavor due to the accumulation of the proper concentrations and types of free fatty acids hydrolytically cleaved from milk fat under the catalytic influence of the lipases normally present in milk. 

The development of a rancid flavor in milk and some other fluid products is usually undesirable and detracts from their market value. In contrast, the popularity of certain dairy products, notably some varieties of cheese, as well as some confectionery items containing milk as an ingredient, is thought to be partially due to the proper intensity of the rancid flavor. Hence, knowledge of the factors involved in the development of rancidity is of great practical importance to several industries. 

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. 

An inhibitory effect of rancid milk on the growth of Streptococcus lactis has been reported. Early reports (Schwartz 1974) claimed that rancid milk significantly inhibits the growth of bacteria in general and of Streptococcus lactis in particular. It has been stated that rancidity in milk may reach such a degree as to actually render the product sterile. (Schwartz 1974). Tarassuk and Smith (1940) attributed the inhibitory effect of rancid milk to changes in surface tension, but Costilow and Speck (1951) believe that the inhibition is due to the toxic effect of the individual fatty acids. 

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). 

Two types of enzymes in milk are important those useful as an index ol heat treatment and those responsible tor bad flavors. Phosphatase is destroyed by the heat treatments used to pasteurize milk hence its inactivation is an indication of adequate pasteurization. Lipase catalyzes the hydrolysis of milk fat which produces rancid flavors. It must be inactivated by pasteurization or more severe heat treatment to safeguard the product against off-flavor development Other enzymes reported to have been found in milk include catalase, peroxidase, protease, diastase, amylase, oleinase. reductase, aldehydrase. and lactase. 

---