Fluid milk flavored

Milk and milk products purchased by the consumer in liquid or semiliquid form generally are classified as fluid milk or cream. Fluid milks include all of the plain milk products, with fat contents varying from those of whole to skim milk, as well as flavored and fermented milks. Creams include products varying in fat content from half and half to heavy whipping cream to fermented sour cream. Products from each category are described briefly, with information on their composition. 

Sweet acidophilus milk differs from conventional acidophilus milk in that a high concentration of viable L. acidophilus organisms is added to cold pasteurized milk and kept cold. At the low storage temperature (4.4 °C) these organisms do not multiply, so the flavor and other properties of sweet acidophilus are identical to fresh fluid milk. The inoculated milk is promoted largely because it contains several million viable L. acidophilus cells per milliliter. 

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). 

With the advent of noncorrodible dairy equipment, oxidative deterioration in fluid milk as a result of copper contamination has decreased significantly, although it has not been completely eliminated (Rogers and Pont 1965). However, the incidence of spontaneous oxidation remains a major problem of the dairy industry. For example, Bruhn and Franke (1971) have shown that 38% of samples produced in the Los Angeles milkshed are susceptible to spontaneous oxidation Potter and Hankinson (1960) have reported that 23.1% of almost 3000 samples tasted were criticized for oxidized flavor after 24 to 48 hr of storage. Significantly, certain animals consistently produce milk which develops oxidized flavor spontaneously, others occasionally, and still others not at all (Parks et al. 1963). Differences have been observed in milk from the different quarters of the same animal (Lea et al. 1943). 

The natural copper content of milk originates in the cow s food and is transmitted to the milk via the bloodstream (Haase and Dunkley 1970). The studies of Dunkley and co-workers (1968A) and Riest et al (1967) suggest that an animal s feed can influence the natural copper content of its milk—a view which is not shared by others (Mulder et al 1964). Nevertheless, the total natural copper content of a milk is not the overall deciding factor in the spontaneous development of an oxidized flavor in fluid milk. 

Samuelsson (1966) concluded, on the basis of his studies, that the close proximity of a copper-protein complex to the phospholipids which are also associated with the fat globule membrane is an important consideration in the development of an oxidized flavor in fluid milks. Haase and Dunkley (1970) stated that although some aspects of catalysis of oxidative reactions in milk by copper still appear anomalous... the mechanism of oxidized flavor development with copper as catalyst involves a specific grouping of lipoprotein-metal complexes in which the spatial orientation is a critical factor. ... 

However, its presence is not the only determinant of whether or not oxidative deterioration occurs. Olson and Brown (1942) showed that washed cream (free of ascorbic acid) from susceptible milk did not develop an oxidized flavor when contaminated with copper and stored for three days. Subsequently, the addition of ascorbic acid to washed cream, even in the absence of added copper, was observed to promote the development of an oxidized flavor (Pont 1952). Krukovsky and Guthrie (1945) and Krukovsky (1961) reported that 0.1 ppm added copper did not promote oxidative flavors in milk or butter depleted of their Vitamin C content by quick and complete oxidation of ascorbic acid to dehydroascorbic acid. Krukovsky (1955) and Krukovsky and Guthrie (1945) further showed that the oxidative reaction in ascorbic acid-free milk could be initiated by the addition of ascorbic acid to such milk. Accordingly, these workers and others have concluded that ascorbic acid is an essential link in a chain of reactions resulting in the development of an oxidized flavor in fluid milk. 

Storage Temperature. The role of storage temperature in the oxidative deterioration of dairy products is anomalous. Dunkley and Franke (1967) observed more intense oxidized flavors and higher TBA values in fluid milks stored at 0°C than at 4° and 8°C. The flavor intensity and the TBA values decreased with increasing storage temperature. Other conditions being equal, condensed milk stored at - 17°C is more susceptible to the development of oxidized flavor than is condensed milk maintained at -7°C (Parks 1974). 

Although other dairy products have not been studied extensively, reports suggest that titratable acidity as well as hydrogen ion concentration tend to influence the development of oxidative deterioration. A relationship was found between the titratable acidity and the development of an oxidized flavor in milk (Parks 1974). While milks developed an oxidized flavor at a titratable acidity of 0.19%, the deteriorative mechanism was inhibited when the milks were neutralized to acidities of 0.145% or less. An increase in pH of 0.1 was sufficient to inhibit the development of oxidized flavors in fluid milks for 24 hr (Parks 1974). In addition to fluid milk, Dahle and Folkers (1933) attributed the development of oxidized flavors in strawberry ice cream to the presence of copper and the acid content of the fruit. 

Homogenization. Homogenization was found to inhibit the development of an oxidized flavor in fluid milk by Tracey et al. (1933). Subse-... 

Fruity flavor in dairy products is the result of ethyl ester formation, usually catalyzed by esterases from psychrotrophic or lactic acid bacteria. Ester formation by P. fragi involves liberation of butyric and ca-proic acids from the one and three positions of milk triglycerides and the subsequent enzymatic esterification of these fatty acids with ethanol (Hosono et al. 1974 Hosono and Elliott 1974). Consequently, among the esters formed, ethyl butyrate and ethyl hexanoate predominate. Pseudomonas-produced fruity flavor can occur in fluid milk, cottage cheese, and butter. 

Fluid milk is commonly subjected to a combination steam injection/in-fusion and vacuum flash evaporation process to remove volatile off-flavor compounds. The process is designed to remove the same amount of water by the flash treatment as is added during steam injection/infu-sion, so that the composition of the milk remains unchanged. This treatment is most effective for removing volatile, water-soluble flavor compounds, such as those from weeds and feed consumed by the cow. The additional heat from this process usually provides further improvement in product shelf life. 

Dimick, P.S. 1982. Photochemical effects on flavor and nutrients of fluid milk. Can. Inst. Food Sci. Technol. J. 15, 247-256. 

The process for cholesterol removal from anhydrous milkfat was patented by General Mills (41). Fractionment Tirtiaux also disclosed the development of a vacuum steam distillation system called the LAN cylinder (38). The steam distillation process (Figure 2) was commercialized, producing a 90-95% cholesterol reduction in anhydrous milkfat with a 95% yield that was reconstituted into 2% fat fluid milk (42). The major disadvantage to the process is that it strips or removes most all volatile flavor components from the fat. These flavor components must be captured (i.e., vacreation) before the distillation process to attempt to reproduce the delicate flavors so desired for reconstitution into a butter product. 

Milk Products. Cardboardy or metallic or tallowy flavors resulting from oxidative changes in the cream fraction of cows milk are distasteful in fluid milk and dairy products. These flavors develop more intensely in certain lots of fluid milk when the cows are fed dry rations. 

The practical use of added ascorbic acid has proved to be of benefit to the dairy industry 311,321). The amounts of ascorbic acid or sodium ascorbate used vary between twenty and several hundred milligrams per liter, 30-50 mg usually being suflScient for fresh fluid milk. Discrepancies in the results of some workers attempting to elucidate the value of ascorbic acid in the development of off-flavor may be due to their examination of an incomplete system of oxidative reactions. It has been dem-... 

Raw fluid milk is routinely processed by exposure to a thermal regimen in order to render it safe for human consumption. The extent to which the milk is exposed to heat generates a range of recognized off-flavors. Duration of the treatm t varies according to the intended use of the processed product. These treatments range in intensity from simple pasteurization (13 seconds at 75 ""C) to retort sterilization (30 minutes at 110 C) (3). 

The basic constituents of milk - protein, lipid, carbohydrate - can serve as precursor substrates for the formation of a wide variety of flavor compounds. Nearly two hundred volatiles have reportedly been found in fresh and processed milk (4). Numerous research efforts have focused on the conditions and mechanisms of off-flavor development in milk. The chemical compounds responsible for these off-flavors have been characterized (3,5-7). Most fluid milk processing is carefiiUy controlled so that the appearance of caramelized and scorched flavor notes rarely occurs. The rich flavor associated with the thermal formation of diacetyl and various lactones is not objectionable to most consumers and, therefore, not a serious concern. Conversely, the sulfurous off-flavor in cooked milk is of concern and is especially prevalent in freshly processed UHT milk. 

Flavor defects in milk and cream are collectively referred to as the oxidized flavor , in butter as metallic or tallowy . Oxidized flavors are noticeable in stored milk at very low levels of oxidation, usually at a peroxide value less than 1, because the low molecular weight off-flavor compounds are more readily volatilized in the predominantly aqueous matrix of milk. The development of oxidized flavor is most evident in fluid milk, cream and butter, because they have mild and more delicate flavors. 

Although in fluid milk the phospholipid fraction is more susceptible to oxidation than the triacylglycerol fraction, in dry milk products, the triacylglycerol fraction is more susceptible to oxidation and the phospholipids act as antioxidants. Thus, solvent-extracted milkfat containing phosphohpids is much more stable to oxidation than milkfat free of phospholipids, obtained by melting churned butter (also called butter oil). The susceptibility of milk phospholipids to oxidation appears to be dependent on whether they are suspended in water or fat. This difference of oxidahve stability influences the development of different flavor defects in various dairy products. With butter, which is a water-in-oil emulsion system containing an aqueous phase of phospholipids dispersed in fat, the phosphohpids oxidize more readily than the triacylglycerol components. 

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. 

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