Milk aroma defect

Strecker degradation, oxidation to indolylacetic acid and decarboxylation. The oxidative cleavage of skatole yields o-aminoacetophenone (cf. Formula 5.36), which has an animal odor and is the key aroma substance of tortillas and taco shells made of com treated with lime (Masa corn). In the case of milk dry products, o-aminoacetophenone causes an aroma defect (cf. 10.3.2). Its odor threshold of 0.2pg/kg (water) is very low. On the other hand, p-amino-acetophenone has an extremely high odor threshold of 100 mg/kg (water). 

During the concentration and drying of milk, reactions that are similar to those described for heat-treated milk (cf. 10.1.3.5 and 10.3.1) occur, but to a greater extent. Thwefore, like the aroma of UHT milk (cf. 10.3.1 and Table 10.38), the aroma of condensed milk is also caused by Maillard reaction products. The stale flavor that appears when condensed milk is stored for longer periods is due especially to the presence of the degradation product of tryptophan, o-aminoacetophenone, which is aroma active in concentrations >lpg/kg. A mbbery aroma defect results from higher concentrations of benzothiazole. 

The content of free but)tric and caprylic acid as well as (Z)-3-hexenal rises when cream is whipped (Table 10.39). Pasteurization results in the formation of 2-acetyl-2-thiazoline in whipped cream and the content of (E,Z)-2,6-nonadienal is greatly increased. A model corresponding to Table 10.39 (without No. 12, 14, 17 and 20) approaches the aroma of whipped pasteurized cream and reproduces especially the "creamy" note. Maillard reaction products are also characteristic of the aroma of milk powder. The development of aroma defects during the storage of whole milk powder is due to products of lipid peroxidation, e. g., (Z)- and (E)-2-nonenal. 

Rancid, soapy aroma defects, which occur in butter samples with very low concentrations of free fatty acids, can be due to contamination with anionic detergents (sodium dodecyl sulfate, sodium dodecyl benzosulfonate). Detergents of this type are used to disinfect the udder and the milking machine. 

As already indicated, aroma defects can arise in milk and milk products either by absorption of aroma substances from the surroundings or by formation of aroma substances via thermal and enzymatic reactions. 

Exogenous aroma substances from the feed or cowshed air enter the milk primarily via the respiratory or digestive tract of the cow. Direct absorption apparently plays only a minor role. Metabolic disorders of the cow can cause aroma defects, e. g., the acetone content of milk is increased in ketosis. 

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. 

Recent studies of photooxidized butter and butter oil identified by aroma extract dilution analysis, 3-methylnonane-2,4-dione, a potent volatile compound derived from furanoid fatty acids (see Section C.4) (Figure 11.7). Six different furanoid fatty acids were established as dione precursors, and were found in various samples of butter made from either sweet cream (116 76 mg/ kg), or from sour cream (153-173 mg/kg), or from butter oil (395 mg/kg). Similar precursors of the dione were identified in stored boiled beef and vegetable oils. This flavor defect arising by photooxidation of butter or butter oil is apparently different from the light-activated flavor in milk,that involves the interaction of sulfur-containing proteins and riboflavin. However, more sensory comparisons are needed to distinguish between these two flavor defects due to light oxidation. 

Citrate is present in milk, fruit, and vegetables. It can be co-metabolized with sugars by citrateutilizing LAB. Citrate utilization results in an excess of pyruvate, which is thus converted to diacetyl (2,3-butanedione), acetoin (2-hydroxy-3-butanone), and 2,3-butanediol to equilibrate the redox balance of cellular metabolism (Collins 1972 Bartowsky and Henschke 2004). Some LAB can also synthesize 2,3-pentanedione from pyruvate and threonine (Ott et al. 2000). Diacetyl and 2,3-pentanedione are associated with a buttery aroma, which positively contributes to the flavor of a range of fermented dairy products such as butter (MalUa et al. 2008), yogurt (Routray and Mishra 2011), and cheese (Curioni and Bosset 2002). Diacetyl also contributes to wine style, while it is responsible for flavor defects in beer. Diacetyl is widely produced by LAB, including species of the Lactococcus, Streptococcus, Leuconostoc, Lactobacillus, Pediococcus, and Oenococcus genera. 

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