SummaryThe bacteriological formation of 2-butanone and 2-butanol was investigated. The data suggest that 2-butanol is formed in cheese via 2-butanone in 3 steps, each step being carried out specifically by a different microbial species. Suitable species have been isolated from a single cheese. The first step could be carried out (in the present investigation) either by certain starter organisms or by a strain ofPediococcus cerevisiae. This strain formed substantial amounts of 2,3-butylene glycol in milk, presumably from citrate, but could not metabolize this compound further. A second micro-organism, a strain ofLactobacillus plantarum, was isolated which, in Cheddar cheese, could convert 2,3-butylene glycol into 2-butanone. The final reduction of 2-butanone to 2-butanol could be readily carried out in MRS medium by a strain ofL. brevis. Results from cheese made aseptically indicate that 2-butanone is formed in Cheddar cheese only when non-starter organisms are present.

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Diacetyl, acetoin, 2,3-butyleneglycol, 2-butanone and 2-butanol concentrations have been determined in ripened Cheddar cheese made using Streptococcus diacetilactis DRC1 or Str. cremoris AM 2 or HP as starter culture. Diacetyl content was found to be initially higher in the DRC 1 cheese than in the other 2 cheeses, presumably because this starter readily ferments citrate. The disappearance of acetoin appears to be related to the formation of 2,3-butyleneglycol in DRCl and AM 2 cheeses, but not in HP cheese. Only trace amounts of 2-butanone and 2-butanol were observed in the 3 cheeses even though the concentration of a postulated precursor, 2,3-butyleneglycol, was present in considerable concentration in the DRC 1 and AM 2 cheeses. Analysis of a number of mature AM 2 and HP cheeses selected at random indicated the presence of 2-butanone in only a few of the AM 2 cheeses. It is suggested that this compound may have arisen from the presence of a micro-organism which entered the cheese vat at the time of cheese-making.

Land cress (Coronopus didymus) is a cruciferous weed which imparts a burnt, unclean flavor to milk when ingested by dairy cows. This off-flavor, unlike many weed taints, is intensified by heat treatment and cannot be eliminated from milk or cream by conventional vacuum pasteurization techniques. Land cress contains the glucosinolate, glucotropaeolin, from which benzyl cyanide, benzyl isothiocyanate, and benzyl thiocyanate are liberated enzymically when the plant is crushed. Benzyl methyl sulfide and benzyl mercaptan have been considered responsible for the off-flavor, the former having been isolated from cress-tainted cream.Benzyl thiocyanate appears to be a key intermediate in offflavor formation. Complete elucidation of the chemical nature of the land cress off-flavor and its precursors has important practical value in the development of methods for control or removal of a taint which has caused significant economic loss to the dairy industries of several countries. Certain feeds and weeds consumed by dairy cows have long been recognized for their contribution to the appearance of abnormal and undesirable flavors in milk and its products. Several general reviews of this subject are available (Babcock, 1938; Conochie, 1950;Parks, 1967; Strobel et a/., 1953). Onions, fermented silage, alfalfa, cabbage, turnips, rape, beet tops, musty hay, and distillers' grains impart off-flavors to milk, but these can usually be avoided by controlled feeding practices. Many varieties of weeds, when ingested by cows, impart taints to milk, and in some cases only minimal consumption of a weed is required to produce a strong off-flavor. The consumption of weeds by dairy cows is more difficult to control than the basic feeding regime. Nevertheless, the incidence of weed taints in milk can be greatly reduced by good pasture management practices.

TWO chemical tests were used to measure the extent of lactose degradation by Maillard browning in casein and skim-milk powder during storage. At the same time these products were evaluated organoleptically. Highly significant correlations between age, flavour score and level of browning in skim-milk powder were in contrast to the lack of any relationship between the flavour score or age of casein and its level of browning. Maillard browning occurring during storage of skim-milk powder was accompanied by increasing levels of cereal and malty flavours. A stale flavour, which closely resembled the musty flavour common to casein, was not observed in skim-milk powder until after storage for 9 months. A distinct difference in the flavour stability of rennet casein and acid casein could not be accounted for in terms of the relative extent of lactose degradation. Evaluation of casein samples with different lactose contents suggested that Maillard browning does not contribute to flavour defects during storage unless the curd is so insufficiently washed during manufacture as to produce an abnormally high level of lactose in the dried product.For many years commercial casein has been used primarily in the preparation of paper coatings, adhesives and plastics, with the result that properties such as viscosity, adhesive strength, solubility and ability to bind water and pigments have received considerable attention. Recently, however, the more widespread use of casein as a high-quality protein in compounded foods has required a more searching evaluation of its flavour stability.A common defect in casein may be described as a musty or stale flavour, which is sometimes detectable immediately following the drying of fresh casein curd and which becomes more pronounced during storage. Organoleptic evaluation of many casein samples has demonstrated that the musty flavour is intensified by heattreatment and that acid-precipitated caseins are considerably more susceptible to musty flavour development than are caseins precipitated by rennet.In 2 recent publications, Australian workers have investigated the nature and origin of the so-called 'gluey' flavour in casein and co-precipitates. In the first study, Ramshaw & Dunstone (1969 a) demonstrated positive correlations between gluey flavour, lactose content and the extent of Maillard browning as measured by the ferricyanide reducing value. Furthermore, the gluey flavour was not detectable by 15-2