SUMMARYFlavour, aroma and appearance of milks fermented with commercially available kefir grains were compared with similar characteristics of heat treated (90 ΰC/30 min) milks that had been fermented with aroma producing starters in addition to the kefir grains. The starters used were either a mixture ofStreptococcus lactissubsp.diacetilactisandLeuconostoc cremoris(lactic culture) or ofStr. thermophilusandLactobacillus bulgaricus(yogurt culture).Two methods of fermentation were investigated involving removal of the grains followed by (i) a second fermentation of the kefir with the aroma-producing organisms and (ii) combining the kefir with aroma-producing organisms for use as a starter to ferment a further sample of heat-treated milk. Products were assessed by a taste panel and marked for acceptability. Addition of lactic or yogurt cultures resulted in products with increased mean scores for aroma and more acceptable flavours.

The mixed flora of yeasts and lactobacilli of kefir is held together in non‐dispersible structures which build up into large grains. The fibrillar extracellular material of the matrix in which the microflora is embedded was stained by ruthenium red and periodic acid‐thiosemicarbazide silver proteinate, indicating that it was largely composed of carbohydrate. It is suggested that the carbohydrate is of bacterial origin and that this is produced by a population of lactobacilli which resides within the matrix and which separates non‐carbohydrate‐producing populations of lactobacilli and yeasts so that sheet‐like structures are formed which show asymmetry, with yeasts predominating on one side and lactobacilli on the other.

SummaryCell-free extracts of both Lactobacillus bulgaricus and L. acidophilus demonstrated threonine aldolase activity, the end product of which was acetaldehyde, the major flavour compound of yoghurt. L. acidophilus also possessed an alcohol dehydrogenase activity capable of reducing acetaldehyde so that little yoghurt flavour was present in milks fermentation with this organism. Addition of threonine to fortified milk before fermentation with L. acidophilus increased acetaldehyde production and resulted in a well flavoured product similar to that of yoghurt made with L. bulgaricus. The contribution of these 2 enzymes to flavour production is discussed.

SummaryLactoperoxidase (LP), thiocyanate (SCN-), pH and somatic cell counts (SCC) were measured in mammary secretions from 20 cows collected 14 d before drying-off, 7 and 21 d after drying-off, and 3–18 d postcalving. The inhibitory activity of the secretions onStreptococcus uberiswas determined and the susceptibility of the udder to infection by this organism was tested by intramammary infusion of 250 colony forming units at the above stages. LP, SCN-, pH and SCO increased during involution and fell postcalving. The secretions collected before drying-off, 7 d after drying-off and postcalving inhibited growth ofStr. uberis.; those collected 21 d after drying-off did not. Inhibitory activity in pre-drying-off secretions was destroyed by heating and restored by addition of LP, glucose and glucose oxidase, but addition of these substances to secretion 21 d after drying-off did not provide a full inhibitory system. The growth ofStr. uberisin the secretions was correlated with intramammary susceptibility, since challenges withStr. uberisat 14 d before drying-off, at 7 and 21 d after drying-off and postcalving led to 43·8, 25·0, 81·3 and 37·5% of quarters becoming infected. It is suggested that the LP/SCN-/H2O2system plays a role in protecting the lactating mammary gland from infection withStr. uberisbut becomes ineffective as involution progresses.

SUMMARYYoghurts were made with single strain starter organisms Lactobacillus bulgaricus and Streptococcus thermophilus, L. bulgaricus being cultured in milk + 50 ppm Na formate or in autoclaved milk and Str. thermophilus in milk treated with the proteinase Maxazyme(R) or with added casein hydrolysate. They were assessed chemically and organoleptically and compared with conventional mixed starter yoghurt. The single starter yoghurts all had acceptable acidities and acetaldehyde levels and keeping quality was good. The coagulum of each was firm and when stirred no syneresis occurred; the yoghurts remained smooth and all had an excellent appearance.