We examine the process of rennet gel formation by visually analyzing displacement of the fat globules in milk. Since the motion of fat globules is affected by the viscoelastic properties of the surrounding medium, the rennet gel formation process can be visually investigated by tracking the fat globules. Optical investigations by the use of particle image velocimetry (PIV) method reveal that the gelation process of the milk proceeds in two stages. The first of these stages is due to reduced mobility of the fat globules resulting from the formation of the network of casein micelles. After reduction of the mobility, most of the fat globules are entrapped in the gel networks, while there still remains active globules jittering in localized spots. Active fat globules confined in those spots specify an emergence of meso-scale heterogeneity in the rennet gel structure. PIV analysis allowed us to follow the time evolution of the mobility of the fat globules and to find appearance of meso-scale heterogeneity in the rennet gel for the first time. Our results are relevant for the understanding of the dynamics of a cheese curd formation in the milk.

 We quantified a kinetics of the rennet-induced gelation by applying an image analysis technique, differential variance analysis (DVA). Previous study showed that the rennet gel formation proceeded with two stages, the coagulation of the micelles and an emergence of meso-scale vacancy due to the casein micelle network formation, respectively. DVA method quantitatively revealed that the first stage was associated with the slowing down of the mobility of the fat globules because of the increase of the microscopic viscosity. Such viscous contribution will arise from the cluster formation of the casein micelles. In the comparison with the rheology, the second stage was characterized by the appearance and continuous increase of the elasticity without reaching to the plateau value. The continuous increase may be attributed to aging of the casein network structure. Crossover from first to second stage was clearly identified by introducing a frozen rate of the fat globules φ_F, which could be a measure of the extent of the gelation. Frozen rate φ_F showed that viscous contribution dominates the kinetics of the gelation until 90 % of the fat globules were entrapped in the casein networks. Image analysis and rheology can be treated to study meso-scale dynamics of the milk gel complementarily.

 The characterization of Lactobacillus helveticus AHU 1049 (AHU 1049), the documented source is dairy cream of Tsukigata-Hokkaido, Japan in 1935, was performed; the draft genome sequence (DGS) to extract gene coding proteolytic enzymes and the preliminary application study for semi-hard and hard cheeses production. DGS revealed that AHU 1049 is harboring one cell envelope proteinase, prtH3, and other peptidases of seven pepN, three pepP, three pepO, four pepD, two pepT, two pip, and one pepX, pcp, pepF, and prpV, indicating potentially act as a free amino acids and low molecular peptides producer. The preliminary cheese production was done using 10 kg pasteurized milk. Semi-hard type (cooking temperature 40℃) with LD, and LD and AHU 1049, and hard type, the scalding temperature at 50℃, 55℃, and 60℃, with LD and AHU 1049 were prepared. The cheeses were ripened for 12 months. The semi-hard type cheese with LD and AHU 1049 had a close moisture content and low amount of degraded peptides, but high amount of free amino acids compared to that with sole LD. As for hard type cheeses, scalding lead to decrease of the moisture content and production of degraded peptides, but the free glutamate and glutamine were similar level to the semi-hard type cheeses without scalding. Those results indicated that AHU 1049 is potentially applicable as a peptidases source for semi-hard and hard type cheeses and that AHU 1049 is an adjunct starter candidate for adjunct starter designated as Japanese origin.

 The aim of this study was to evaluate the flavor characteristics of dairy products. In this study, fermented butter-like products were prepared from the cream of cow's milk that had been fermented using dairy starters. The volatile compounds including short-chain fatty acids and lactones, were analyzed using gas chromatography-mass spectrometry. The preparation using two kinds of lactic acid bacteria (Lactococcus lactis subsp. lactis and Lc. lactis subsp. cremoris) had the highest butanoic acid, and n-decanoic acid content. In contrast, two preparations fermented with five kinds of lactic acid bacteria (Lc. lactis subsp. lactis, Lc. lactis subsp. cremoris, Leuconostoc mesenteroides, Leu. pseudomesenteroides, and Lc. lactis subsp. lactis biovar diacetylactis) produced δ-lactones. It is postulated that Lc. lactis subsp. lactis and Lc. lactis subsp. cremoris were related to produce some fatty acids by the digestion of triacylglycerol, whereas Lc. lactis subsp. lactis biovar diacetylactis and Leuconostoc spp. (especially Leu. mesenteroides) could produce lactones. Therefore, it may be possible to manufacture fermented dairy products according to several purposes by using different combinations of lactic acid bacteria.

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