Yogurt is a milk curd produced all over the world, obtained by a lactic fermentation of a milk base enriched with milk proteins, and sometimes sugars and thickeners. One of the most important sensorial attributes for yogurt is texture, which could be assessed by sensory or instrumental analysis. A lot of work has been published in studying the contribution of milk base, starter, and process on yogurt texture in order to develop new textures, or simply to reduce fat content, or the level of addition of protein and thickener in milk. However, these studies are limited to only a few factors. The topic of this review was to synthesize the data of literature, with the aim of extracting and classifying factors on the basis of their influence on yogurt texture. Three factors, milk base heating, starter, and yogurt shearing after fermentation, respectively, play a key role in the elaboration of texture. The control of these three parameters allows the improvement of the textural attributes of yogurts by 2 to 15 times.

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In the present work, the compared effect of milk base and starter culture on acidification, texture, growth, and stability of probiotic bacteria in fermented milk processing, was studied. Two strains of probiotic bacteria were used, Lactobacillus acidophilus LA5 and L. rhamnosus LR35, with two starter cultures. One starter culture consisted only of Streptococcus thermophilus ST7 (single starter culture); the other was a yogurt mixed culture with S. thermophilus ST7 and L. bulgaricus LB12 (mixed starter culture). For the milk base preparation, four commercial dairy ingredients were tested (two milk protein concentrates and two casein hydrolysates). The resulting fermented milks were compared to those obtained with control milk (without enrichment) and milk added with skim milk powder. The performance of the two probiotic strains were opposite. L. acidophilus LA5 grew well on milk but showed a poor stability during storage. L. rhamnosus LR35 grew weakly on milk but was remarkably stable during storage. With the strains tested in this study, the use of the single starter culture and the addition of casein hydrolysate gave the best probiotic cell counts. The fermentation time was of about 11 h, and the probiotic level after five weeks of storage was greater than 106 cfu/ml for L. acidophilus LA5 and 10(7) cfu/ml for L. rhamnosus LR35. However, an optimization of the level of casein hydrolysate added to milk base has to be done, in order to improve texture and flavor when using this dairy ingredient.

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The physicochemical characteristics and the renneting properties of milks from Lacaune ewes are reported. The ewes belonged to divergent genetic lines termed ‘low’ or ‘high’ and were selected on a combination of fat and protein yield. The 179 second to fifth parity ewes were sampled three times during lactation at about 50, 135 and 175 d in milk. Physicochemical and renneting variables were principally influenced by days in milk and somatic cell count, whereas β-lactoglobulin genotype, parity and divergent lines had relatively few effects. Fat and protein content increased throughout lactation, whereas rennet gel firming rate decreased. Salt distribution and micellar characteristics were also significantly affected by days in milk. Milks with high somatic cell count were characterized by high values of pH and soluble nitrogen content and rennet coagulation properties were significantly affected at >3×105−5×105 cells/ml milk. Divergent lines had a strong effect on milk yield, but no effect on milk composition consistent with the selection criteria. Gel firming rate was slightly higher in milks from high yield ewes. Protein content, Ca concentration, pH and micelle characteristics were the main factors related to variations in the renneting properties of the milks.

Lactoprotéines caprines et aptitudes technologiques Influence du polymorphisme génétique de la caséine as1 caprine sur les caractéristiques physico-chimiques et technologiques du lait

Résumé -Une étude systématique a été entreprise, portant sur la composition protéique et miné-rale et le comportement vis-à-vis de la présure de 115 échantillons de laits individuels provenant de 40 chèvres.La grande variabilité des caractéristiques physico-chimiques et technologiques des laits individuels a été mise en évidence. La recherche, par des méthodes statistiques appropriées, des relations entre les caractères physico-chimiques des laits et leur comportement vis-à-vis de la présure, montre qu'un petit nombre de variables physico-chimiques permet d'expliquer une grande partie des variations d'aptitude à la coagulation par la présure des laits. Ces variables sont: les concentrations en caséine totale, en calcium total et colloïdal, le rapport calcium soluble sur calcium total, le diamètre moyen des micelles, et le rapport caséines o.s sur caséine 13. D'autre part, il apparaît que l'écart de fermeté entre les coagulums bovin et caprin est essentiellement dû aux différences des caractéristiques micellaires des caséines (diamètre moyen et degré d'hydratation) des deux laits.L'ensemble des résultats fournit des indications intéressantes pour la recherche de moyens permettant d'améliorer le comportement fromager du lait de chèvre, par correction technologique ou sé-lection génétique.lait de chèvre -caractères physico-chimiques -aptitude à la coagulation par la présure Summary -A study of the relations between physico-chemical characteristics of goat milks and their rennetlng properties. A systematic study of the protein and minerai composition and rennet coagulating properties of 115 individual milk sampi es from 40 goats was undertaken. A great variability in physico-chemical characteristics of individual milks has been observed. Study of the relationships between physico-chemical characteristics of milks and their coagulating properties utilising suitable statistical methods shows that a few physico-chemical variables can explain a major part of the variation in the renneting properties of individual goat milks. These variables are: casein content, total and colloidal calcium content, soluble calcium/total calcium ratio, mean diameter of casein micelles, o.s caselns/â casein ratio. On the other hand, the gap between cow's and goat's rennet curd firmness is essentially due to the differences in the physical properties of casein micelles between the two milks (mean diameter and solvation).These results provide interesting informations on the ways to find out methods for improving the cheesemaking capacity of goat milk, by technological adjustment or genetic selection.