I. Sodini

The Relative Effect of Milk Base, Starter, and Process on Yogurt Texture: A Review

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.

Preliminary observations on the effects of milk fortification and heating on microstructure and physical properties of stirred yogurt

The aim of this work was to study how milk fortification and heating affect yogurt microstructure (micellar characteristics, protein network) and physical properties (viscosity, water-holding capacity (WHC), and graininess). Milk was fortified with skim milk powder (control), whey protein concentrate (WPC), caseinate, or a mixture of caseinate and whey protein. Two heat treatments were applied, giving average whey protein denaturation levels of 58% and 77%. For caseinate-enriched yogurts, the heating effect was negligible. When milk was enriched with WPC, heating led to a high level of cross-linking within the gel network. Heating increased yogurt viscosity and WHC, but also graininess. When milk was fortified with a blend of WPC and caseinate giving a whey protein-to-casein ratio of 0.20, the yogurt viscosity was greatly improved, while graininess was kept low. The results show a relationship between micelle solvation and yogurt microstructure, as well as micelle size in milk base and yogurt graininess.

Microbial dynamics of co- and separately entrapped mixed cultures of mesophilic lactic acid bacteria during the continuous prefermentation of milk.

Four strains of mesophilic lactic acid bacteria were separately or coentrapped in kappa-carrageenan/locust bean gum gel beads and used for continuous prefermentation of UHT skim milk in a stirred-tank bioreactor. Lactic acid and cell productivities of the immobilized cell bioreactor were particularly high and remarkably stable during eight weeks of continuous milk prefermentation (about 18 g h-1 l-1 of lactic acid and 4.9 x 10(12) CFU h-1 l-1, respectively, but important variations of the bacterial populations is prefermented milk and gel beads occurred in any case (co-or separate entrapment). The strain Lactococcus lactis subsp. lactis biovar diacetylactis CDII became dominant, accounting for approx. 90% (released cells) and 78% (immobilized cells) of the total population. Microscopic observations of sections of gels beads showed a progressive destructing of the bead surface with rupture and release of entrapped viable cells from peripheral cavities of the gel. It is believed that these cavities close again after releasing all or part of their cell content, entrapping the different strains of the mixed culture and initiating a new colonization step and a cross-contamination of the beads. On the other hand, experimentations over seven-week periods with pasteurized milk showed the high resistance of the immobilized cell bioreactor to psychrotrophic contamination.

Use of an immobilized cell bioreactor for the continuous inoculation of milk in fresh cheese manufacturing

A system was developed to continuously acidify and inoculate skim milk for the production of fresh cheese. Four strains of mesophilic lactic acid bacteria were entrapped separately in κ -carrageenan/locust bean gum gel beads and used in a stirred bioreactor operated at 26°C with a 25% (v/v) gel load. The pH in the reactor was controlled at 6.0 by adding fresh milk using proportional integrated derived regulation. The bioreactor was operated during 8-h daily cycles for up to 7 weeks with different milks (heat treatment, dry matter content) and differing starting procedures. The heat treatment of the milk was an important factor for process performance: a dilution rate increase of 57% and an inoculation level decrease of 63% were observed with sterilized UHT skim milk (142°C – 7.5 s) compared with pasteurized skim milk (72°C – 15 s). The dry matter content of the milk (8–13% w/w) had no detectable effect on these parameters. A convenient starting procedure of the system was tested; steady-state was reached in less than 40 min following an interruption period of 16–60 h. These results combined with our published data on process performance show the feasibility of using an integrated immobilized cell bioreactor for milk prefermentation in cheese manufacture.

Identification of interacting mixed cultures of lactic acid bacteria by their exclusion from a model predicting the acidifying activity of non-interacting mixed cultures

Abstract A model predicting the acidifying activity of mixed cultures of lactic acid bacteria and based on the lack of interaction between the strains has been investigated to identify interacting cultures. Three mixed cultures with Streptococcus thermophilus TH3 and ST7 and Lactobacillus delbrueckii ssp. bulgaricus LB10 were grown on milk. The acidifying activities of the two mixed cultures TH3/LB10 and TH3/ST7 were predicted accurately by the model, with mean prediction errors of 7.7% and 14.1%, respectively. However, the model underestimated the acidifying activity of the mixed culture ST7/LB10, with a mean prediction error of 43.5%, which provides evidence of positive interaction between the strains ST7 and LB10 during acidification.

Effect of citrate on growth of Lactococcus lactis subsp. lactis in milk

Abstract The effect of citrate on the growth of Lactococcus lactis subsp. lactis var. diacetylactis in milk has been investigated. Five strains of Lactococcus lactis subsp. lactis var. diacetylactis were compared to their citrate-negative variants, which lack the plasmid coding for citrate permease. In most cases, acidification kinetics and the final bacterial concentration of pure cultures of parental and variant strains did not differ significantly. Co-cultures of parental and variant strains, however, systematically tended towards the predominance of parental strains. Citrate metabolism is responsible for this change, since the predominance of citrate-positive strains was not observed in the absence of citrate. Continuous culture in milk enabled the difference in growth rates between the parental strain Lactococcus lactis subsp. lactis var. diacetylactis CDI1 and its citrate-negative variant to be quantified by following changes in the populations of the two co-cultured strains. At 26 °C, the growth rate of the parental strain was 7% higher than that of its citrate-negative variant. These results show that citrate metabolism slightly stimulates the growth of lactococci in milk.