Yogurt fermentation is a homolactic fermentation. Glucose metabolism by ST, LB, and Lb. acidophilus proceeds by the EMP pathway. Lactose utilization in ST, LB, and bi-fidobacteria involves lactose transport into the cells via cytoplasmic proteins (permeases). This translocation of lactose takes place without its chemical modification. This unphos-phorylated lactose is hydrolyzed by h-galactosidase to glucose and galactose. Glucose is catabolized and galactose is secreted from the cells (23,53). The lactose permease is an active transport system, and the energy is provided in the form of a proton motive force developed by expulsion of protons. The excreted amount of galactose is proportional to the amount of lactose taken up. The current model for lactose transport in these bacteria is that a single transmembrane antiport permease simultaneously translocates lactose molecules into the cytoplasm and galactose out of the cell. The energy generated through galactose efflux thus supports lactose uptake into these cells. The average lactose content of yogurt mix of 13% milk solids-not-fat was about 8.5% (54). During fermentation it was reduced to about 5.75%. The initial galactose content of the mix was a trace but increased to 1.20% during fermentation. Only a trace amount of glucose was noted (54). In commercial yogurts, lactose ranged from 3.31% to 4.74% and galactose varied from 1.48% to 2.50% (54).
There are a number of inhibitors for the yogurt culture that can impede or slow down lactose fermentation. Some of these are listed here; for further details, refer to (5,20,27,53). These are heat-sensitive lactenins, lactoperoxidase/thiocyanate/hydrogen peroxide (LPS), agglutinins, mastitic milk, antibiotic residues, hydrogen peroxide, detergents and santizer residues, and bacteriophages. Many of the inhibitors mentioned here may be seasonal and sporadic or accidental. Bacteriophages, on the other hand, are pernicious and can be devastating if not managed properly.
Bacteriophages (phages) are viruses that can infect bacteria and destroy one or more components of the yogurt culture. Phage are differentiated into virulent (lytic) and temperate phage, which reflect different growth responses in the bacterial host. Phage that infect and lyse the host cell are called virulent phage, whereas those that do not necessarily lyse their bacterial hosts, but instead insert their genome into the host cell chromosome, are called temperate phage. The propagation of virulent phage in the bacterial cell is called the lytic or vegetative cycle of phage multiplication and results in the release of new infectious phage progeny (55). Bacteriophages multiply much faster than the bacteria. A bacteriophage with a burst size of 100 can destroy a culture within a couple of generations. This can cause huge economic losses and result in inferior yogurt. Due to explosive growth of yogurt and mozzarella cheese production, a greater incidence of phage against ST has been reported (29). It is also noted that phage for lactobacilli appear less frequently (29,56). Recent work has shown that ST phages are closely related, at both the genetic and morphological level, making differentiation difficult. Electron microscopy studies revealed that both temperate and lytic phages were nearly identical, having small isometric heads and long noncontractile tails (57,58). Some lysogenic strains were autolytic at 45°C (58). New strategies to develop phage-resistant strains include antisense RNA technology and origin-conferred phage-encoded resistance (PER) (57).
Both lytic and temperate bacteriophages have been found in Lactobacillus delbrueckii subsp. bulgaricus and subsp. lactis (59,57). Strains of Lactobacillus acidophilus isolated from dairy products harbored temperate phages and some produced bacteriocins. One induced phage lysed nine other dairy lactobacilli including LB (60). Some of the LB were also sensitive to the bacteriocin produced by Lb. acidophilus.
Primary yogurt cultures, ST and LB should be carefully chosen and evaluated with respect to phage and compatibility with other adjuncts. Since many of the organisms used in yogurt may harbor temperate phage, propagated starter culture, yogurt, and environment should be monitored for phage. Use of aseptic techniques for propagation and production by properly trained personnel along with proper mix heat-treatment sufficient to kill phage are essential to keep phage under control.
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