INDUSTRIAL SIGNIFICANCE OF LACTOCOCCAL BACTERIOPHAGES

The major functions of starters in dairy fermentations are shown in table 1. See the section on starters also.

Function	Result/Mechanism
Acid production	Gel formation Whey expulsion (syneresis) Preservation Flavour development
Flavour compound production	Formation of diacetyl and acetaldehyde
Preservation	Lowering of pH and redox potential Production of antibiotic substances e.g. bacteriocins such as nisin Production of hydrogen peroxide Formation of D-leucine Production of lactate/lactic acid Acetate formation
Gas formation	Eyehole formation Production of openness to facilitate 'blue veining'
Stabiliser formation	Body and viscosity improvement Increase cheese yield? Result in reduced use of milk powder in yoghurt manufacture
Lactose utilisation	Reduce potential for gas and off-flavour development Make products more acceptable to the lactose intolerant
Lowering of redox potential	Preservation Aids flavour development

Although lactic acid production is the major task required of starters in many milk fermentations, starters have other - sometimes equally important - roles.

The preservative properties of starters have been used to extend the shelf life of Cottage cheese and to inhibit the growth of psychrotrophic micro-organisms in silo milk in cheese factories. The mechanism for these effects involves competition for nutrients, decrease in pH, the formation of inhibitory compounds, lowering of redox potential and possibly activation of the lactoperoxidase system.

The production of openness e.g. slits or seams, is important in some varieties of blue-veined cheese such as Cambazola or Blue Brie and can be achieved by the use of starters containing strains of Lc. lactis subsp. lactis biovar. diacetylactis and/or Leuconstoc species.

Some starters produce complex polysaccharide-type materials which can be utilised by technologists to improve the body or viscosity of products such as yoghurt or to produce the essential product characteristics (ropiness) of Finnish long milk - Viili.

Infection with bacteriophage is the major single cause of fermentation failure or of problems in fermentation processes utilising lactic acid bacteria.

In cheesemaking, lysis of starter cells can result in problems which range from slow acid production to completely lost vats. Lysis of starters during the early stages of cheesemaking may result in cheese of high pH, high lactose content, high redox potential and low lactic acid and lactate content.

These factors may lead to major product quality problems including those resulting from the growth of pathogens e.g. Staphylococcus aureus, especially if raw milk has been used.

Cheddar cheese at the start of pressing contains about 0.5% (w/w) lactose. Failure of starters to utilise residual lactose (due to phage-induced cell lysis) during pressing and in the first few weeks of maturation may result in sweet cheese and/or gas production by heterofermentive non-starter bacteria.

Because acid production by the starter influences syneresis, variations in starter activity may result in variations in the moisture content of the final cheese. Obviously these effects have the potential to affect the profitability of dairy plants.

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