Proteolysis in cheese

It was long argued that milkfat was the primary constituent responsible for cheese flavor.While it is certainly true that many cheese flavors are either evolved from the lipid fraction or are soluble in the lipid phase, it is now generally accepted that, for most cheeses (the main exception being the blue mold-type cheeses), it is the protein fraction that makes the more important overall contribution (Table 5-2). Cheeses made under controlled conditions in which proteoly-sis does not occur develop neither the flavor nor texture of a normal cheese.The recognition that proteolysis has such a profound influence on cheese flavor and ripening has led to a detailed understanding of many of the specific steps involved in the degradation of milk proteins and the metabolism of the hydrolysis products (Figure 5-9).

For the most part, the relevant organisms responsible for these activities are lactic acid bacteria, either those added as part of the starter culture or those present as part of the ordinary microflora. Lactic acid bacteria cannot synthesize amino acids from ammonia and instead require pre-formed amino acids for protein biosynthesis and cell growth. However, milk contains only a small amount of free amino acids (<300 mg/L), which are quickly assimilated.Thus, growth of lactic acid bacteria in milk depends on the ability of these cells to metabolize proteins in milk. The means by which large casein molecules are broken down by these bacteria into their component amino acids (and beyond) consists of four main steps: (1) hydrolysis of casein; (2) transport of casein-derived amino acids, peptides, and oligopeptides into the cytoplasm of starter and non-starter lactic acid bacteria; (3) intracel-lular hydrolysis of accumulated peptides; and (4) metabolism of amino acids.

In Cheddar-type cheeses, starter culture lactic acid bacteria, notably, L. lactis subsp. lactis, produce a cell-wall anchored proteinase that hydrolyzes specific casein subunits (Box 5-1), producing as many as 100 or more different peptides varying in length from four to thirty amino acid residues. Although proteinases are produced by several strains of L. lactis subsp. lactis and other lactic acid bacteria, the enzymes are structurally quite similar (in terms of their DNA and amino acids sequences).

Table 5.2. Sources of flavor compounds in cheese1.

Protein (casein)

Carbohydrate

Lipid

peptides

lactate

fatty acids

amino acids

acetate

keto acids

sulfur compounds

pyruvate

esters

ammonia and amines

ethanol

methyI ketones

pyruvate

diacetyl

Iactones

acetate

acetoin

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