《乳品生物化学》（英文版） 10 Chemistry and biochemistry of cheese and fermented milks

DAIRY CHEMISTRY AND BIOCHEMISTRY but the lower its stability. Syneresis is promoted by cutting the curd finely, e.g. Emmental (fine cut)versus Camembert(large low pH (Figure 10.10b); calcium ions increasing the cooking temperature( Camembert, c 30C; Gouda, c 36C; Cheddar, c 38C; Emmental or Parmesan, 52-55C)(Figure 10. 10a); stirring the curd during cooking fat retards syneresis, while increasing the protein content(up to a point improves it; at high protein concentrations, the gel is too firm and does not synerese(e.g. UF retentate) Gels prepared from heated milk synerese poorly(assuming that the milk does coagulate). Such reduced syneresis properties are desirable for fer mented milk products, e.g. yoghurt(milk for which is severly heated, e.g 90°C×10min) but are undesirable for cheese Good analytical methods for monitoring syneresis are lacking. Principles that have been exploited include: dilution of an added marker, e.g. a dye, which must not adsorb on to or diffuse into the curd particles, measurement of the electrical conductivity or moisture content of the curd or by measuring the volume of whey released (probably the most commonly used method although only one-point values are obtained) 10.2.3 Acidification Acid production is a key feature in the manufacture of all cheese varieties the pH decreases to about 5(+0.3, depending on variety) within 5-20h, at a rate depending on the variety (Figure 10. 11). Acidification is normally achieved via the bacterial fermentation of lactose to lactic acid, although an acidogen, usually gluconic acid-o-lactone, alone or in combination with acid, may be used in some cases, e.g. Mozzarella Traditionally, cheesemakers relied on the indigenous microfora of milk for lactose fermentation, as is still the case for several minor artisanal varieties. However, since the indigenous microfora varies, so does the rate of acidification and hence the quality of the cheese; the indigenous micr flora is largely destroyed by pasteurization. 'Slop-back'or whey cultures (starters; the use of whey from today ' s cheesemaking as an inoculum for tomorrows milk) have probably been used for a very long time and are still sed commercially, e. g. for such famous cheese as Parmigiano-Reggiano and Comte. However, selected'pure'cultures have been used for Cheddar and Dutch-type cheeses for at least 80 years and have become progressively more refined over the years. Single-strain cultures were introduced in New Zealand in the 1930s as part of a bacteriophage control programme Selected phage-unrelated strains are now widely used for Cheddar cheese

394 DAIRY CHEMISTRY AND BIOCHEMISTRY but the lower its stability. Syneresis is promoted by: 0 cutting the curd finely, e.g. Emmental (fine cut) versus Camembert (large 0 low pH (Figure 10.1Ob); 0 calcium ions; 0 increasing the cooking temperature (Camembert, c. 30°C; Gouda, c. 36°C; Cheddar, c. 38°C; Emmental or Parmesan, 52-55OC) (Figure 10.1Oa); 0 stirring the curd during cooking; 0 fat retards syneresis, while increasing the protein content (up to a point) improves it; at high protein concentrations, the gel is too firm and does not synerese (e.g. UF retentate). Gels prepared from heated milk synerese poorly (assuming that the milk does coagulate). Such reduced syneresis properties are desirable for fer￾mented milk products, e.g. yoghurt (milk for which is severly heated, e.g. 90°C x 10min) but are undesirable for cheese. Good analytical methods for monitoring syneresis are lacking. Principles that have been exploited include: dilution of an added marker, e.g. a dye, which must not adsorb on to or diffuse into the curd particles, measurement of the electrical conductivity or moisture content of the curd or by measuring the volume of whey released (probably the most commonly used method although only one-point values are obtained). cut); 10.2.3 Acidification Acid production is a key feature in the manufacture of all cheese varieties - the pH decreases to about 5 (k0.3, depending on variety) within 5-20h, at a rate depending on the variety (Figure 10.11). Acidification is normally achieved via the bacterial fermentation of lactose to lactic acid, although an acidogen, usually gluconic acid-6-lactone, alone or in combination with acid, may be used in some cases, e.g. Mozzarella. Traditionally, cheesemakers relied on the indigenous microflora of milk for lactose fermentation, as is still the case for several minor artisanal varieties. However, since the indigenous microflora varies, so does the rate of acidification and hence the quality of the cheese; the indigenous micro￾flora is largely destroyed by pasteurization. ‘Slop-back’ or whey cultures (starters; the use of whey from today’s cheesemaking as an inoculum for tomorrow’s milk) have probably been used for a very long time and are still used commercially, e.g. for such famous cheese as Parmigiano-Reggiano and Comte. However, selected ‘pure’ cultures have been used for Cheddar and Dutch-type cheeses for at least 80 years and have become progressively more refined over the years. Single-strain cultures were introduced in New Zealand in the 1930s as part of a bacteriophage control programme. Selected phage-unrelated strains are now widely used for Cheddar cheese;

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 395 Time(h) igure 10 11 ph profile of Cheddar during cheese manufacture although selected by a different protocol, highly selected cultures are also used for Dutch and Swiss-type cheeses Members of three genera are used as cheese starters. For cheeses that are cooked to a temperature below about 39C, species of Lactococcus, usually Lc lactis ssp. cremoris, are used, i. e for Cheddar, Dutch, Blue, surface mould and surface-smear families. For high-cooked varieties, a thermophilic L tobacillus culture is used, either alone(e.g. Parmesan) or with Streptococcus salivarius ssp. thermophilus (e.g. most Swiss varieties and Mozzarella) Leuconostoc spp. are included in the starter for some cheese varieties, e.g Dutch types; the function is to produce diacetyl and co2 from citrate rather than acid production The selection, propagation and use of starters will not be discussed here The interested reader is referred to Cogan and Hill(1993) The primary function of cheese starter cultures is to produce lactic acid at a predictable and dependable rate. The metabolism of lactose is sum marized in Figure 10.12. Most cheese starters are homofermentative, i.e produce only lactic acid, usually the L-isomer; Leuconostoc species are heterofermentative. The products of lactic acid bacteria are summarized in Table 10.4 Acid production plays several major roles in cheese manufacture Controls or prevents the growth of spoilage and pathogenic bacteria. Affects coagulant activity during coagulation and the retention of active coagulant in the curd

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 395 2 5 Time (h) Figure 10.11 pH profile of Cheddar during cheese manufacture. although selected by a different protocol, highly selected cultures are also used for Dutch and Swiss-type cheeses. Members of three genera are used as cheese starters. For cheeses that are cooked to a temperature below about 39"C, species of Lactococcus, usually Lc. lactis ssp. cremoris, are used, i.e. for Cheddar, Dutch, Blue, surface mould and surface-smear families. For high-cooked varieties, a thermophilic Lac￾tobacillus culture is used, either alone (e.g. Parmesan) or with Streptococcus saliuarius ssp. therrnophilus (e.g. most Swiss varieties and Mozzarella). Leuconostoc spp. are included in the starter for some cheese varieties, e.g. Dutch types; the function is to produce diacetyl and CO, from citrate rather than acid production. The selection, propagation and use of starters will not be discussed here. The interested reader is referred to Cogan and Hill (1993). The primary function of cheese starter cultures is to produce lactic acid at a predictable and dependable rate. The metabolism of lactose is sum￾marized in Figure 10.12. Most cheese starters are homofermentative, i.e. produce only lactic acid, usually the L-isomer; Leuconostoc species are heteroferrnentative. The products of lactic acid bacteria are summarized in Table 10.4. Acid production plays several major roles in cheese manufacture: 0 Controls or prevents the growth of spoilage and pathogenic bacteria. 0 Affects coagulant activity during coagulation and the retention of active coagulant in the curd

396 DAIRY CHEMISTRY AND BIOCHEMISTRY Larrooccr te ociobacrlir Lactose CYTOPLASM Galactcse-6-P Glucose.GP Fructose-6-P Tagatose. L. 6-biP Fructose-1,5-hiP xylulose- 5-P Dihydroxyacetone-P= Glyceraldehyde. ACETYL ACETYLALDEHYDE Leloir Figure 10.12 Metabolism of lactose by lactic acid bacteria; man bacillus species/ strains can not metabolize galactose(from Cogan and Hill, 1993) texture; rapid acid production leads to a low level of calcium in the chee Solubilizes of colloidal calcium phosphate and thereby affects and a crumbly texture (e.g. Cheshire)and vice versa( e.g. Emmental/3e Promotes syneresis and hence influences cheese composition Influences the activity of enzymes during ripening, and hence affects cheese quality

396 DAIRY CHEMISTRY AND BIOCHEMISTRY LQcrococcr Some lacrobocrlii ood srreprococci kuronosrocs EXTERNAL Lactose Lactose Lactose ENVIRONMENT CELL WALL MEMBRANE I’MF CYTOI’LASM Laciore-P i Lactose J. Laclose I t Glucose Lactose Glucose Galactose-6-P p4-t ADI’ Glucose-6-P - Galactose-I-P GlUCoSe-I-P KT1’ t AUI’ + - Glucore.6.P LlDP {C:::: 6-Phosphogluconate 1 Fructose-6-P J Tagatose.6-P k”, K ::, $ t co2 ADI’ t $. Ribulose-5-P Tagatose.1.6.blP Fructose-1’6-hiP Xylulose.5-P p, v Acetyl-P 1 CuASH Dthydroxyacetone-P -1 Glyceraldehyde-3-P t :::,yp 1.3-Diphosphoglycerate 3-Phosphoglycerate 24 ACETY L-CoA CnASH t ?-Phosphoglycerate I Tagatose pathway Phosphoenolpyruvate A,.. ‘\I I’ 2 Fyruvare Lactate Glycolytic pathway ACETYLALDEHYDE NADH K l\AIl’ Ethanol Leloir Phosphoketolase pathway pathway Figure 10.12 Metabolism of lactose by lactic acid bacteria; many Lactobacillus species/strains can not metabolize galactose (from Cogan and Hill, 1993). Solubilizes of colloidal calcium phosphate and thereby affects cheese texture; rapid acid production leads to a low level of calcium in the cheese and a crumbly texture (e.g. Cheshire) and vice versa (e.g. Emmental). Promotes syneresis and hence influences cheese composition. Influences the activity of enzymes during ripening, and hence affects cheese quality

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 397 Table 10.4 Salient features of lactose metabolism in starter culture organisms(from Cogan and Hil1993) Organism Transport" enzyme Pathway I-I lactose) GLY 4 L-Lactate Leuconostoc spp. 2 D-Lactate +2 ethanol + 2C0 GLY 2L-Lactate subsp thermophilus PMF? Bgal GLY 2D-Lactate GLY Lb. helveticus PMF? Bgal GLY 4 L(mainly)+D-lactate PTS, phosphor system; PMF, proton motive force "These etabolize only the glucose moiety of lactose The primary starter perfor cal functions in addit production, especially reduction of the redox +250 mv in milk to -150 mv in cheese), and, most importantly, plays a major, probably essential, role in the biochemistry of cheese ripening. Many strains produce bacteriocins which control the growth of contaminating micro-organisms. The ripening of many varieties is characterized by the action, not of the primary starter, but of other micro-organisms, which we will refer to as a econdary culture. Examples are Propionibacterium in Swiss-type cheeses, Penicillium roqueforti in Blue cheeses, Penicillium camemberti in surface mould-ripened cheeses, e.g. Camembert and Brie, Brevibacterium linens and yeasts in surface smear-ripened cheese, Lactococcus lactis ssp. lactis biovar diacetylactis and Leuconostoc spp. in Dutch-type cheeses. The specific function of these micro-organsims will be discussed in section 10.2.7 on ripening. Traditionally, a secondary culture was not used in Cheddar-type cheeses but there is much current interest in the use of cultures of selected bacteria, usually mesophilic Lactobacillus spp or lactose-negative lactococ cus spp, for Cheddar cheese with the objective of intensifying or modifying favour or accelerating ripening; such cultures are frequently referred to as adjunct cultures 10.2.4 Moulding and shaping When the desired pH and moisture content have been achieved, the curds are separated from the whey and placed in moulds of traditional shape and size to drain and form a continuous mass; high-moisture curds form a

CHEMISTRY AND BIOCHEMISTRY OF CHEESE AND FERMENTED MILKS 397 Table 10.4 Salient features of lactose metabolism in starter culture organisms (from Cogan and Hill, 1993) ~ Organism Cleavageb Products Transport” enzyme Pathway‘ (mol mol- lactose) Lactococcus spp. PTS ppgal GLY 4 L-Lactate Leuconostoc spp. ? Bgal PK 2 D-Lactate + 2 ethanol + 2C0, Str. salicarius PMF GLY 2 L-Lactated Lb. delbrueckii PMF? /?gal GLY 2 D-Lactated Lb. delbrueckii PMF? jgal GLY 2 D-Lactated Lb. helveticus PMF? Pgal GLY 4 L- (mainly) + D-lactate subsp. thermophilus subsp. lactis subsp. bulgarrcus OPTS, phosphotransferase system; PMF, proton motive force. *ppgal, phospho-8-galactosidase; pgal, 8-galactosidase. ‘GLY, glycolysis; PK. phosphoketolase. dThese species metabolize only the glucose moiety of lactose. The primary starter performs several functions in addition to acid production, especially reduction of the redox potential (Eh, from about +250mV in milk to - 150mV in cheese), and, most importantly, plays a major, probably essential, role in the biochemistry of cheese ripening. Many strains produce bacteriocins which control the growth of contaminating micro-organisms. The ripening of many varieties is characterized by the action, not of the primary starter, but of other micro-organisms, which we will refer to as a secondary culture. Examples are Propionibacterium in Swiss-type cheeses, Penicillium rogueforti in Blue cheeses, Penicillium camemberti in surface mould-ripened cheeses, e.g. Camembert and Brie, Breuibacterium linens and yeasts in surface smear-ripened cheese, Lactococcus lactis ssp. lactis biovar diacetylactis and Leuconostoc spp. in Dutch-type cheeses. The specific function of these micro-organsims will be discussed in section 10.2.7 on ripening. Traditionally, a secondary culture was not used in Cheddar-type cheeses but there is much current interest in the use of cultures of selected bacteria, usually mesophilic Lactobacillus spp. or lactose-negative Lactococ￾cus spp., for Cheddar cheese with the objective of intensifying or modifying flavour or accelerating ripening; such cultures are frequently referred to as ‘adjunct cultures’. 10.2.4 Moulding and shaping When the desired pH and moisture content have been achieved, the curds are separated from the whey and placed in moulds of traditional shape and size to drain and form a continuous mass; high-moisture curds form a

398 DAIRY CHEMISTRY AND BIOCHEMISTRY Figure 10. 13 A selection of cheese va howing the diversity of cheese size, shape and opearance. continuous mass under their own weight but low-moisture varieties are pressed Cheeses are made up in traditional shapes(usually flat cylindrical, but also sausage, pear-shaped or rectangular) and size, ranging from around 250g(e.g. Camembert)to 60-80kg(e.g. Emmental; Figure 10.13). The size of cheese is not just a cosmetic feature; Emmental must be large enough to prevent excessive diffusion of CO2, which is essential for eye development while Camembert must be quite small so that the surface does not become over-ripe while the centre is still unripe(this cheese softens from the surface to the centre) Curds for the Pasta filata cheeses, e.g. Mozzarella, Provol Halloumi, are heated in hot water(70-75oC), kneaded and stretched when the pH reaches about 5.4; this gives the cheeses a characteristic fibrous structure 10.2.5 salting All cheeses are salted by mixing dry salt with the drained curd ( confined largely to Engl rieties), rubbing dry salt on the surface of the pressed cheese(e.g. Re or Blue cheeses), or by immersion of the pressed cheeses in brine (most varieties). Salt concentration varies from c.0.7%(c. 2% salt- in-moisture)in Emmental to 7-8%(c. 15% salt-in- moisture)in Domiati