Plasmin (EC 3.4.21.7) was added to cheese milk to assess its contribution to Cheddar cheese ripening; the activity of plasmin in the cheese was increased by levels ranging from 1-5 to 6 times that in the control cheeses. Even at the highest level of added plasmin, no activity was found in the whey. /?-Casein was degraded faster in the experimental cheeses than in the controls, and the concentration of y-caseins increased concomitantly. The total N in the water-soluble extract was up tõ 20 % higher in the experimental than in the control cheeses but phosphotungstic acid-soluble N was not affected by the plasmin activity in the cheese. Several differences were apparent in the gel electropherograms of the water-soluble extracts of the experimental and control cheeses; some peptides were present at higher concentrations in the former, others in the latter, suggesting that plasmin contributes to both the formation and degradation of water-soluble peptides in cheese. The organoleptic quality of the plasmin-enriched cheeses was judged superior to that of the controls and ripening was considerably accelerated; a plasmin level 3-4 times the indigenous value appeared to be optimal. No bitterness was detected in any of the cheeses.

Sodium reduction in cheese can assist in reducing overall dietary Na intake, yet saltiness is an important aspect of cheese flavor. Our objective was to evaluate the effect of partial substitution of Na with K on survival of lactic acid bacteria (LAB) and nonstarter LAB (NSLAB), pH, organic acid production, and extent of proteolysis as water-soluble nitrogen (WSN) and protein profiles using urea-PAGE, in Cheddar cheese during 9mo of storage. Seven Cheddar cheeses with molar salt contents equivalent to 1.7% salt but with different ratios of Na, K, Ca, and Mg cations were manufactured as well as a low-salt cheese with 0.7% salt. The 1.7% salt cheeses had a mean composition of 352g of moisture/kg, 259g of protein/kg and 50% fat-on-dry-basis, and 17.5g of salt/kg (measured as Cl(-)). After salting, a faster initial decrease in cheese pH occurred with low salt or K substitution and it remained lower throughout storage. No difference in intact casein levels or percentage WSN levels between the various cheeses was observed, with the percentage WSN increasing from 5% at d 1 to 25% at 9mo. A greater decrease in intact αs1-casein than β-casein was detected, and the ratio of αs1-casein (f121-199) to αs1-casein could be used as an index of ripening. Typical changes in bacteria microflora occurred during storage, with lactococci decreasing gradually and NSLAB increasing. Lowering the Na content, even with K replacement, extended the crossover time when NSLAB became dominant. The crossover time was 4.5mo for the control cheese and was delayed to 5.2, 6.0, 6.1, and 6.2mo for cheeses with 10, 25, 50, and 75% K substitution. Including 10% Mg or Ca, along with 40% K, further increased crossover time, whereas the longest crossover time (7.3mo) was for low-salt cheese. By 9mo, NSLAB levels in all cheeses had increased from initial levels of ≤10(2) to approximately 10(6)cfu/g. Lactococci remained at 10(6) cfu/g in the low-salt cheese even after 9mo of storage. The propionic acid concentration in the cheese increased when NSLAB numbers were high. Few other trends in organic acid concentration were observed as a function of Na content.

Cheese is a highly nutritious food with many diverse flavours and textures. It can be used as a snack or as part of a dish or prepackaged convenience food. The diversity of cheese is due to an increasing knowledge of the technology of cheesemaking, and the biochemistry and microbiology of cheese ripening. Professor Pat Fox's research career over the past 40 years has contributed to our understanding of cheese technology, biochemistry and microbiology. This paper highlights Pat Fox's contributions in the area of cheese technology.

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SummaryLevels of plasmin activity in various commercial cheeses are reported. The effects of some processing conditions (pH, cooking temperature and method of salting) on plasmin activity in cheese were investigated. Neither draining pH nor method of salting affected the activity. However, plasmin activity in cheese, rennet curd and micellar casein dispersions increased with increasing cooking temperature. The results suggest that plasmin activity in cheese depends on cooking temperature used during manufacture.