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Chymosin-Mediated Proteolysis, Calcium Solubilization, and Texture Development During the Ripening of Cheddar Cheese

¹ Department of Food and Nutritional Sciences, University College, Cork, Ireland
² Department of Food Science, University of Wisconsin-Madison, 1605 Linden Drive, Madison 53706

Corresponding author: Paul McSweeney; e-mail: p.mcsweeney{at}ucc.ie.

Full fat, milled-curd Cheddar cheeses (2 kg) were manufactured with 0.0 (control), 0.1, 1.0, or 10.0 µmol of pepstatin (a potent competitive inhibitor of chymosin) added per liter of curds/whey mixture at the start of cooking to obtain residual chymosin levels that were 100, 89, 55, and 16% of the activity in the control cheese, respectively. The cheeses were ripened at 8°C for 180 d. There were no significant differences in the pH values of the cheeses; however, the moisture content of the cheeses decreased with increasing level of pepstatin addition. The levels of pH 4.6-soluble nitrogen in the 3 cheeses with added pepstatin were significantly lower than that of the control cheese at 1 d and throughout ripening. Densitometric analysis of urea-PAGE electro-phoretograms of the pH 4.6-insoluble fractions of the cheese made with 10.0 µmol/L of pepstatin showed complete inhibition of hydrolysis of _S1-casein (CN) at Phe₂₃-Phe₂₄ at all stages of ripening. The level of insoluble calcium in each of 4 cheeses decreased significantly during the first 21 d of ripening, irrespective of the level of pepstatin addition. Concurrently, there was a significant reduction in hardness in each of the 4 cheeses during the first 21 d of ripening. The softening of texture was more highly correlated with the level of insoluble calcium than with the level of intact _S1-CN in each of the 4 cheeses early in ripening. It is concluded that hydrolysis of _S1-CN at Phe₂₃-Phe₂₄ is not a prerequisite for softening of Cheddar cheese during the early stages of ripening. We propose that this softening of texture is principally due to the partial solubilization of colloidal calcium phosphate associated with the para-CN matrix of the curd.

Key Words: proteolysis • texture • Cheddar cheese • insoluble calcium

Abbreviation key: CCP = colloidal calcium phosphate, FAA = free amino acids, LAB = lactic acid bacteria, NSLAB = nonstarter lactic acid bacteria, PC = principal component, RP = reversed phase, SN = soluble nitrogen, TPA = texture profile analysis.

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