Effects of heat load gradient occurring in moulding on characterization and ripening of Grana Padano

Abstract

Summary - A centripetal temperature gradient takes place in Grana Padano (GP) during moulding because of the slow heat transfer within the cheese and the fast cooling of the outer part. This gradient, in combination with the low pH value, induces a centripetal inactivation of alkaline phosphatase (ALP). Throughout the ripening process, the activity of the enzyme keeps > 3 105 mU/kg cheese in the peripheral zones and < 300 mU in the core. The objective of the present study was to determine whether the presence of heat load gradients and ALP inactivation improves the analytical distinction between GP and imitation hard cheeses and affects the ripening behaviour. DifferentIy ripened sampies of traditional GP, grana cheese of reduced size, or made from pasteurized milk following GP technology, or collected at the market were submitted to a stratigraphie study. For each cheese sample, subsequent portions collected from the rind to the core were analyzed for moi sture and water activity, aw' ALP activity, furosine, free ami no acids (FAA), casein phosphopeptides (CPPs) and free fatty acids (FFA). Regarding cheese characterization, thermal treatments of cheese milk can be recognized in GP only by determining the enzyme activity in the most peripheral portions. When the size of the cheese is reduced, cooling in moulding is faster and ALP inactivation in the core is lower. When the moulded cheese is heated, inactivation of the enzyme also takes place in the outer parts. The furosine values, which describe the extent of heat load in dairy systems, increase from the outer part to the GP core (from 10 mg to 60 mg/lOO g protein). This gradient is unaffected by prolonged ripening, but changes when modifications in technoIogical parameters such as pasteurization of milk, forced heating in moulding and reduction of cheese size are introduced. The distribution of ALP and furosine values within the cheese constitutes a distinctive parameter between GP and its imitation products. Regarding chee se ripening, ail the values of chemical indices adopted for describing the ripening of GP show a progressive increase from the inner to the outer portions of nine- or 15month-old chee se samples. These patterns are more pronounced for most of the FAA, and are marked for free serine which increases in the outer zones up to 50% more than in the core. The amount of 'enzyme resistant' CPP ~-CN(f16-22)3P, the accumulation of whieh requires the presence of active phosphatases and aminopeptidases, has the same stratigraphie behaviour as that of FAA. The close