Electrophoretic ripening index for the evaluation of proteolysis and the deduction of the age of Parmesan cheese

Abstract

We have established an electrophoretic method to evaluate proteolysis in Parmesan cheese by means of an objective ripening index. The separation of caseins by alkaline polyacrylamide gel electrophoresis (PAGE) (12% T, 2.6% C, pH 8.9, 5 M urea) is followed by the densitometric analysis of theγ- andβ-casein fractions. The relationship between the resulting coefficients (γ-Cn/β-Cn) and the age of reference samples of Original Italian Grana Padano (6–22 months) was linear up to 15 months, allowing an evaluation of the extent of proteolysis and therefore a deduction of the age of the Parmesan samples analysed. Based on this calibration we propose to use a threshold level of 1.3 (γ-Cn/β-Cn) to verify the required age, i.e. 12 months, of Parmesan cheese, retailed as a loaf or prepacked slices. Commercially grated Parmesan samples may contain ≤20% cheese rind, which has a very low degree of casein breakdown. So a threshold level of 0.8 (γ-Cn/β-Cn) is proposed for all grated Parmesan products. The preparation of reference solutions with defined coefficients (γ-Cn/β-Cn), corresponding to the proteolysis indices of reference samples of different aged Grana Padano is described. The coefficients (γ-Cn/β-Cn) as well as theβ-casein content of two additional series of reference samples and of 49 commercial Parmesan samples taken from retail outlets in Austria and Italy are presented.