Short communication: Evidence for methylglyoxal-mediated browning of Parmesan cheese during low temperature storage.

Brown pigmentation can occasionally form in Parmesan cheese during the ripening process, creating an unappealing appearance and associated off-flavors. The browning reactions proceed at refrigerated temperatures and in the relative absence of reducing sugar, contrary to typical Maillard browning. Residual sugars, lipid oxidation products, byproducts of fermentation, and (or) enzymes may react with primary amines in cheese to form brown pigmentation and concomitantly generate volatile compounds unique to each of these reactions. Determining the volatile profiles provides a means of understanding the potential substrates involved and causative reaction pathways. This work is divided into 2 segments. The first portion characterized the pigmentation and the browning index of Parmesan cheeses with and without extensive browning. The second phase examined the volatile character of the cheeses using gas chromatography-mass spectrometry. Various pyrazines, such as 2,3,5-trimethylpyrazine and 3,5-diethyl-2-methylpyrazine, were found in the brown cheeses but were not present in the white samples. The formation of pyrazines is proposed to result from the spontaneous condensation of aminoacetone. Aminoacetone can be formed by the Strecker degradation of amino acids with methylglyoxal, the latter a byproduct of sugar fermentation. Evidence is provided to support a browning pathway mediated by the production of methylglyoxal in Parmesan cheese.