Effect of freezing on organic acid contents and lipolytic index of plain soft and Monterey Jack goat milk cheeses

Three lots of plain soft (PS) goat cheeses were purchased, and three lots of Montery Jack (MJ) goat milk cheeses were manufactured at the University dairy plant. The two varieties were subdivided into two equal portions to study effects of freezing and thawing on changes in organic acid profiles and lipolytic properties. One portion was stored at 4 °C as the nonfrozen control and the other half was immediately frozen (−20 °C) and subsequently thawed the next day at 4 °C and stored at 4 °C for 0, 14 and 28 days. Organic acids were quantified using a HPLC (Hewlett-Packard; LC-1100 Series) equipped with auto sampler, quaternary pump, vacuum degasser and diode array detector. The column was reverse phase Hewlett-Packard ODS Hypersil 5 μm (125 mm × 4 mm), and solvent was 0.5% ammonium phosphate. Freezing MJ cheese reduced (P < 0.05) lactic, orotic, citric acids, while butyric isomers were significantly increased. Changes in other organic acids and their isomers in MJ were not significant. Acetic and propionic acids in PS were increased (P < 0.05) by freezing, whereas other acids were not significantly affected. The PS had no pyruvic acid, while MJ contained no tartaric acid, but several unknown large peaks appeared between propionic and butyric acids. At the initial stage, there were no differences in pH and acid degree values (ADV) between fresh (nonfrozen) and frozen–thawed treatment groups of both cheeses. However, ADVs gradually increased as the refrigerated storage extended up to 4 weeks, indicating that lipolysis increased with the extended refrigeration storage at 4 °C. It was concluded that freezing influenced some organic acid contents, and did not for the pH and ADVs of initial cheeses, while the subsequent refrigeration aging elevated the lipolytic index of both goat cheeses.