Organic acids as a freshness indicator for tofu (soybean curd).

The aim of this study was to evaluate organic acids as potential indicators of tofu freshness. To achieve this, relationships between organic acids concentrations and the growth of microorganisms in fresh tofu were investigated. The levels of microorganisms (total bacterial count) and organic acids (phytic acid, oxalic acid, citric acid, lactic acid, formic acid and acetic acid) were analyzed in tofu (packed and unpacked) every 3 days during 15 days of storage at different temperatures (4, 10, and 25 °C). Organic acids were analyzed by high performance liquid chromatography and microbial analysis was conducted by plate counting method. The levels of oxalic acid, citric acid and formic acid decreased significantly during the storage period, while the levels of lactic acid and acetic acid increased significantly when stored at 10 °C. The acetic, lactic and formic acids showed significant correlation to the levels of microorganisms in packed tofu, suggesting the use of these organic acids as potential freshness quality indicators of tofu. Current study demonstrated the effective way of predicting freshness of tofu by utilizing organic acid analysis, as opposed to traditional method relying on microbial count.

Formation and reduction of furan in maillard reaction model systems consisting of various sugars/amino acids/furan precursors.

To investigate the effect of the temperature on the formation of furan, various furan models were conducted at 90, 121, and 150 °C. A total 15 models, including alanine (ALA), serine (SER), ribose (RIB), RIB/ALA, RIB/SER, glucose (GLU), GLU/ALA, GLU/SER, sucrose (SUC), SUC/ALA, SUC/SER, furoic acid (FUR), GLU/FUR, acetaldehyde (ACET), and GLU/ACET, were prepared. The maximum level of furan was detected in the GLU/SER and GLU/ALA models at a molar ratio of 0.5:0.5. The formation of furan was proportional to the temperature in all models. The RIB/SER model generated the greatest amount of furan among the 11 models ranging from 2.1 to 4931.9 ng mL(-1) under all temperature conditions. Among the precursor models, the FUR model formed the greatest amount of furan ranging from 1058.2 to 13 927.9 ng mL(-1) at all temperatures.

Volatile compounds as markers of tofu (soybean curd) freshness during storage.

Volatile compounds in packed and unpacked commercial tofu were extracted by solid phase microextraction fibers and analyzed by gas chromatography-mass spectrometry. The tofu samples were stored at 4 °C for 15 days to measure freshness and quality changes during storage. Totals of 41 and 35 volatile compounds were identified from packed tofu and unpacked tofu, respectively. Peak area ratio (PAR) of hexanal in packed tofu increased from 11.44 (day 0) to 496.30 (day 9) and finally decreased to 11.06 on day 15 of storage. PAR of ethanol and 1-hexanol increased from 5.41 and 0.76 (day 1) to 66.93 (day 9) and 47.46 (day 15), respectively. Changes in these volatiles in unpacked tofu were similar to those in packed tofu. The results show that hexanal, ethanol, and 1-hexanol are characteristic volatile compounds for providing quantitative and qualitative information regarding deterioration of tofu.