Abstract:
The objectives of this research were to study the conditions for the production of hard soybean curd using lime juice (Citrus aurantifolia Swingle), Somjeed juice (C.mitis Blanco) and tangerine juice (C.reticulata Blanco) as coagulants. Firstly, the chemical composition of dehulled soybean (Glycine max Merrill) was analysed and the results showed that there were 8.75% moisture, 42.25% protein, 19.32% fat, 0.97% crude fiber, 5.49% ash and 31.96% carbohydrate in the duhulled soybean. The total acidity (as citric acid) of lime juice, Somjeed juice and tangerine juice were 7.5%, 5.21% and 2.14% respectively. The results of HPLC analysis showed that lime juice contained 6.86% citric acid and 0.52% L-malic acid while Somjeed juice contained 4.54% citric acid, 0.27% L-malic acid and tangerine juice contained 2.43% citric acid, 1.07% L-malic acid. The water to bean ratio of 7:1 was the suitable condition for the preparation of soy milk. The effect of coagulation temperature, stirring rate, weight for pressing the curd and the amount of citrus juices used as coagulants on the qualities of hard soybean curd was studied. It was found that the optimum condition for preparation of hard soybean curd was coagulation temperature of 90 ํC, stirring rate of 150 rpm, weight for pressing the curd of 3 kg and the amount of lime juice, somjeed juice and tangerine juice were 1, 2 and 5% (w/w) of soy milk. The texture and chemical composition of the soybean curd obtained was similar to those curds obtained by using 1% CaSO4.2H2O, 1% MgSO4.7H2O, 0.5% CaCl2.2H2O and 0.5% MgCl2.6H2O as coagulants. The results of sensory analysis using 9 points Hedonic Scale showed that the soybean curd using citrus juices as coagulants had highest score of total acceptability. However, the results showed that the total acceptability score of soybean curds using situs juices as coagulants was different from the curds using CaSO4.2H2O and MgSO4.7H2O as coagulants nonsignificantly but the score was different from soybean curds using CaCl2.2H2O and MgCl2.6H2O as coagulants significantly. Studying of microstructure of the soybean curds using Scanning Electron Microscope (SEM) showed that the curds using lime juice and CaSO4.2H2O as coagulants had coarse structure but the curds using Somjeed juice, tangerine juice, MgSO4.7H2O, CaCl2.2H2O and MgCl2.6H2O as coagulants had fine structure. The shelf-life of the soybean curds citrus juices as coagulants was also studied. The results indicated that the curds stored at 4-10 ํC in PS tray and wrapped with PVC film could stay as long as 7 days without any changes in physical properties and sensory qualities. The microbiological results also showed that the curds were in the standard of Thai Industrial Standard Institute.