Effects of single food components such as sodium chloride, sugars and milk protein on freeze concentration by freezing and thawing treatment were evaluated in this study. This method has a different concept from traditional freeze concentration whereby complete freezing is essential prior to thawing. The concentrated fractions were separated upon thawing the frozen sample in a chamber at 10°C. Various thawing rates were obtained when thawing the food components of different concentrations. The concentration of the solute affects the freezing point depression of the aqueous solution. The concentration index was used to evaluate the effectiveness of freezing and thawing on concentrating the food components within the range of 1 to 20%. The 1% of sodium chloride, glucose, fructose, lactose and sucrose solutions were concentrated 4-5 times in the first fraction, except dextrin (less than 2.5 times) . As the concentration of the food components increased, the concentration effect reduced as shown by the reducing index approaching unity. However, dextrin and milk protein solutions were not effectively concentrated by freezing and thawing due to their smaller freezing point depression.
Bean cured refuse is major sub-product of tofu; it has been a foodstuffs with plentiful nutrition, but presently it is mostly disposed to be reclaimed or burned. In the present study we aim to dissolve the bean curd refuse by applying the super- or sub- critical water as the reaction solvent under a high temperature and high pressure and to extract useful components from the dissolved products. We applied the super- or sub-critical water to the bean curd refuse at an approaching temperature of 200-450°C and pressure of 15-30 MPa in a high pressure vessel under a batch operation. It is found that rapid heating, short reaction time and lower temperature (up to 250°C) in the sub-critical water treatment are favored for selective production of sugars. Maximum amounts of organic acids and alcohols were produced at an operating temperature of 300°C in the pressure range of 15 to 30 MPa. At an operating temperature exceeding 300°C, amount of solid residue decreased to about 5% on the elemental carbon basis. At operating temperatures beyond 350°C (mainly at supercritical range) amount of gaseous products exceeded 30%.
An electromagnetic cooking device has come into wide use in houses recently. However, a strange phenomenon that the flavor of steamed food come amiss compared with a usual gas heating has been observed. This phenomenon might be caused by the water quality change during cooking. Therefore, tap water was heated by electromagnetic heating or gas heating, and then the water quality change was analyzed. In electromagnetic heating, water quality changed significantly, that is, pH rose from 7.2 to 8.3, and hardness increased from 19.0 to 33.2 mg/L for the first 10 minutes cooking. On the other hand, in the case of gas heating, it had the minor transition. When distilled water was heated by both methods, transition of water quality was scarcely observed. It is suggested that the components of tap water (Ca, Mg, Na, K) attached to surface of pan as the water scale was eluted into the water by strong electromagnetic heating.
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