| Summary: | 碩士 === 國立中興大學 === 食品暨應用生物科技學系所 === 101 === Freeze concentration is a concentration method conducted at low temperatures. It reduces the negative influence from heat on products, and it can be applied on the concentration of fluid food. In recent years, with the rising living standards and greater emphasis on health, consumers have higher demands for nutrients, flavor and quality of fruit juices, and the needs of high-quality fruit juices has increased. The concentrated fruit juices produced with this method preserve their nutrients and flavor substances at the highest level. Therefore, further researches on this method should be conducted in order to improve the quality of concentrated fruit juices.
This experiment applied discontinuous freeze concentration by layer crystallization on glucose concentration. It investigates the changes of glucose concentration and the distribution coefficient K in tube of various diameters and at different rotations speed (rpm) of mixer in order to determine the optimum conditions for this method and compare the differences between the discontinuous and continuous methods. Moreover, with cranberry juice produced by freeze concentration method in optimum conditions, this study discusses the concentration changes of each ingredient in cranberry juice with different starting concentration and at various temperatures.
The result of comparing tubes with different diameters indicates that tubes with 8 cm diameter have the smallest distribution coefficient K, and when its smallest value is at rotation speed of mixer at 300-400 rpm, and the solid content is at 300 rpm, the concentration is of the best effect. Under the operation conditions in this experiment, the discontinuous and the continuous progressive freeze concentration methods have no obvious differences.
This study used discontinuous progressive freeze concentration method on making concentrated cranberry juice, and the result showed an inverse ratio between the concentration efficiency and the starting concentration; low starting concentration has better concentration efficiency. The result indicates that in juice obtained with double concentration, the quantity of its fruit solids increased to 192%, its citric acid elevated to 250%, and the absorbance of its pigment increased by 198% .
When the temperature of the refrigerant decreased from -10℃ to -15℃, the result suggested that after double concentration, the soluble solids, the citric acid and the absorbance of its pigment in solution increased to 232%, 280% and 272%, respectively, and both concentration values are higher than the values at -10℃. The observation showed that decrease of temperatures caused higher concentration of the soluble solids, citric acid and the absorbance of its pigment but distribution coefficient K has higher value at -15℃than at -10℃. Therefore, temperature will affect the concentration efficiency, we can study the factor in depth to obtain the proper temperature conditions.
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