| Browning and haze in apple juice concentrate (AJC) have been long-standing problems in processing and there is no reliable solution to them. So, how to safely better cope with these problems and to protect the nutrition of juice is important to produce high quality AJC.Effect of storage on browning and haze of AJC is investigated and some inducements are determined in the paper; effect of processing on browning and haze is researched; the change of color and turbidity during producing season is carefully observed; kinetics on the adsorption of dark colored compounds from apple juice using LSA-800B adsorbent resin is studied; also micro-filtration is attempted to retard browning and haze development by using metal membrane. Theories and data are supplied to resolve the problems in AJC processing. The results of the work are listed as follows:1.First pasteurization of the raw juice may inactivate or denature polyphenoloxidase (PPO), so in the following processing and storage, accumulation of brown color is mainly due to Maillard reaction.2. During storage time, haze is liable to form at high temperature but low temperature. Haze development during storage reveals three distinct phases which may be labeled as follows: a lag phase, a growth phase, and a stable phase. The rate of haze formation at all three temperatures 5℃, 20℃, and 37℃, as measured by the increase 'in turbidity, can be described by first-order reaction kinetics.3. The rate of browning at all three temperatures fit first-order kinetic model. The Maillard reaction occurs preferably at high temperature, for the rate of this reaction at high temperature is faster than that at low temperature. The browing increases of juice samples which are stored at 5℃, 20℃, and 37℃ are 12.45%, 46.86%, and 415.28%, respectively. According to colour value (T440) and browning index (A420) changing, the rates of browning at all three temperatures can be described by first-order reaction kinetics.4.The contents of soluble protein (Pro), amino acid nitrogen (AAN), polyphenol (PP), and Tannin (Tan) in AJC decrease by increasing storage time and the decreasing rate at 37℃ is evidently greater than that at 5℃ and 20℃.5. The relation between 5-hydroxymethylfurfural(HMF)and browning of juice can be described as exponential and the regression coefficient is larger than 0.90. So, the reason why AJC becomes browning during storage is Maillard reaction; browning changes greatly by little change of HMF, At the beginning of storage, lower temperature can prevent browning of juice. AAN contents of the AJC samples decrease as a function of storage and temperature. The linear relation between AAN and browning is also indicated that the cause of browning is Maillard reaction.6. At initial processing stage, low temperature and instant high temperature sterilization, Maillard reacts slowly, which has no obvious effect on the browing. The rate of Maillard reaction is increase at second pasteurization for the sake of high temperature and long-playing. Ultrafiltration (UF) has hardly effect on AAN, however, other unit operations can reduce the content of ANN especially in concentration, adsorption and second pasteurization. The tendency of PP and Tan changing is declining after enzymolysis during processing. Solubled protain can be reduced markedly by enzymolysis as well as by UF and adsorption, but concentration and second pasteurization.7. The initial turbidity of hazed AJC is remarkably higher than that of normal AJC, thereby, controlling the initial turbidity of juice is important.8. The adsorption of color compounds on LSA-800B adsorbent resin was an endothermic process, and controlled by physical mechanisms only. The initial equilibrium time of adsorption is average 9.73h at 25 ℃ for all concentrations of adsorbent resin, and may tend to decrease as the adsorption temperature increases. Within the adsorbent resin concentration of 2-4g/L juice, the effect of adsorption is distinct; exceeding the scope, the efficiency per unit adsorbent decrease...
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