Influence of temperature and residence time of main liming process on the reduction of a-amino acids during beet juice purification

During beet raw juice purification, an appreciable reduction in the content of α-amino acids results from destruction reactions, i.e. saponification of amides, which chiefly occur during the main liming process owing to alkaline conditions at elevated temperatures. Hence, from the point of view of optimum degradation conditions in main liming, the aim of this study was to investigate the influence of temperature (i.e. 80, 85 and 90°C) and duration (i.e. 15, 25, 35 and 45 minutes) on a-amino acid degradation. In this research, the classical method was used to purify the beet raw juice. The α-nitrogen content of both raw and thin juices was measured using a Betalyser analysing system. In this study, invert sugar content of thin juices was also evaluated since reducing sugars not only give rise to formation of coloured and acid products during main liming process but also tend to react with amino acids to form browning products in technical juices. The resultant thin juices were concentrated in an atmospheric pressure evaporator until a brix of 60 was obtained. Limed juices heated at 80°C exhibited larger α-amino nitrogen content than those at higher main liming temperatures and the greatest residual α-amino nitrogen content (13.14%) of the original level in raw juice was found at 80°C and the minimum residence time (15 min). High temperatures and long residence times of main liming decreased the residual α-amino nitrogen content, although they resulted in increased colour of thin juices. Nevertheless, these juices subsequently exhibited a lower colour formation in beet thick juice during evaporation compared with thin juices subjected to milder conditions of main liming.