Apple juice concentrate amongst other uses is used to formulate concentrated and single strength fruit juice blends. An apple juice concentrate whose quality has deteriorated during storage poses problems for the processing industries as it limits its use in the formulation of certain products. A dark coloured apple juice concentrate cannot be used to formulate light coloured beverages while an apple concentrate contaminated with microbes can only be incorporated into products with a pasteurisation treatment. It is important to evaluate the deterioration in the quality of apple juice concentrate stored at normal and at accelerated temperatures.
The physicochemical parameters and microbial counts were monitored in apple juice concentrate stored at 10 ºC (normal temperature) and at 25 ºC and 35 ºC (accelerated temperatures) over 12 weeks after which modelling of the experimental data was done. The Biolog system and (GTG)5 PCR fingerprinting were used to identify isolated yeasts.
There was a significant change (p≤0.05) in L*, b*, a*, hue, chroma, total colour difference (TCD), non-enzymatic browning index (A420nm) and 5-hydroxymethylfurfural concentration with storage time and temperature. The Q10 values for L*, b*, non-enzymatic browning index (A420nm) and 5-hydroxymethylfurfural were 1.53, 1.20, 1.81 and 5.25, respectively and activation energies were 8.17, 7.70 and 14.8 kcal mol-1 for L*, b* and non-enzymatic browning index (A420nm), respectively. Microbial growth was not observed in apple juice concentrate stored at 10 ºC while yeast and mould growth was observed after 5 weeks of storage at 25 ºC and 35 ºC. After fitting the yeast growth data into the Baranyi and Roberts model, no significant difference was observed in the lag phase for yeast growth at 25 ºC and 35 ºC while there was a significant difference p≤0.05 in the maximum specific growth rate. The theoretical minimum temperature of growth extrapolated from the Ratkowsky model was about 20 ºC. The activation energy and Q10 values for the maximum specific growth rate were 39.95 kcal mol-1 and 4.47, respectively. The isolated yeasts were identified as Kluyveromyces delphensis, Saccharomyces dairensis, Zygosaccharomyces bailii, Rhodotorula glutinis and Metchnikowia reukaufii and strain variability was observed for the species K. delphensis and S. dairensis. The higher Q10 and activation energy values for the maximum specific growth rate of yeasts in comparison with the Maillard browning parameters (L*, b* and A420nm) signify that loss in quality due to yeast growth is more rapid compared to Maillard browning.
The results of this study show that under refrigerated storage temperatures deterioration in the quality of apple juice concentrate is due to Maillard browning while at accelerated storage temperatures quality loss is due to both Maillard browning and microbial growth of yeasts. At accelerated storage temperatures, darkening of the apple juice concentrate starts immediately and continues throughout the storage period. However growth of spoilage yeasts is only detected after 5 weeks of storage and proceeds rapidly thereafter resulting in spoilage.