Attempts to increase storage stability of strawberry yoghurt by combination treatments

The aim of the experiments was to establish whether the microbiological stability of strawberry yoghurt might be improved by decreasing the microbial load of the fruit. The effect of heat treatment, freezing, irradiation and various combinations of these treatments upon cell count and sensory quality was investigated. It was established that none of the individual treatments was entirely satisfactory. Surfacial heat treatment at 550C, freezing and irradiation with 0.4-0.6 Mrad substantially increased the storage life of strawberries or that of the yoghurt prepared with this fruit; when compared to yoghurt made with frozen strawberries by the dairy factory, the increase was 2.5 fold at 150C and 3.5 fold at 20C. The relative increase of storage life was lower at lower yeast-cell counts. The strawberries irradiated with doses above 0.2 Mrad showed aroma and flavour changes immediately upon treatment. This effect, however, was eliminated after some days. The yoghurt made with strawberries given a radiation treatment of 0.57 Mrad did not differ organoleptically from the yoghurt made with untreated strawberries. In the knowledge of the survival rate of yeasts after irradiation the D10 values were established. These were found in the dose range between 0.043 and 0.087 Mrad. It was established that the applied heat treatment, freezing and irradiation at these dose levels and at 103-104 cells per gram were not sufficient from the point of view of microbiological stability. (F.J.)