Quality properties of set yoghurt as affected by types of Acacia gum and storage period

Abstract

The effect of types of Acacia gums added at the level of 0.3%, and storage periods on quality properties of set yoghurt prepared using cow milk was investigated. The types of Acacia gums included Acacia senegal, Acacia leata, Acacia polyacantha, Acacia seyal var. seyal and Acacia seyal var. fistula were used for stabilizing the texture of the set yoghurt. Starter culture (5%) was added after the pasteurization of the milk at 85°C for 10 minutes. The milk was incubated at 45°C for four hours, thereafter cooled to a temperature of 6±2°C,and stored for 20 days. The physicochemical, rheological properties, minerals content and acceptability were determined at storage intervals of 0, 4, 8, 12, 16, and 20 days. The physicochemical determinations revealed that, all test samples recorded decrease in pH-values that occurred during all storage intervals where the highest value (4.18) was recorded by the sample treated with Acacia leata and the lowest (3.62) was recorded by the control. The addition of acacia gums has significantly (P? 0.05) lower levels of the titratable acidity in all samples of set yoghurt compared to the control. The test sample of set yoghurt treated with containing Acacia leata has recorded the highest (19.25%) for the total solids, protein (4.22%) and ash (1.21%). The highest fat content (3.41%) was obtained by yoghurt containing Acacia senegal. The viscosity of the control sample expressed the lowest value being (2116 cps) while the highest (2505 cps) was obtained by set yoghurt treated with Acacia senegal. The results of the rheological analyses indicated that, the control sample has provided the highest wheying-off and synersis (0.92 ml, 2.98%) while the lowest (0.38 ml, 1.85% respectively) were given by the gum of the acacia leata. However, little increase in the volume of the wheying-off and synersis were obtained in all test samples as the storage period progressed. All types of Acacia gums have invariably caused an increase in phosphorus content of all samples until the day 8 of storage period. The highest value (122.48 mg/100g) being for the samples treated with the Acacia gum, while the lowest value (94.40 mg/100g) for the control. The senegal gum had imposed similar increasing trends of calcium and magnesium recording the highest values (192.30 and 194.73 mg/100g respectively), while the control gave the lowest values (141.73 and 129.74 mg/100g respectively). Generally, the contents of the minerals determined in yoghurt have decreased during storage periods. For acceptability, the use of leata gum a manifested the best appearance (5.50), texture (5.77), flavour (5.97), and acceptability (5.78), followed by Acacia senegal (5.45 5.73, 5.95 and 5.74), Acacia polyacantha (5.28, 5.57, 5.75 and 5.75), Acacia seyal var. seyal (4.98, 5.50, 4.43and 5.31), Acacia seyal var.fistula (4.93, 5.46, 45.40, 5.30) and finally the control (3.98, 4.86, 5.00 and 4.67). The storage period affected the acceptability of set yoghurt in terms of appearance, texture and overall acceptability in that the three sensory parameters gave the best qualities at the beginning of the storage period and the worst at the end. Regarding the flavour, remarkable development was observed after 12 days from the beginning of the storage period by recording the highest value (5.94), thereafter, reached the lowest level (4.83) at the end of the storage period