Effect of Inlet-Air Temperature on Physico-Chemical and Sensory Properties of Spray-Dried Soy Milk

Abstract

Samples of spray-dried soy milk powder were produced at various spray-dryer inletair temperatures and characterized. Soybean seed (Glycine max TAX 1448 – 2E Var.) was sorted, boiled for 40 min, manually dehulled, wet milled using plate mill and sieved with muslin cloth to obtain water soluble extract (soy milk). The soy milk was divided into two portions (samples A and B) and spray-dried using co-current spray dryer at a constant feed rate (20.5 ml/sec) but at air-inlet temperatures of 204oC and 260oC, respectively. Preliminary investigation carried out on this study showed that samples produced at air inlet temperatures below 200oC exhibited wet and agglomerated particles. The recovered powdered samples were analyzed for proximate composition, pH, available lysine, total solids, pack bulk density, viscosity, solubility and wettability at different reconstituting water temperatures, and sensory properties. Results showed that 38.60% and 45.55% yield (soy milk powder) were achieved at the end of the process for samples A and B respectively. The samples showed no significant differences (P ≥ 0.05) in some of these evaluated parameters such as fat, ash and pH. Soy milk powder showed high protein content (62.05±0.23%), fat (19.92±0.08%), ash (1.41±0.02 %) and available lysine (5.02±0.29%), but low carbohydrate content (12.85±0.01 %) and moisture (3.66±0.23%). The physical properties showed that the mean total solid of the samples was 10.33±0.33%, pack bulk density (0.57±0.00 g/ml), while the mean viscosity was 47 mpas. The sample spray-dried at 204oC had solubilities of 48% and 78% at reconstituting water temperatures of 40oC and 80oC, respectively while the sample produced at 260°C showed lower solubility of 38.46% and 45.01% when temperature of reconstitution were 40^oC and 60^oC, respectively. However, the sample produced at 260^oC exhibited decreased solubility when the reconstituting water temperature was raised above 60oC. Its solubility was 40.39% at reconstituting water temperature of 70^oC which further decreased to 38% at 80oC. The wettability of the samples steadily decreased as the reconstituting water temperature increased from 40 to 80^oC. The wettability of the sample spray-dried at 204oC decreased from 36 to 22 sec, while that of sample spray-dried at 260^oC decreased from 29 to 18 sec. Sensory scores showed that the sample spray-dried at 204^oC was preferred to the sample spray-dried at 260^oC.

Keywords: Soy milk, spray-drying, proximate, physical, sensory