Effect of operating parameters on the physical and flow properties of spray dried low-phenylalanine whole camel milk: A response surface methodology approach

Camel milk has many nutritional and therapeutic properties. Additionally, due to the absence of beta-lacto globulin, it does not cause allergic reactions. Phenylketonuria is a hereditary metabolic disease that unable to metabolize the amino acid phenylalanine. The most severe clinical complication of this disease is irreversible mental retardation. Nowadays, using food sources without phenylalanine is the only way to cure. The aim of this research was to produce spray dried low-phenylalanine whole camel milk in a laboratory scale with optimum flow ability to be used in the diet of these patients.

Fresh camel milk was prepared from dromedary camels (Golestan, Iran) and its amino acid phenylalanine (Phe) content was measured by HPLC method. Then, whole camel milk was hydrolyzed by using two types of protease (Aspergillus oryzae and papain) in three different ways. The optimum method for removing the amino acid phenylalanine from whole camel milk was determined by calculating and comparing the analysis of variance of the removal efficiency of phenylalanine. This low-Phe sample was dried in a spray dryer with three variables parameters of inlet temperature in three levels 120, 140 and 160°C and feed flow rate in three levels 400, 600 and 800 mL/min and air flow rate in three levels 15, 20 and 25 m3/h. The experimental design in the drying stage was response surface method (RSM) and Box-Benken experimental design.

The results showed that by using the combined method of two enzymes of Aspergillus oryzae and papain with equal amount and using activated carbon of 0.9 gr/gr protein, a 99.33% decrease in the amino acid content of phenylalanine was observed in whole camel milk. The sample of hydrolyzed camel milk with the lowest amount of Phe was used in a spray dryer at three temperatures from 120 to 160°C and three feed flow rates from 400 to 800 mL/min and three air flow rates from 15 to 25 m3/h. The results showed that the parameters used in spray dryer had significant effect on the flow ability characteristics of low-Phe whole camel milk powder produced in the research including bulk and tapped density, filling and emptying repose angles, dispersibility, wettability, cohesiveness, Particle size distribution and color parameters.

Analysis of variance (ANOVA) of the results showed that the effect of inlet temperature parameters on bulk and tapped density of low-Phe camel milk powder was significant (P>0.05). The highest bulk and tapped density related to the sample was dried at 120°C and the feed flow rate of 600 mL/min and the air flow rate of 25 m3/h (520 and 630 kg/m3, respectively) And the lowest of them was 172 and 340 kg/m3 for the sample was dried at 140°C, feed flow rate of 800 mL/min and air flow rate of 15 m3/h. The Carr Index of low-Phe camel milk powder showed that the sample was dried at 160°C and a feed flow rate of 600 mL/min and an air flow rate of 20 m3/h had very good flowability. The same sample according to Hassner's Ratio was very good powder. The angle of repose of filling was directly related to the cohesiveness of powders, so that the angle of repose of filling increased with the increase of cohesiveness. The falling repose angle was also inversely related to the flowability of the samples, and by reducing the repose angle, their flowability was done more easily. Increasing the inlet temperature of the spray dryer also increased the size of powder particles and the amount of a* and b*, while the amount of l* of the sample of whole camel milk powder decreased.