Study on the effect of osmotic pretreatment on the quality of air-dried potato sticks using response surface methodology

Potato is the fourth most important agricultural product after rice, wheat, and corn. Potato produces more dry matter, protein, and minerals per unit area in comparison with other crops. Many of the ingredients in potato are important, due to their beneficial effects on health. Hence, this product is very suitable for human consumption. Osmotic dehydration is used to remove a portion of water from foods such as fruits and vegetables, by immersion in high osmotic solutions such as sugars and salts.  Osmotic dehydration is a relatively simple and economical process that improves the texture and rehydration properties of the products and prevents enzymatic browning by preventing the activity of polyphenol oxidases. In this process, the acid is removed and the sugar is absorbed, thus the composition of the fruits is changed and the taste and total acceptance of the product are improved. Osmotic dehydrating method has been used for drying various products such as carrots, mangoes, pineapples, strawberries, bananas, apples, apricots, and many other fruits. In this research, the effects of osmotic pre-treatment on qualitative and sensory properties of dried potatoes were investigated. In this study, Agria cv. potato samples were used and after washing, were completely peeled and cut into pieces of 3.5 × 0.5 × 0.5 cm. The treatments used in this study included osmotic solution concentration (at 0, 10 and 20% sodium chloride levels), osmotic solution temperature (30, 45 and 60°C), and immersion time in osmotic solution (at three levels 60, 240 and 720 minutes). The proportion of potato sticks to osmotic solution in all experiments was 1:6. After applying osmotic pre-treatment, samples were dried in an oven with a temperature of 60°C and a flow rate of 1.5 m/s to reach a moisture content of 8-10%. The moisture content, rehydration capacity, shrinkage, browning, color indices, and sensory properties (taste, color, shape, strength, and total acceptance) of potato samples were evaluated after the drying process and the response surface methodology (RSM) based on central composite design (CCD) were used to determine the optimal conditions for osmotic pre-treatment. The results of statistical analysis of the data showed that osmotic time and osmotic solution concentration had a significant effect on moisture content of potato slices. By increasing the time and decreasing the concentration of osmotic solution, moisture content of dried potato samples increased. The immersion time and osmotic solution temperature did not have any significant effect on the rehydration of potato slices, but the effect of osmotic solution’s concentration was statistically significant. By increasing the concentration of osmotic solution, the rehydration rate of potato samples decreased. The results showed that the interaction of temperature and concentration of osmotic solution had a significant effect on the degree of shrinkage of potato slices. Osmotic time and osmotic solution concentration had statistically significant effects on the browning index of potato slices, in a way that by increasing the concentration of osmotic solution, the browning index of potato samples decreased initially and then increased. The results of statistical analysis of the color indices showed that the osmotic solution temperature had a significant effect on the brightness (L*) of the potato slices, while the concentration of osmotic solution significantly affected their yellowness (b*) and the temperature and concentration of osmotic solution had a significant effect on the redness (a*) of the potato slices. By increasing the concentration of osmotic solution, the overall change in the color (E) of potato samples at high temperatures of osmotic solution first increased and then decreased. Sensory evaluation results showed that concentration, time, and temperature of osmotic solution had no significant effect on the sensory properties of potato sticks, except for taste. By increasing the temperature and time of immersion in osmotic solution, the taste scores of the samples increased.  According to the results of optimization by the surface response method, the concentration of 2%, temperature of 46° C and dipping time of 173 min was introduced as favorable conditions.