The effect of osmotic dehydration before drying on the physicochemical and sensorial characteristics of dried onion slices

Introduction: Drying is one of the oldest methods of food and agricultural products preservation, which increases their shelf life by removing a part of the product water content. Long-term storage, minimizing the storage and packaging needs, reducing the transport costs, and producing a variety of products are the most important reasons of food drying. Application of some pre-treatments before drying such as osmotic dehydration is suggested to improve the quality of the dried food products. Osmotic dehydration is a process that used for partially dehydration of fruits and vegetables, by immersing them in a concentrated solution of sugar or salt (Sutar and 2007). Reducing the drying time, limiting the heat damages, improving the textural quality and vitamin retention, flavor enhancement and color stabilization are some advantages of the osmotic pretreatments (Sutar and Sutar 2013). Osmotic dehydration removes water from the fruits and vegetables up to a specific level, which is still high for food preservation, so this process must be followed by further processes in order to lose the most portion of the fruits and vegetables water content. Air drying can be used to remove the remaining water from the osmo-dehydrated product (Shahade et al 2015). Salts and sugars are the common solutes that mostly used for osmotic treatment. The quality characteristics of final product in salt-treated samples are higher than sugar-treated products and the preventing of the undesired reaction products formation are from advantages of salt over other solutes (Shahade et al 2015). Onion is a famous spice that is grown all over the world and consumed in various forms. This vegetable is used as an appetizing, healthful and digestive food because of its high nutritional quality and therapeutic properties. Onion is a product that is consumed throughout the year, but its production is seasonal. This product is a perishable food due to its high moisture content. Some environmental factors such as microorganisms, enzymes, and surface vaporization of moisture content leads to spoilage of onion during the storage (Shahade et al 2015; Sahu et al 2017). Drying is a major processing operation in the food industry that can be used to enhance the storability, improve the quality and minimize packaging and handling costs of onion. Drying of onion with the usual drying method causes undesirable changes in sensorial properties such as texture, color, taste and serious decreases in nutritional value of dried product. Pre-drying treatments, such as osmotic dehydration has been suggested to improve the quality of dried onion. It seems that the osmo-air drying can be an appropriate method to increase the shelf life and improve the quality of dried onion without adversely affecting its sensorial and nutritional value (Sahu et al 2017). The effect of osmotic dehydration process parameters on onion dehydration kinetics and optimization of process conditions has been investigated by some researchers (Alam et al 2013; Hussein et al 2018; Patil et al 2012; Revaskar et al 2014; Shahade et al 2015). But the few studies on the characteristics of dried onion with osmotic pretreatment have been done. The purpose of this study was investigating the effect of osmotic solutions compositions on dehydration amount and quality attributes of osmo-air dried onion slices. Material and methods: Fresh yellow onion was procured from the local market. The onions were peeled carefully and washed and then cut into 3.0±0.1mm thick slices using a manual stainless steel cutter. The osmotic solutions of different compositions were prepared by dissolving required amounts of salt, sugar and citric acid in distilled water. Five solutions of different compositions were used for osmotic dehydration of onions (T2 (10% NaCl), T3 (10% NaCl and 10% sucrose), T4 (1% citric acid), T5 (10% NaCl and 1% citric acid) and T6 (10% NaCl, 10% sucrose, and 1% citric acid). Solution to sample ratio was kept as 2.5:1 in each experiment. The samples were immersed in osmotic solutions for 1 h at room temperature (25 °C). All Samples were weighed after removal from osmotic solutions and dried in a laboratory oven at 65 °C until to reach 6% moisture content (Shahade et al 2015). The moisture content of the fresh and osmotically dehydrated onion slices was measured by oven drying method at 105 °C and the amount of water loss, solid gain and weight reduction of the samples were calculated. The dried onion slices were evaluated in terms of physicochemical (rehydration rate, pH, color, vitamin C and total phenolic compounds) and sensorial (Color and appearance, texture, Taste and flavor and Overall acceptability) properties. The rehydration ratio of dried onion was calculated after flooding 10 g of dried samples in 1000 mL of distilled water at 22±1 °C for 5 h (Patil et al 2012). The pH value was measured by a digital pH meter. The color measurement was done using a colorimeter (TES-135A, Taiwan). In order to determine the total phenolic contents, the Folin–Ciocalteu method was employed and measurements were done at 765 nm with a spectrophotometer (UNICO 2802, China) (Taga et al 1984). The vitamin C content was determined by Indophenol method and measurements were done at 515 nm with a spectrophotometer (Klein and Perry 1982). The sensorial evaluation was carried out by 5-point Hedonic Scale. The dried onion slices were investigated for the sensorial attributes by 20 trained panelists. The panelists were asked to evaluate the samples and score them between 1 (most disliked) to 5 (most liked). The statistical model that used was completely randomized design. The experiments were performed in 3 replications and analysis of variance (ANOVA) of data was conducted using MSTAT-C software. Differences between the means values were also determined using Duncan’s Multiple Range test (p < 0.05). Results and discussion: The results of this research showed that the compositions of osmotic treatments had a significant effect on weight loss, water loss, solid gain, physicochemical characteristics and sensorial properties of dried onion slices. The highest weight loss, water loss and solid gain were related to the pretreated onion by the solution containing three solutes (salt, sucrose and citric acid). The presence of citric acid in the osmotic solutions significantly decreased the pH value of the final product. Although the osmotic pretreatments decreased the L* value, but the dried onion slices pretreated with sucrose and salt solution had the highest b* value. The Millard's browning reactions produced brown pigments in the dried product and the evaluation of the browning index showed that the application of the osmotic pretreatments especially with citric acid containing solutions increased the browning index of the dried onion slices. The least undesirable color changes were observed in onion slices pretreated with salt solution. Also, the osmotic dehydration with T6 treatment (salt, sucrose and citric acid) had the greatest effect on preserving of vitamin C and total phenolic compounds. As compared with control the use of the osmotic pretreatment reduced the rehydration ratio of the product. The most rehydration ratio after control was related to onions treated with citric acid solution (T4). Also, the results showed that the presence of sucrose in the osmotic solution increased the rehydration ratio of dried onion slices.Althoughthecolorscoreofosmoticdriedonionsliceswaslowerthanthecontrol, butthe osmoticpretreatmentsimprovedthetasteandincreasedtheoverallacceptabilityofthedriedproduct. Conclusion: Osmotic pretreatment of onion slices with a multi-component osmotic solution (salt, sucrose and citric acid) caused the optimal results in terms of sensory and physicochemical properties.