Catalytic infrared radiation improving peeling effect and quality of cherry tomatoes

Tomatoes are regarded as an important agricultural commodity in the world due to their essential role in the diet as a vital source of vitamins (C and E), phenolics and flavonoids. Tomatoes is available in various products including juice, purees, pastes, ketchup, sauces, salsas, soups and canned tomatoes. However, tomato skin, which consists of a thin cuticle layer, a single layer of epidermal cells, and two to four layers of hypodermal cells, is very tough and undesirable to consumers. Therefore, during the processing of tomato products, peeling is the first and particularly important unit operation. The traditional peeling methods including mechanical, chemical and hot soaking peeling have been applied for cherry tomatoes. These peeling methods with high energy and water consumption, serious product salinity and wastewater disposal problems are not environmental friendly. Therefore, sustainable and non-chemical peeling methods have been desired by tomato processors for a long time to reduce water, energy and chemical reagent consumption, meanwhile obtain high quality peeled products. Recently, infrared and ultrasound technologies had been studied as alternatives to food processing technologies. Thus, catalytic infrared radiation and ultrasound technologies were chosen for peeling cherry tomatoes in this study. The effects of different catalytic infrared radiation parameters and ultrasonic durations on the peeling effect of cherry tomatoes were investigated. In addition, the comparison of traditional peeling methods (hot water and lye peeling) on the peeling effect, lycopene content, flesh color and texture of cherry tomatoes was also carried out. Microstructural changes in tomato epidermal tissues under catalytic infrared radiation were also compared with those of control, hot water, hot lye and ultrasonic treated samples. The results showed that the optimal catalytic infrared radiation parameters were as follows: radiation distance of 30 cm and duration of 3 min. In addition, as the peeling times of the hot water, lye and ultrasonic peeling methods increased, the peeling effects of cherry tomatoes enhanced. Compared to the hot water, lye and ultrasonic peeling methods, the peeling effect of catalytic infrared radiation significantly improved (P<0.05), and the infrared peeling significantly reduced the flesh loss of cherry tomatoes (P<0.05); Moreover, the infrared peeling well maintained the integrity of the peeled tomato shape. The hardness of catalytic infrared peeled tomatoes under the radiation distance of 30 cm was significantly (P<0.05) higher than that of hot water, lye and ultrasonic peeled tomatoes, respectively. The results of scanning electron microscopy showed that microstructural destruction in tomato epidermal tissues by ultrasonic treatment was the most severe; so many small holes could be clearly observed in the picture of ultrasonic treated tomatoes. The ratio of skin to flesh of catalytic infrared peeled tomatoes was significantly (P<0.05) lower than that of hot water, lye and ultrasonic peeled tomatoes, suggesting that catalytic infrared peeling method had a better peeling effect than other peeling methods. Therefore, the results of this research provided scientific evidence of the benefits of catalytic infrared peeling in comparison to the hot water, lye and ultrasonic peeling methods and demonstrated the potential of catalytic infrared peeling as an alternative to conventional cherry tomato peeling methods.