Investigating the Effect of Different Concentrations of Salicylic Acid and Citric Acid on some Quality Characteristics and Microbial Load of Fresh Cut Watermelon (Citrullus lanatus)

Introduction Watermelon (Citrullus lanatus) (Thunb) is known as a rich source of various vitamins (such as vitamin A) and phytochemical compounds that have high antioxidant activity. Studies show that characteristics such as taste, nutritional value, cost, and convenience in consumption have a direct impact on consumer choice, therefore, in recent decades, the desire to consume and buy cut products has been increasing. Also, due to the large size of the watermelon, to avoid waste, this product can be offered sliced. Cutting causes damage that not only elevates respiration and ethylene production but also accelerates other biochemical reactions, leading to changes in color, taste, aroma, texture, and nutritional quality. Additionally, cutting and removing the natural protective covering of the fruit create conditions that promote an increase in microbial load. In order to increase the natural resistance of fruits and vegetables as well as maintain the sensory and nutritional quality of fresh products, it is recommended to use environmentally friendly technologies such as salicylic acid (SA). Citric acid, like salicylic acid, is considered a safe compound, and as an organic acid, it can be used as an approved food additive. Due to the positive effects of salicylic acid and citric acid on the quality properties of fresh products, no information has been found regarding the post-harvest use of these compounds on sliced watermelon. Therefore, in the present study, our aim was to investigate the effects of citric acid and salicylic acid treatments on sensory properties, quality, microbial load and color changes of cut watermelon fruit during the storage period. Our findings can provide a new strategy for maintaining the quality of sliced watermelon fruit. The microbial load and fruit tissue softening resulting from fruit cutting lead to a decline in quality due to increased fruit respiration and water loss, which are limiting factors for the post-harvest shelf life of cut watermelon fruits. In the present study, the impact of citric acid and salicylic acid on some quality indices of cut watermelon pieces during the storage period was investigated.   Materials and Methods The harvested fruits were transported to the laboratory and their external surface was disinfected with sodium hypochlorite (200 μL.L-1) and the skin of the fruits was removed by a sharp and sterile knife. The harvested fruits were separated from their peels, and then the fruit flesh was cut into 4 cm by 4 cm pieces. The pieces were immersed in salicylic acid solutions (1 or 2 mM) and citric acid solutions (0.5 and 1 mM) for 2 minutes. Immersion in water was also introduced as a control. Subsequently, the fruits were packaged in polyethylene containers with cellophane coating and stored for 14 days at a temperature of 4 degrees Celsius.       Results and Discussion Based on the results of variance analysis, immersion of freshly cut watermelon fruits in different concentrations of citric acid and salicylic acid had a significant effect on the firmness of the fruit tissue at the 1% level. The interaction effects of measurement time and immersion in solutions were significant at the 1% probability level. The highest amount of tissue stiffness was related to the treatments of 1 mM citric acid (4.58 newtons) and 2 mM salicylic acid (4.69 newtons), and the lowest value was obtained from the control samples (3.54 newtons). The highest weight loss was related to the control samples and the lowest amount was obtained from 1 mM citric acid and 2 mM salicylic acid. During the maintenance period, the highest and lowest weight loss was obtained from the control and 2 mM salicylic acid treatments, respectively. The highest amount of soluble solids was obtained from the control treatment and the lowest amount was obtained from the 2 mM salicylic acid treatment. The highest amount of microbial load was obtained from the control samples (6.11), and the lowest amount was obtained from the 2 mM salicylic acid treatment (4.02), followed by the 1 mM salicylic acid treatment (4.17). Also, with the passage of storage time, the amount of microbial load increased significantly. This study was conducted with the aim of investigating the effect of salicylic acid and citric acid on the quality and microbial characteristics of cut watermelon. This experiment included the use of different concentrations of salicylic acid and citric acid to evaluate their effect on quality parameters and microbial load in watermelon slices. Quality characteristics, including color, firmness, sweetness and overall visual appeal, were measured using standard methods. In addition, the microbial load, including both bacterial and fungal populations, was determined to evaluate the antimicrobial potential of the applied acids. The results showed significant effects of salicylic acid and citric acid on increasing some quality traits and reducing microbial contamination in watermelon slices. This research provides valuable insights into the use of salicylic acid and citric acid as potential agents to improve the quality and safety of cut watermelon. The results indicated that salicylic acid at both concentrations (1 or 2 mM) and citric acid at 1 mM led to a significant reduction in microbial load and weight loss. Moreover, the mentioned treatments restrained the increase in soluble solids content resulting from the post-harvest handling of cut watermelon fruits, contributing to the preservation of fruit tissue strength. The results of evaluating color indices and organoleptic properties indicated that salicylic acid treatments at both concentrations (1 or 2 mM) and citric acid at 1 mM preserved the fruit quality to the best extent. In general, among the treatments used in this experiment, 2 mM salicylic acid yielded the best results in preserving the quality of cut watermelon fruits during cold storage. Subsequently, treatments with 1 mM salicylic acid and 1 mM citric acid are recommended.   Conclusions The results indicated that salicylic acid at both concentrations (1 or 2 mM) and citric acid at 1 mM led to a significant reduction in microbial load and weight loss. Moreover, the mentioned treatments restrained the increase in soluble solids content resulting from the post-harvest handling of cut watermelon fruits, contributing to the preservation of fruit tissue strength. The results of evaluating color indices and organoleptic properties indicated that salicylic acid treatments at both concentrations (1 or 2 mM) and citric acid at 1 mM preserved the fruit quality to the best extent. In general, among the treatments used in this experiment, 2 mM salicylic acid yielded the best results in preserving the quality of cut watermelon fruits during cold storage. Subsequently, treatments with 1 mM salicylic acid and 1 mM citric acid are recommended.