Setting a safe target internal atmosphere for starch-based coated fruits and vegetables

Abstract

Fruit surface coatings reduce gas exchange and modify the internal atmosphere composition. However, to set a safe internal atmosphere target, an optimum starch concentration must be determined for each fruit. To avoid the risk of anaerobiosis, surface coatings must be designed considering specificities in the structure and physiology of the product to be coated, properties of the coating solution, and coating-fruit surface interaction. In this study, it was developed a method for estimating the effective proportion of a starch-based coated surface using digital image processing. The starch-coated areas were stained with iodine solution and quantified using coated/uncoated region color image segmentation. To understand the need for surfactant and the optimal temperature for adding surfactant, 3% cassava starch coatings were prepared with and without Tween 40® as a surfactant. Tween 40® was added into starch matrix coating at 18 and 60 °C. These coatings were applied to bananas, mandarin oranges, and bell peppers. The coatings without and with Tween 40® (added at a warm coating) showed weak adhesion to the fruit peels, especially those with high cutin and wax levels (mandarin and pepper). However, adding Tween 40® to a cooled coating enhanced its adhesion regardless of the structure of the fruit peel surfaces. Thus, these results show for the first time that the temperature for adding the surfactant markedly affects the coated percentage area of fruits. Following this, cassava starch coatings at 1.5, 2.0, 2.5, and 3.0% with Tween 40® added at 18 °C were tested on banana and papaya. The internal levels of O 2 and CO 2 were assessed to target safe, coated fruit internal atmosphere. Increasing the starch coating content increased the fruit's coated area, resulting in a decrease in O 2 and increase in CO 2 internal levels. The internal atmosphere build-up with the highest starch content (3.0%) and Tween 40® for papaya was O 2 (5.8 kPa) and CO 2 (6.6 kPa), which is suitable for postharvest papaya storage. However, for bananas, this level of starch coating resulted in very low O 2 (1.9 kPa) and high CO 2 levels (48.6 kPa). These results demonstrate that the internal atmosphere build-up for coated fruits with similar starch matrix coating contents is fruit-specific. • An approach for estimating fruit's starch-coated area was developed. • Image segmentation of stained starch-coated fruit was used to assess the coated area. • Cooling starch matrix before adding surfactant enhances fruit coated surface area. • Internal atmosphere build-up at similar starch coating content depends on fruit type.