Effect of Maturity on the Quality of Preserved Strawberries

This study investigated the differences in the contents of free sugars, organic acids, water-soluble pectin, anthocyanins and phenolics, water distribution, texture, color difference, microstructure, and flavor composition of preserved strawberries made from Monterrey strawberries harvested at different ripening stages (pink, half red, and full red). The results showed that the major free sugars in both fresh and preserved strawberries were fructose, glucose, and sucrose at total contents ranging from 51.14 to 73.47 g/100 g and 77.20 to 85.85 g/100 g dry mass, respectively. The increase in total sugar content in preserved strawberries was mainly due to the increase in sucrose content. The contents of citric acid, malic acid, and water-soluble pectin were significantly lower in preserved strawberries than in fresh ones at each maturity. Low-field nuclear magnetic resonance (LF-NMR) spectroscopy showed that the relaxation times of bound water, semi-bound water, and free water in fresh strawberries were significantly longer at the full red ripening stage than at the other two ripening stages, and the peak area of semi-bound water, which was the major water state in preserved strawberries, was significantly higher in the sample made from fully red ripe strawberries. Texture analysis showed that the hardness of both fresh and preserved strawberries decreased significantly with increasing maturity, and so did the elasticity of preserved strawberries, while the opposite trend was observed for the adhesiveness. For both fresh and preserved strawberries, the color parameter a* value significantly increased with maturity. For fresh strawberries, the contents of catechin, tannin and quercetin decreased significantly with maturity. Anthocyanins were detected in neither fresh nor preserved strawberries at the pink ripening stage. The contents of phenolics and anthocyanins decreased substantially in preserved strawberries compared with fresh ones. For both fresh and preserved strawberries, the cell area increased with maturity, the cell roundness decreased and the porosity increased, indicating irregular cell morphology and reduced integrity. Electronic nose analysis showed that with increasing ripeness, the response values for nitrogen oxides and methyl-containing compounds increased in both fresh and preserved strawberries, and the difference in volatile composition between them was mainly reflected in sulfur compounds. This study provides a reference for selecting the appropriate ripeness of strawberries in the processing of preserved strawberries.