Mucilage of Pereskia aculeata Miller as Cryoprotectant of Frozen Strawberries: Preservation of Bioactive Compounds and Microstructure

Abstract

Whole strawberries were frozen after being impregnated with solutions of ora-pro-nóbis mucilage (MOPN) or pectin at concentrations of 0%, 0.5%, and 1.0% (w/v), CaCl₂ at concentrations of 0.5% and 1.0% (w/v), and sucrose at concentrations of 5.0% and 10% (w/v). The strawberries were vacuum impregnated for 2 h with a controlled pressure of 64.3 kPa, and the rheological behavior of the solutions was evaluated before and after vacuum impregnation. In the strawberries, the texture and cell fluid losses, total soluble solids, pH, titratable acidity, and bioactive compounds were analyzed before and after processing. The microstructures of the frozen samples were evaluated by low-vacuum electron microscopy. The impregnation process influenced the rheological parameters, and the solutions showed Newtonian or non-Newtonian (n < 1) behavior. The treatment with MOPN efficiently reduced the cell fluid loss and texture loss in the thawed strawberries. Compared with pectin, these samples more effectively preserved the anthocyanin content and showed less reduction and oxidation of β-carotene from the β-carotene/linoleic acid system. For the treatments with MOPN or pectin combined with sucrose and CaCl₂, the values for the milligrams ascorbic acid/100 g of strawberry and 1,1-diphenyl-2-picrylhydrazyl (DPPH) were statistically similar to those of the fresh fruit. Microstructural analysis by low-vacuum SEM confirmed that the vacuum impregnation process was able to incorporate the solutions inside the microstructure of whole strawberries. Compared to pectin solutions, MOPN solutions have potential for use as cryoprotectants of fruits.