Vacuum impregnation of firming agents in red raspberries

Red raspberries are a delicate and highly perishable fruit with a fragile pulp tissue. In this study we used vacuum impregnation (VI) methods to incorporate pectin and calcium chloride into whole red raspberries to improve their firmness. Specifically, we impregnated low methoxyl pectin (LMP) at 10 g of pectin kg^-1 of solution and calcium chloride (CaCl₂ ·2H₂ O) at 30 g calcium kg^-1 of pectin, and on the other side pectin methylesterase (PME) at 10 g of enzyme kg^-1 of solution, and (CaCl₂ ·2H₂ O) at 10 g of calcium kg^-1 of solution, into whole red raspberries. We tested three vacuum levels 33.9, 50.8, and 67.8 kPa, three vacuum impregnation times 2, 7, and 15 min, and two temperatures, 20 and 40 °C, during VI treatment. Maximum force (F_M ) and gradient (G_C3 ) were evaluated to assess raspberry firmness.

A vacuum level of 50.8 kPa, processing time of 7 min, and a LMP and calcium infusion at 20 °C resulted in the firmest fruit compared to the other treatments. At these VI treatment conditions, F_M and G_C3 values of red raspberries obtained were 28 N, and 8.4 N mm^-1 , respectively.

The optimal VI conditions identified in this study can be used to improve firmness and structural integrity of red raspberries by infusion of LMP and calcium. Findings on vacuum-impregnated red raspberries may be used to develop dehydrofrozen berries for incorporation into bakery and dairy products. © 2018 Society of Chemical Industry.