The response of some vegetables to vacuum impregnation

Abstract

The response of several vegetables (beetroot, carrot, eggplant, zucchini, mushroom and oyster mushroom) to vacuum impregnation (VI) treatments, in terms of sample volume deformation and impregnation levels, was studied. The coupling of the hydrodynamic mechanism (HDM), which implies sample impregnation, with the sample volume deformation–relaxation phenomena (DRP) was analysed at the end of the two typical VI steps, the former at vacuum pressure and the second at atmospheric pressure. The length of these process steps ranged between 5 and 15 min. During the vacuum period, all vegetables lost native liquid since a negative impregnation level, from −3% for mushroom to −18% for carrot, was detected. Likewise, samples showed volume deformation ranging from −1.8% for eggplant to 7% for oyster mushroom. At the end of the treatment, total impregnation varied from 7% for beetroot to 34% for mushroom, while volume deformations ranged between −37% for eggplant and 10% for oyster mushroom. Diced zucchini samples showed VI responses different to sliced samples, in agreement with its heterogeneous tissue structure. For each vegetable product, the effective porosity to HDM action (ε e) was calculated and compared with the initial gas volume fraction (ε), determined through sample densities. Except in beetroot and zucchini, the ε e values were greater than ε values due to the internal free liquid substitution by the external solution. Beetroot and eggplant showed, respectively, the lowest and the highest values of ε e (4.47 and 64%). Microstructural changes, induced by VI, were analysed by Cryo-SEM. Industrial relevance Little work exists on vacuum impregnation of vegetables. Such impregnated vegetables could offer an interesting array of new product lines. The authors have studied the coupling of hydrodynamic mechanisms and the deformation–relaxation phenomena of viscoelastic product matrices in some vegetables to determine the feasibility of applying vacuum impregnation to modify vegetable composition and structure. Vacuum impregnation made it possible to effectively fill intercellular spaces of even non-porous vegetables such as carrots with an impregnation liquid.