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Advanced food processing methods that accomplish inactivation of microorganisms but minimize adverse thermal exposure are of great interest to the food industry. High pressure (HP) and pulsed electric field (PEF) processing are commercially applied to produce high quality fruit and vegetable products in the United States, Europe, and Japan. Both microbial and plant cell membranes are significantly altered following exposure to heat, HP, or PEF. Our research group sought to quantify the degree of damage to plant cell membranes that occurs as a result of exposure to heat, HP, or PEF, using the same analytical methods. In order to evaluate whether new advanced processing methods are superior to traditional thermal processing methods, it is necessary to compare them. In this review, we describe the existing state of knowledge related to effects of heat, HP, and PEF on both microbial and plant cells. The importance and relevance of compartmentalization in plant cells as it relates to fruit and vegetable quality is described and various methods for quantification of plant cell membrane integrity are discussed. These include electrolyte leakage, cell viability, and proton nuclear magnetic resonance ( 1 H-NMR).

The integrity of onion cells and its impact on tissue texture after high pressure and thermal processing was studied. The contribution of cell membranes and the pectic component of cell walls on the texture properties of onion tissue were analyzed. Neutral red (NR) staining of onion parenchyma cell vacuoles was used for the evaluation of cell membrane integrity and microscopic image analysis was used for its quantification. The content of methanol in tissue as a result of pectin methylesterase activity was used to evaluate the pectin component of the middle lamella and cell walls and the hardening effect on the tissue after processing. High pressure treatments consisted of 5-min holding times at 50, 100, 200, 300, or 600 MPa. Thermal treatments consisted of 30-min water bath exposure to 40, 50, 60, 70, or 90 °C. In the high pressure treatments, loss of membrane integrity commenced at 200 MPa and total loss of membrane integrity occurred at 300 MPa and above. In the thermal treatments, membrane integrity was lost between 50 and 60 °C. The texture of onions was influenced by the state of the membranes and texture profiles were abruptly modified once membrane integrity was lost. Hardening of the tissue corresponded with pressure and temperature PME activation and occurred after membrane integrity loss.The texture of vegetables is an important quality attribute that affects consumer preference. Loss of textural integrity also indicates that other biochemical reactions that affect color, flavor, and nutrient content may occur more rapidly. In this study, we analyzed changes in the texture of onions after preservation with heat and high pressure.

A cell viability assessment method based computer vision analysis of the uptake of neutral red dye was used to quantify cell membrane integrity in raw and processed parenchyma cells of onion tissues. The presence of stained vacuoles was used as an indicator of tonoplast membrane integrity and photomicrographs were acquired for microscopic image analysis and cell integrity quantification. Two different image analysis methods, involving the analysis of the saturation and green components of RGB (red, green, blue) images, were compared to the conventional cell count method. Use of the saturation component of RGB images allowed for the visualization and quantification of viable and inviable cells as well as extracellular air spaces. The combination of neutral red uptake, as visualization by light field microscopy, and saturation image analysis, allowed for quantitative determination of the effects of high pressure processing on onion cell integrity.Preservation of vegetable tissues may involve heating or other methods that result in the loss of tissue integrity and potentially quality deterioration. In this study, we stained unprocessed and processed onion tissues with neutral red dye and then used a microscope and a computer imaging program to quantify how many cells were intact or ruptured.

Proton nuclear magnetic resonance (¹H-NMR) relaxometry was used to study the effects of high pressure and thermal processing on membrane permeability and cell compartmentalization, important components of plant tissue texture. High pressure treated onions were subjected to pressure levels from 20 to 200 MPa at 5 min hold time at initial temperatures of 5 and 20 °C. Thermally treated onions were exposed for 30 min at temperatures from 40 to 90 °C. Loss of membrane integrity was clearly shown by changes in transverse relaxation time (T(2)) of water at temperatures of 60 °C and above. Destabilization effects on membranes exposed to high pressure were observed at 200 MPa as indicated by T(2) measurements and cryo-scanning electron microscopy (Cryo-SEM). T(2) relaxation successfully discriminated different degrees of membrane damage based on the T(2) shift of the vacuolar component. Analyses of the average water self-diffusion coefficient indicated less restricted diffusion after membrane rupture occurred in cases of severe thermal treatments. Milder processing treatments yielded lower average diffusion coefficients than the controls. ¹H-NMR proved to be an effective method for quantification of cell membrane damage in onions and allowed for the comparison of different food processes based on their impact on tissue integrity.