Top Editors ... Pressure and temperature bring texture improvements

Title: Pressure and temperature bring texture improvements
Date: 21/07/2008
Autor: LFT, Katholieke Universiteit Leuven, Belgium

The texture of carrots is improved with the use of combined high pressure/high temperature treatments compared with traditional high temperature treatment alone, according to the latest research findings from the EC, Framework 6, Priority 5 ‘Food Quality & Safety, Integrated Project NovelQ’

High pressure technology has the potential to produce foods that meet many of the consumer demands for fresh tasting foods, free from, additives, microbiologically safe and with an extended shelf life1. The technology uses pressures (in the range of 400-600 MPa / 0-50°C for pasteurisation and in the range of 500-800 MPa / 60-90°C for sterilisation) to inactivate enzymes and microorganisms. Pressure is transmitted uniformly and instantaneously throughout the food, which allows very homogeneous products to be obtained with minimal negative effects on food quality (ie colour, flavour and nutritional value). Currently, high pressure processing is successfully
applied on a commercial scale for pasteurisation of a whole range of food products, e.g. fruit juices, guacamole, oysters and ham.
Given the technical progress being made in high pressure equipment, commercial high pressure sterilisation is feasible in the near future. This process would involve both elevated pressure and elevated temperature contributing to sterilisation by inactivating spores and enzymes. To explore the potential of high pressure/high temperature (HP/HT) processing of plant tissue, the effect of combined HP/HT treatments on the texture of carrots was compared with that of high temperature (HT) treated samples2.
Carrot disks (~1 cm3) were on the one hand subjected to 80°C and 100°C in a water bath and on the other hand to 80°C in combination with 600 MPa in multivessel high pressure equipment (Resato, Roden, The Netherlands).
Subsequently, these carrots were investigated at macroscopic, mesoscopic and molecular level.

Macroscopic
The hardness of the processed carrots was evaluated by a compression test with a TA-XT2i Texture Analyzer (Stable Micro Systems, Surrey, U.K.) (Figure 1). In each case, there was a rapid loss in the initial hardness due to membrane damage and turgor pressure loss. The hardness of the thermally treated carrots decreased further with increasing treatment time, probably due to beta-eliminative degradation of pectin. In contrast, the high pressure/high temperature treated carrots did not undergo further softening, indicating that the beta-elimination reaction of pectin is inhibited with combined high pressure/high temperature treatments.

Mesoscopic
The carrot tissue was examined with an Olympus BX-41 light microscope (Olympus, Optical Co. Ltd, Tokyo, Japan).
Raw carrot tissue showed well defined, conspicuously well stained cell walls. The intensity of the cell wall staining decreased progressively with increasing thermal impact accompanied by increased cell wall thickening. These observations are probably due to heat induced solubilisation of the intercellular cementing pectin facilitating cell wall loosening. In contrast, high pressure/high temperature treated tissue showed a close resemblance to the raw tissue, indicating that pectin solubilisation was limited.