A.M. Matser

Combined high-pressure and thermal treatments for processing of tomato puree: evaluation of microbial inactivation and quality parameters

The effects of combined high-pressure thermal treatments on consistency, viscosity, colour, lycopene content, enzyme activity and micro-organisms were determined, and compared to conventional pasteurisation and sterilisation processes of tomato puree. High-pressure processing at ambient temperature (HPP) improved the colour and viscosity compared to heat pasteurisation, while the water binding capacity and lycopene content were unaffected by HPP. Products treated at 700 MPa, 20 °C resulted in inactivation of the natural flora to a level below the detection limit. After pressure treatment and during chilled storage a increase in viscosity was observed. HPP caused partial inactivation of polygalacturonase (70%), but activation of pectin methylesterase. After high-pressure sterilisation treatments combined with elevated starting temperatures (≥80 °C, HPS, one or two pulses) an ambient stable product was obtained. HPS (one pulse, 700 MPa, 30 s, 90 °C) reduced B. stearothermophilus spore contamination level in inoculated meatballs in tomato puree with at least 4.5 log units. HPS resulted in more than 99% inactivation of polygalacturonase and pectin methylesterase. HPS resulted in a lower viscosity compared to conventional sterilised samples, whereas the water binding capacity was improved. Colour appreciation was improved and lycopene content was retained compared to a 40% loss after conventional sterilisation.

Quality and storage-stability of high-pressure preserved green beans

The effects of high-pressure technology on naturally present microbial flora, texture, color, ascorbic acid content and peroxidase activity of whole green beans were evaluated and compared to conventional preservation techniques. High-pressure processing (HPP) and two-pulse pressure treatment (pHPP) for achieving, respectively, pasteurization and sterilization on the product quality of intact green beans were under investigation. After 1-month storage there was no significant outgrowth of microorganisms for both HPP and pHPP. Both HPP and pHPP treatment showed a large retention of firmness and ascorbic acid for green beans compared to conventional preservation methods. pHPP resulted in marked changes in greenness. During storage after pressurization or conventional processing significant changes in ascorbic acid and color occurred. pHPP resulted in more than 99% inactivation of peroxidase, whereas after HPP 76% of the initial peroxidase activity remained and further decreased during storage.

Effect of high pressure on colour and texture of fish

Abstract The effect of high pressure on the colour of fish species was investigated to evaluate the range of high pressure treatments that did not influence the appearance of the fish. High pressure treatments during 5 minutes higher than 150-200MPa resulted in a cooked appearance of pollack (Pollachius virens), mackerel (Scomber scombrus), tuna (Thunnus thynnus), cod (Gadus morhua), salmon trout (Salmon trutta), carp (Cyprinus carpio), plaice (Pleuronectus platessa) and anglerfish (Lophius piscatorius). Only octopus (Octopus vulgaris) retained a raw appearance till 400-800MPa. The influence of high pressure on the texture of cod (Gadus morhua) was evaluated after frozen storage up to 6 months. The hardness increased as a result of high pressure processing at 200 and 400 MPa. During storage only minor changes of the hardness occurred. Even the non-pressurised sample showed only a small increase in the hardness during storage.

Effects of high isostatic pressure on mushrooms

Abstract The possibility of high isostatic pressure treatment as an alternative to conventional blanching of mushrooms was investigated. An important condition for successful application is a sufficient inactivation of the browning enzyme polyphenoloxidase. The experiments showed that this enzyme is very pressure-resistant in mushrooms. The activity increased after pressure treatments at 600 MPa. Higher pressures up to 950 MPa are necessary for inactivation. The effects on texture, colour and yield of mushrooms were evaluated at these high pressures. If fresh mushrooms are used, pressure treatment results in a dark brown colour. However, if mushrooms are evacuated before pressure treatment, the colour expressed in L, a* and b* values is comparable with conventionally blanched mushrooms. The stiffness of the pressurised mushrooms is larger than that of blanched mushrooms. The yield is roughly the same for the conventional blanched mushrooms and the pressurised mushrooms.

High pressure-temperature processing as an alternative for preserving basil

In this study the effect of sterilisation by high pressure (HP) on the quality of basil was compared to conventional processing techniques. By means of freezing, or blanching followed by drying, microbial reduction of spores was maximal one-log. Pulsed HP-temperature treatment yielded a reduction of the natural occurring flora of more than 3 log CFU/g product. The essential oil content in basil consisted mainly of methylchavicol and linalool. HP sterilisation preserved these compounds in contrast to the equivalent conventional heat sterilisation process, which reduced the linalool and methylchavicol content to respectively 21 and 3% of the original amount. Drying and freezing also resulted in a significant reduction of the essential oil content.

Buffer selection for HP treatment of biomaterials and its consequences for enzyme inactivation studies

Biochemical systems are best studied under buffered conditions and it is, therefore, necessary to have good knowledge on the pressure-induced changes in buffer pH. For experiments conducted at a set temperature, it is possible to use a relatively stable pressure buffer. However, when experiments have to be performed in a relatively large window of both pressure and temperature, it is not possible to find a buffer that is both pressure and temperature independent. An example of such an experiment is the measurement of pressure and temperature stability of enzymes. Here, we present the pressure dependence of pH of several buffers. We show that the pH of MES buffer pH 6.0 in a P–T plane from 0.1 to 1000 MPa and 10–90 °C has a pH range from 5.5 to 6.5. The effect of pH changes on enzyme inactivation is illustrated by an inactivation experiment with the β-glycosidase from the hyperthermophile Pyrococcus furiosus. Within the measured range, a decrease in pH of 0.5 units causes the enzyme inactivation constant to increase by an additional factor 2–3.

High pressure pasteurization and sterilisation of tomato puree

The effects of high-pressure (HP) pasteurisation and sterilisation on the microbial safety and quality of tomato puree were compared to conventional heat treatments. HP pasteurisation was achieved by a HP treatment at ambient temperature and 700 MPa. This treatment also reduced the naturally present spores to a level below the detection limit. HP sterilisation was reached by a two pulse treatment at 700 MPa and a starting temperature of 90iaC. HP pasteurisation and conventional pasteurisation resulted in an activation of pectin methylesterase and a reduction of the polygalacturonase activity. Both enzymes were inactivated after conventional and HP sterilisation. HP treatments resulted in an equal or improved retention of the red colour compared to the heat treatments. Heat with or without pressure induced a reduction in the viscosity, while HP pasteurisation at ambient temperature increased the viscosity.

High pressure effects on fruits and vegetables

The chapter provides an overview on different high pressure based treatments (high pressure pasteurization, blanching, pressure-assisted thermal processing, pressure-shift freezing and thawing) available for the preservation of fruits and vegetable products and extending their shelf life. Pressure treatment can be used for product modification through pressure gelatinization of starch and pressure denaturation of proteins. Key pressure–thermal treatment effects on vitamin, enzymes, flavor, color, and texture of fruits and vegetable are discussed.