The effects of high hydrostatic pressure at subzero temperature on the quality of ready-to-eat cured beef carpaccio.

We compared the application of high hydrostatic pressure (HHP) on unfrozen carpaccio (HHP at 20 °C) and on previously-frozen carpaccio (HHP at -30 °C). HHP at 20 °C changed the color. The pressure increase from 400 to 650 MPa and the time increment from 1 to 5 min at 400 MPa increased L* and b*. a* decreased only with 650 MPa for 5 min at 20 °C. The prior freezing of the carpaccio and the HHP at -30 °C minimized the effect of the HHP on the color and did not change the shear force, but increased expressible moisture as compared to the untreated carpaccio. HHP at 20 °C was more effective in reducing the counts of microorganisms (aerobic total count at 30 °C, Enterobacteriaceae, psychrotrophs viable at 6.5 °C and lactic acid bacteria) than HHP at -30 º C. With HHP at 20 °C, we observed a significant effect of pressure and time on the reduction of the counts.

Formation risk of toxic and other unwanted compounds in pressure-assisted thermally processed foods.

Consumers demand, in addition to excellent eating quality, high standards of microbial and chemical safety in shelf-stable foods. This requires improving conventional processing technologies and developing new alternatives such as pressure-assisted thermal processing (PATP). Studies in PATP foods on the kinetics of chemical reactions at temperatures (approximately 100 to 120 °C) inactivating bacterial spores in low-acid foods are severely lacking. This review focuses on a specific chemical safety risk in PATP foods: models predicting if the activation volume value (V(a) ) of a chemical reaction is positive or negative, and indicating if the reaction rate constant will decrease or increase with pressure, respectively, are not available. Therefore, the pressure effect on reactions producing toxic compounds must be determined experimentally. A recent model solution study showed that acrylamide formation, a potential risk in PATP foods, is actually inhibited by pressure (that is, its V(a) value must be positive). This favorable finding was not predictable and still needs to be confirmed in food systems. Similar studies are required for other reactions producing toxic compounds including polycyclic aromatic hydrocarbons, heterocyclic amines, N-nitroso compounds, and hormone like-peptides. Studies on PATP inactivation of prions, and screening methods to detect the presence of other toxicity risks of PATP foods, are also reviewed.

Modeling thermophysical properties of food under high pressure.

A set of well-known generic models to predict the thermophysical properties of food from its composition at atmospheric conditions was adapted to work at any pressure. The suitability of the models was assessed using data from the literature for four different food products, namely tomato paste, potato, pork, and cod. When the composition of the product considered was not known, an alternative was proposed if some thermal data at atmospheric conditions were available. Since knowledge on the initial freezing point and ice content of food are essential for the correct prediction of its thermal properties, models for obtaining these properties under pressure were also included. Our results showed that good predictions under pressure, accurate enough for most engineering calculations can be made, either from composition data or using known thermal data of the food considered at atmospheric conditions. All the equations and coefficients needed to construct the models are given throughout the text, thus readers can compose their own routines. However, these routines can also be downloaded free at http://www.if.csic.es/programas/ifiform.htm as executable programs running in Windows.

Prediction of ice content in biological model solutions when frozen under high pressure.

High pressure is, at least, as effective as cryoprotective agents (CPAs) and are used for decreasing both homogenous nucleation and freezing temperatures. This fact gives rise to a great variety of possible cryopreservation processes under high pressure. They have not been optimized yet, since they are relatively recent and are mainly based on the pressure-temperature phase diagram of pure water. Very few phase diagrams of biological material are available under pressure. This is owing to the lack of suitable equipment and to the difficulties encountered in carrying out the measurements. Different aqueous solutions of salt and CPAs as biological models are studied in the range of 0 degrees C down to -35 degrees C, 0.1 up to 250 MPa, and 0-20% w/w total solute concentration. The phase transition curves of glycerol and of sodium chloride with either glycerol or sucrose in aqueous solutions are determined in a high hydrostatic pressure vessel. The experimental phase diagrams of binary solutions were well described by a third-degree polynomial equation. It was also shown that Robinson and Stokes' equation at high pressure succeeds in predicting the phase diagrams of both binary and ternary solutions. The solute cryoconcentration and the ice content were calculated as a function of temperature and pressure conditions during the freezing of a binary solution. This information should provide a basis upon which high-pressure cryopreservation processes may be performed and the damages derived from ice formation evaluated.

The initial freezing temperature of foods at high pressure.

The Pure water (P,T)-phase diagram is known in the form of empirical equations or tables from nearly a century as a result of Bridgman's work. However, few data are available on other aqueous systems probably due to the difficulty of high-pressure measurements. As an alternative, six approaches are presented here to obtain the food phase diagrams in the range of pressure 0.1-210 MPa. Both empirical and theoretical methods are described including the use of an artificial neural network (ANN). Experimental freezing points obtained at the laboratory of the authors and from literature are statistically compared to the calculated ones. About 400 independent freezing data points of aqueous solutions, gels, and foods are analysed. A polynomial equation is the most accurate and simple method to describe the entire melting curve. The ANN is the most versatile model, as only one model allows the calculation of the initial freezing point of all the aqueous systems considered. Robinson and Stokes' equation is successfully extended to the high pressures domain with an average prediction error of 0.4 degrees C. The choice of one approach over the others depends mainly on the availability of experimental data, the accuracy required and the intended use for the calculated data.

Anisakis simplex larva killed by high-hydrostatic-pressure processing.

Anisakis simplex is a common nematode parasite present in many marine fish, including finfish and squid. It can pose a public health problem if it is not destroyed during food processing. Anisakis larvae were isolated from fish tissue, and their survival of high-pressure treatments in distilled water and physiological isotonic solution was assayed. Treatment at a pressure of 200 MPa for 10 min at a temperature between 0 and 15 degrees C kills all Anisakis larvae, with a lack of motility being used as an indicator of larval death. Lower pressures can be successfully employed down to 140 MPa, but with lower pressures, the treatment time must be increased by up to I h to kill all larvae. Meanwhile, most larvae treated for >10 min at pressures of >120 MPa were dead, with the autofluorescence method being used to determine death. Cycles of compression and decompression increase the destruction of larvae compared with a single pressure treatment for a similar treatment time. Our results indicate that high-pressure treatment is an alternative nonthermal method for killing this nematode. The possible mechanism of death and damage by pressure is discussed, and uses for this treatment in food processing are suggested.

High-pressure shift freezing. Part 1. Amount of ice instantaneously formed in the process.

A mathematical model to calculate the amount of ice formed instantaneously after a rapid expansion in high-pressure shift processes (HPSF) was developed. It considers that when water is expanded it does not extend over its melting curve but reaches a metastable state (supercooled water), which also occurs in practice. Theoretical results appear to agree with experimental data.

High-presssure shift freezing. Part 2. Modeling of freezing times for a finite cylindrical model.

A comprehensive vision of the heat transfer process involved in high-pressure shift freezing (HPSF) is shown in comparison to the process at atmospheric pressure. In addition, a mathematical model to predict the freezing times is presented. This model takes into consideration the dependence of the thermophysical properties relating to temperature and pressure and the supercooling reached by liquid water at atmospheric pressure after adiabatic expansion in the HPSF process. Experimental and theoretical data appear to agree.

Response to high-pressure, low-temperature treatment in vegetables: determination of survival rates of microbial populations using flow cytometry and detection of peroxidase activity using confocal microscopy.

Application of high hydrostatic pressure (200, 300, 350 and 400 MPa) at 5 degrees C for 30 min to different micro-organisms, including Gram-positive and Gram-negative bacteria, moulds and yeasts, proved to be more effective in inactivating these organisms than treatments at 20 degrees C for 10 min and at 10 degrees C for 20 min. Moulds, yeasts, Gram-negative bacteria and Listeria monocytogenes were most sensitive, and their populations were completely inactivated at pressures between 300 and 350 MPa. The same conditions of pressure, temperature, and time were applied to different vegetables (lettuce, tomato, asparagus, spinach, cauliflower and onion), achieving reductions of from 2-4 log units in both viable mesophiles and moulds and yeasts at pressures of between 300 and 400 MPa. Sensory characteristics were unaltered, especially in asparagus, onion, tomato and cauliflower, though slight browning was observed in cauliflower at 350 MPa. Flow cytometry was applied to certain of the microbial populations used in the above experiment before and after the pressurization treatment. The results were indicative of differing percentage survival rates depending on micro-organism type, with higher survival rates for Gram-positive bacteria, except L. monocytogenes, than in the other test micro-organisms. Growth of survivors was undetectable using the plate count method, suggesting that micro-organisms suffering from pressure stress were metabolically inactive though alive. The pressurization treatments did not inactivate the peroxidase responsible for browning in vegetables. Confocal microscopic examination of epidermal tissue from onion showed that the enzyme had been displaced to the cell interior. Use of low temperatures and moderately long pressurization times yielded improved inactivation of micro-organisms and better sensorial characteristics of the vegetables, and should lower industrial costs.

High pressure response of fruit jams contaminated with Listeria monocytogenes.

High pressure is an alternative to thermal processing and is used to preserve food. Listeria monocytogenes is a bacterium which grows at low temperature, is able to multiply under vacuum, and is responsible for food poisoning. Pressures of 100, 200, 300 and 400 MPa were used for 5, 10 and 15 min at 20 degrees C on pure culture, and on apple and plum jam baby food artificially contaminated with Listeria. Pure culture was also to test pressures of 200, 300, 350 and 400 MPa at 5 degrees C for 30 min. The results were analysed statistically and showed that there were no significant differences between pressures of 100 and 200 MPa at 5, 10 and 15 min. However, at 300 MPa, there were significant differences at 15 min. When the pressure treatment was 400 MPa, significant differences were observed at pressure times of 5, 10 and 15 min. The results were fitted to a linear curve. In pure culture, no viable cells were detected after high pressure treatment of 350 MPa for 30 min at 5 degrees C. The use of low temperature helps to maintain the sensory properties of the product.

Freezing rate simulation as an aid to reducing crystallization damage in foods.

In food freezing processes the presence of large ice crystals is a serious drawback when a good final quality of the product is desired. To study the size and distribution of those crystals, a large piece of pork muscle has been frozen by liquid nitrogen evaporation. A mathematical model to simulate different cooling rates at the surface of the product was solved using a finite element method; this model satisfactorily fitted experimental data and predicted local freezing rates at different locations in the meat tissue. The model was applied to find the freezing rates that led to a good quality product, related to an optimum distribution of small ice crystals located inside and outside the tissue fibres.

Size and location of ice crystals in pork frozen by high-pressure-assisted freezing as compared to classical methods.

In high-pressure-assisted freezing, samples are cooled under pressure (200 MPa) to - 20 °C without ice formation then pressure is released (0.1 MPa) and the high super-cooling reached (approx. 20 °C), promotes uniform and rapid ice nucleation. The size and location of ice crystals in large meat pieces (Longissimus dorsi pork muscle) as a result of high-pressure-assisted freezing were compared to those obtained by air-blast and liquid N(2). Samples from the surface and centre of the frozen muscle were histologically analysed using an indirect technique (isothermal-freeze fixation). Air-blast and cryogenic fluid freezing, having thermal gradients, showed non-uniform ice crystal distributions. High-pressure-assisted frozen samples, both at the surface and at the central zones, showed similar, small-sized ice crystals. This technique is particularly useful for freezing large pieces of food when uniform ice crystal sizes are required.

Effect of high pressure on the reduction of microbial populations in vegetables.

Resistance of micro-organisms to high pressure is variable and directly related to extrinsic and intrinsic factors. Pressures of 100, 200, 300, 350 and 400 MPa were applied at 20 degrees C for 10 min and at 10 degrees C for 20 min using strains of Gram-positive and Gram-negative bacteria, moulds and yeasts, as well as spores of Gram-positive bacteria. The results showed that at pressures of 100 and 200 MPa, decreases in microbial populations were not significant, whereas the populations of all the micro-organisms tested decreased considerably at a pressure of 300 MPa. A pressure of 300 MPa at 10 degrees C for 20 min was required to completely reduce the population of Saccharomyces cerevisiae, and a pressure of 350 MPa was needed to reduce most of the Gram-negative bacteria and moulds. The Gram-positive bacteria were more resistant, and pressures of 400 MPa were unable to completely reduce their populations. The different pressures employed had little effect on the initial numbers of spores. The initial populations of viable aerobic mesophiles and moulds and yeasts in vegetables (lettuce and tomatoes) decreased 1 log unit at pressures of 300 MPa and above under both sets of experimental treatment conditions. However, treatment at that pressure also resulted in alterations in the organoleptic properties of the samples. In the tomatoes, the skin loosened and peeled away, though the flesh remained firm, and colour and flavour were unchanged. The lettuce remained firm but underwent browning; flavour was unaffected. In vegetables use of moderate pressures in combination with other treatment conditions would appear to be required to reduce the populations of contaminating micro-organisms while avoiding the undesirable alterations in organoleptic properties that take place at 300 MPa.