Pedro D. Sanz

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.

Modelling heat transfer in high pressure food processing: A review

Abstract The most claimed advantage of high-pressure food processing as compared to thermal processing is that pressure acts instantaneously and uniformly through a mass of food independently of its size, shape or composition. Nevertheless, thermal gradients are established in the products after compression and cause inhomogeneities in the pursued pressure effect. Modelling heat transfer in high-pressure food processes can be a useful tool to homogenise and optimise these treatments. The main difficulty is the lack of appropriate thermophysical properties of the processed materials under pressure. When modelling high-pressure processes at subzero temperatures, pressure/temperature phase transition data and latent heat are also needed. Those for water are known, but there is a total lack for those corresponding to components relevant to foods. Moreover, the precise mechanisms that rule high-pressure shift freezing and induced thawing are not yet clear and so it hinders modellisation. This review collects the difficulties found and the advances made up to date in modelling heat transfer in high-pressure processes, including those performed at subzero temperatures.

Conventional freezing plus high pressure–low temperature treatment: Physical properties, microbial quality and storage stability of beef meat

Meat high-hydrostatic pressure treatment causes severe decolouration, preventing its commercialisation due to consumer rejection. Novel procedures involving product freezing plus low-temperature pressure processing are here investigated. Room temperature (20°C) pressurisation (650MPa/10min) and air blast freezing (-30°C) are compared to air blast freezing plus high pressure at subzero temperature (-35°C) in terms of drip loss, expressible moisture, shear force, colour, microbial quality and storage stability of fresh and salt-added beef samples (Longissimus dorsi muscle). The latter treatment induced solid water transitions among ice phases. Fresh beef high pressure treatment (650MPa/20°C/10min) increased significantly expressible moisture while it decreased in pressurised (650MPa/-35°C/10min) frozen beef. Salt addition reduced high pressure-induced water loss. Treatments studied did not change fresh or salt-added samples shear force. Frozen beef pressurised at low temperature showed L, a and b values after thawing close to fresh samples. However, these samples in frozen state, presented chromatic parameters similar to unfrozen beef pressurised at room temperature. Apparently, freezing protects meat against pressure colour deterioration, fresh colour being recovered after thawing. High pressure processing (20°C or -35°C) was very effective reducing aerobic total (2-log(10) cycles) and lactic acid bacteria counts (2.4-log(10) cycles), in fresh and salt-added samples. Frozen+pressurised beef stored at -18°C during 45 days recovered its original colour after thawing, similarly to just-treated samples while their counts remain below detection limits during storage.

A neural network approach for thermal/pressure food processing

Abstract High-pressure processing is an interesting technology for the food industry that offers some important advantages compared to thermal processing. But, the results obtained after a pressure treatment depend as much on the applied pressure as the temperature during the treatment. Modelling the thermal behaviour of foods during high-pressure treatments using physical-based models is a really hard task. The main difficulty is the almost complete lack of values for thermophysical properties of foods under pressure. In this work, an artificial neural network (ANN) was carried out to evaluate its capability in predicting process parameters involved in thermal/pressure food processing. The ANN was trained with a data file composed of: applied pressure, pressure increase rate, set point temperature, high-pressure vessel temperature, ambient temperature and time needed to re-equilibrate temperature in the sample after pressurisation. When ANN was trained, it was able to predict accurately this last variable. Then, it becomes a useful alternative to physical-based models for process control since thermophysical properties of products implied are not needed in modellisation.

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.

Thermal effect in foods during quasi-adiabatic pressure treatments

Abstract Large pressure shifts are a part of the emerging high-pressure food treatments. Pressure treatments count among their advantages vs. thermal processes the lack of the undesirable effects associated with temperature changes and a better cost-efficient ratio. Nevertheless, any pressure variation has with it an associated temperature change, whose effect, if not conveniently considered, can alter the product, increase costs by introducing the need of thermoregulation or, at the very least, become an error source in hydrostatic pressure treatment studies. In this article, water and fatty foods are used as models and the thermal changes originated by pressure variations are studied. The range 20–65°C and 0.1–350 MPa was considered, as it is the most interesting for food applications. Results were compared, where possible, to theoretically calculated values and a sufficiently good agreement with them was found. The fatty foods studied (milk fat and cream) yielded negative temperature changes, much higher than that of water, upon quasi-adiabatic expansions of the same magnitude. The absolute value of this temperature change decreased with increasing temperatures and pressures, in opposite sense to the water case. A delay of the thermal effect over its causing pressure shift was consistently found.

Some Interrelated Thermophysical Properties of Liquid Water and Ice. I. A User-Friendly Modeling Review for Food High-Pressure Processing

Referee: Professor Dr. D. Knorr, Department Food Technology and Food Process Engineering, Berlin University of Technology, Konigin Luise Str 22, D-14195 Berlin Germany A bibliographic search yielded a set of empirical equations that constitute an easy method for the calculation of some thermophysical properties of both liquid water and ice I, properties that are involved in the modeling of thermal processes in the high-pressure domain, as required in the design of new high-pressure food processes. These properties, closely interrelated in their physical derivation and experimental measurement, are specific volume, specific isobaric heat capacity, thermal expansion coefficient, and isothermal compressibility coefficient. Where no single equation was found, an alternative method for calculation is proposed. Keeping in mind the intended applications and considering the availability of both experimental data and empirical equations, the limits for the set of equations where set in −40 to 120°C and 0 to 500 MPa for liquid water and −30 to 0°C and 0 to 210 MPa for ice I. The equations and methods selected for each property are described and their results analyzed. Their good agreement with many existing experimental data is discussed. In addition, the routines implemented for the calculation of these properties after the described equations are made available in the public domain.

Freezing processes in high-pressure domains

Abstract Experimental freezing of water in high-pressure domain is studied considering temperature reduction (TRF) as well as high-pressure-assisted freezing (HPAF). The most important advantage of HPAF is that the whole volume of the sample is subcooled when an expansion is made, so a rapid and uniform nucleation and growth of ice crystals are produced. In this work through mathematical modelling the amount of ice appearing instantaneously in the latter freezing, is predicted.

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.

High-Pressure Freezing

Publisher Summary The advantages of high-pressure freezing processes for product quality as compared to conventional methods stem from two main parameters: reduced duration of phase transition and less mechanical stress during formation of ice crystals. When freezing at constant pressure (atmospheric conditions and high-pressure assisted freezing), nucleation occurs only near the sample surface, in direct contact with the cooling medium. When the latent heat of crystallization is removed, ice crystals grow from the surface to the center of the product. The final ice crystals are needle-shaped and radially oriented. High-pressure-assisted freezing to ice I offers some advantages because the latent heat of the water decreases with pressure. However, the freezing point also decreases and so the temperature of the cooling medium must be lowered to keep the T between it and the sample constant. High-pressure-assisted freezing to dense forms of ice produces a quality improvement; however, to preserve that improvement thawing must take place under pressure to avoid solid–solid phase transition to ice I during expansion. Transferring this technology to an industrial scale is therefore a challenge because continuous storage under high pressure at low temperature seems to be a cost-intensive technology.