Low temperature-pressure processing of foods : safety and quality aspects, process parameters and consumer acceptance

Concept development for pilot/industrial scale semi-continuous or continuous HP reactor: development of product & process concepts: feasibility study of batch rectangular HP reactor: the scope of the work was to make feasibility study of development of high-pressure reactor with size and geometry suitable for processing rectangular fish blocks. The requirements for industrial reactor for high-pressure thawing of fish blocks were agreed during the 3rd full meeting in Helsinki in March 2004. The concept was formulated and shaping of reactor was conducted in collaboration with Warsaw University of Technology, as a part of master's thesis (M.Mikolajczyk, Structure and strength analysis of reactor for high-pressure thawing, WUT, 2005). The reactor was designed in CATIA v5 CAD system. It consists of high-pressure chamber, frame and two closures. The chamber of outer diameter of about 1 meter is composed of a sleeve retained by a pack of circular plates, made of high strength steel. In the sleeve four Copper filling inserts, and liner made of Stainless Steel, are placed. The filling inserts and the liner define inner geometry of the chamber. The frame consist of multilayer casing (set of thin steel sheets) and two half-rollers. Strength analysis of the high-pressure reactor was made using ANSYS FEM code. Modeling was made separately for the frame and for the high-pressure chamber. Four FEM models with various inner geometry of the chamber (from square to circular) were made. Calculations were made for three cycles of loading-unloading. The fatigue strength of the chamber was evaluated from the rate of plastic strain per cycle using the Manson-Coffin formula. Concept development of semi-continuous cylindrical HP reactor: for tubular reactor, allowing semi-continuous or continuous processing of foods, new original concept has been formulated. The main requirements for industrial equipment for high-pressure thawing and high-pressure freezing were agreed by the project partners and are: maximum pressure of 400MPa, temperature range: -45 to +25°C. Selected geometry: Cylindrical with cooling/heating jacket. Inner diameter of 60mm, outer of 129mm. The matter of the concept of the proposed tubular reactor is application of a processing pipe, continuously subjected to high pressure and ended with valves. The tubular reactor, allows semi-continuous pressure-supported freezing and thawing. It has a modular structure, which allows free determination of its length depending on results of modelling studies. Feasibility studies of the proposed concept of the tubular reactor are in progress. The potential advantages of PSF and PIT against conventional freezing and thawing processes, or a step further, the potential advantages of PSF and PIT processes performed in the metastable region seems to be directly related to the development of ice crystals in a given degree of size and size distribution. To verify the data collected by means of indirect observations (microstructure studies, drip loss, etc.), the use of the HPLT microscopic cell allowed direct observation of nucleation processes at high pressure.

Different model systems have been identified, characterised and the corresponding Tg-values have been measured using differential scanning calorimetry. These model systems and the measured Tg values can be further used in the project for two purposes: - The model systems provide systems with known Tg values (Tg(carrot) = -32° C, Tg(strawberry) = -43° C) to study enzyme activity and crystal size stability during frozen storage and - The different chemical components studied can be used to be infused in strawberries as a means to change the Tg value of this case study to evaluate the hypothesis that the Tg value of a real food material is of key importance in determining the evolution of texture and crystal size distribution during frozen storage (after regular or high pressure freezing processes). Tg is an important parameter related to food stability. Different values had been reported on fish, being controversy the actual importance of each one on the storage stability. Glass transition on sea bass muscle was analysed at ENITIAA by modulated DSC (Q100 - TA Instruments). Preliminary studies were performed to choose the better DSC parameters. Five different samples of suspensions containing Bacillus subtilis vegetative cells (heat inactivated) were analysed: 104, 106, 108, 1010, and pellet.

To prevent enzyme related quality degradation during frozen storage of fruits and vegetables, blanching is traditionally used to inactivate such enzymes prior to freezing. At the same time this heat treatment can cause undesirable changes in different quality parameters including colour aspects and nutrients. Therefore, the influence of high pressure - low temperature treatments succeeding a regular freezing unit operation on the inactivation of quality related enzymes including pectinmethylesterase, polygalacturonase, lipoxygenase and peroxidase and polyphenoloxidase in model systems, crude enzymes extracts and real food products was evaluated. To inactivate the enzymes in the conventional freezing (air-blasting) a thermal pre-treatment (blanching) is necessary before the freezing treatment. In the PSF treatment is also necessary to have a good flavour and odour. In general vegetable-frozen products by B-PSF) have a good odour, flavour, colour and texture. The texture in PSF products is a parameter that could compete with the conventional frozen products (air blasting). The existence of metastable phases of ice I and liquid in the domain of ice III (as explained by Schlüter et al., 2004), permits the use of optimized paths for the processes PSF and PIT in the domain of ice III without actual crystallization of ice III. Pressure-shift freezing was performed at 240 MPa and around -24°C. At these conditions, although the stable phase is ice III, liquid water was still present in the samples before the pressure release. Lowering the temperature before pressure release means increasing the temperature gradient in the samples for the instantaneous nucleation, that is, the jump from this temperature level of -24°C to the plateau (at -1°C for potato, approximately). If the equipment is able to provide lower temperatures, a lowest level can be also used, until around -28°C, without nucleation of ice III, due to the existence of a metastable liquid phase in this region (the cause of the supercooling phenomenon of around 15K when freezing to ice III). Pressure-induced thawing was performed at 290 MPa. After placing samples in the high pressure vessel, pressure was increased until 290 MPa, and at this pressure level, the temperature profile followed the extension of the phase transition line of ice I into the domain of ice III. At this pressure level, the area of nucleation of ice III is not reached and, therefore, a metastable mixture of ice I and the partially melted water in the surface during pressurization is present in the samples. In this way, a higher driving force due to a higher temperature gradient was reached, being the difference between sample (around -35°C at 290 MPa) and the surrounding medium (+11°C) temperatures of 46K. Compared to the classical PAT, in which the temperature gradient is around 32K (from -21°C from the sample to +11°C from surrounding medium), the gradient is increased in 14K using this process, leading to a shorter processing time. The enzymatic activity of polyphenoloxidase (PPO) was chosen to evaluate the effectiveness of PSF and PIT processes for food preservation, since it is dependent on the cell disruption, which is caused by ice nucleation during freezing, and crystal growth during storing and thawing. Results showed at laboratory and pilot scale that the activity of PPO was not increased after freezing and thawing processes when pressure was applied, being even slightly reduced in the metastable region.

The inactivation of Bacillus subtilis strain PS832, grown in TSB as culture medium, was studied for different pressure, temperature and time combinations. Some of these combinations lead to solid-solid phase transitions after freezing at atmospheric pressure and subsequent pressurisation. Depending on the pressure level, the samples may undergo solid-solid phase transitions from ice I into the domains of ice II, ice III or ice V. Treatments of cells between 250 and 350 MPa at -25°C are the most effective in inactivating vegetative Bacillus subtilis cells. For these conditions, a double effect of extracellular solid-solid (Ice I-III) phase transition and possible intracellular solid-liquid phase transition is suggested to be key in mediating the observed drop in viability. The damages on Bacillus subtilis PS832 vegetative cells induced by subzero temperatures and pressures up to 250MPa in buffer solution was studied by means of flowcytometry with combination of membrane permeability and viability probes such as PI and cFDA. The growth of single cells was traced by measuring the optical density and light scatter of the growth medium. Bacterial cells showed a heterogeneous resistance to the HPLT treatment. The synergistic effects of low high pressure processing, subzero temperature and pH was investigated on Listeria monocytogenes cells in buffer and in smoke salmon marinated. Various conditions of pressure (100, 150 and 200 MPa) at subzero temperature (-10, -14 and -18°C) were applied without freezing due to the phase diagram of water under pressure. Selected pH acid (4.5) and neutral were considered.

Good predictions of plateau times can be made from initial pressure and temperature values, both process parameters that can be readily adjusted in the food industry. Potato phase diagram has been experimentally obtained. The results have been used to define new concepts and processes, like ¿areas of nucleation¿ and ¿metastable phases¿. It can be observed that the phase transition line of ice I presents an extension in the domain of ice III, as well as for ice V in the domain of ice III. The degree of supercooling is clearly higher for ice III than for ice I and the distribution of the points obtained led to the definition of ¿areas of nucleation¿ as a better term than ¿nucleation lines¿. The existence of phase transition and nucleation points of both ice I and ice V into the domain of ice III led to the definition of ¿metastable phases¿. The water phase during the time at which the sample temperature remains below the phase transition point without nucleation must be also considered as a liquid metastable phase. The existence of these metastable phases opened new processing opportunities, especially for pressure-shift freezing and pressure-induced thawing, to optimise the thermal gradients (therefore increasing the amount of instantaneously frozen ice during pressure-shift freezing and reducing the processing time for pressure-induced thawing). Freezing, thawing, phase transition times have been defined. A bench-mark test in the equipment of three different laboratories was performed: a HPLT laboratory vessel placed in Berlin (TUBER), a HP pilot plant placed in Madrid (CSIC-1), a HP laboratory vessel also placed in Madrid (CSIC-2) and a HP vessel placed in Nantes (ENITIAA). Two different processes were carried out, PSF at 200 MPa and PIT at 200 MPa and the processing times were recorded.

Pressureassisted (PAT) and pressureinduced thawing (PIT) experiments were performed in agar gel at pressures 50 to 210 MPa and initial temperatures 5 to 20°C. Lower pressures and temperatures yielded PAT processes in which the sample temperature increases during pressurization: no melting took place. The complete phase transition occurred during the holding time. Higher temperatures and/or higher pressures yielded PIT processes in which partial melting occurred during pressurization, causing decrease of the temperature. Obtaining PAT or PIT depends on the initial temperature of the sample for a given pressure. Common situations were non homogeneous thawing processes. The influence of pressure, individually considering the temperature gradients and latent heats of fusion existing at different pressure levels was studied. New processes, as thawing from pressures in the domain of ice III and ice V or pressure-shift thawing have to be considered in the future, when industrially implementing thawing processes. The existence of extended lines of the melting curves of ice I and ice V and the subsequent existence of metastable phases of ice I and ice V in the domain of ice III are proven. The physical phenomena occurring during thawing are explained through schematic representations of the sample cylinders.

Specific volume, specific heat capacity, thermal expansion coefficient and adiabatic compressibility coefficient of water and ice I have been have been evaluated for the development of an user-friendly software with executable programs that allow the rapid calculation of the mentioned properties at any pressure-temperature combination. Simple analytical models that permit to make an estimation of the thermophysical properties as function of pressure and temperature have been developed for model gels. The procedure consists in assuming that the thermophysical properties at atmospheric conditions are shifted according to the shift of the thermophysical properties of water and of ice (or of their ratio). In other words, the effect of pressure on dry matter is neglected except the shift of the initial freezing point caused by pressure. These analytical models have been applied to the evaluation of enthalpy, apparent specific heat and thermal conductivity of tylose. Data on potato, pork tissue, salmon, sea bass, etc. were obtained together through calorimetry studies and the compilation of data has been done on a CD-Rom distributed by CSIC-IF in which selected thermophysical properties can be calculated for any food material after input of the corresponding water activity.

The aim of the work was to show experimentally if increasing pressure for a given temperature before expansion produce higher amounts of instantaneous frozen water and to study the supercooling phenomenon and its importance in the percentage of ice frozen after fast expansions. Samples were frozen by PSF at 100 MPa (-8.4°C), 150 MPa (-14°C) and 200 MPa (-20°C) as well as conventional air frozen (CAF), and liquid immersion freezing (LIF) at -20/-40°C. In the case of gelatine gel, CAF produced much larger ice crystals than LIF and PSF. LIF generated smaller ice crystals than those formed by PSF at 100 MPa. For the three PSF treatments, a higher pressure resulted in smaller ice crystals. In the case of Atlantic salmon, CAF produced larger and irregular ice crystals, resulting in serious deformation of muscle tissue. The ice crystals produced by LIF were fairly small but the cross-sectional area and roundness had large variations. PSF process significantly improved the microstructure of ice crystals (size, formation and location).

Two standard resistance pressure gauges were manufactured in UNIPRESS: Manganin pressure gauge and semiconductor pressure gauge InSb. Application of these gauges for studies of phase transitions in foods bears new challenges. The pressure gauges should be able for work in water, tolerate non-hydrostatic states induced by phase transitions and not mechanically degrade due to corrosion. Two InSb pressure gauges were made with dimensions 8x0.7x0.7 mm and electric resistance of 29.6 and 35.6 mΩ. The resistance was measured in function of current from 1mA to 100 mA. The resistance was found to be constant, with accuracy better than 10-3. This means that the current of 100 mA does not heat the gauge. Consequently, increasing the cross section of the InSb gauge allowed to increase the operating current and the output voltage, and secured higher pressure sensitivity. Additionally, the behaviour of thermoelectric elements: thermoelectric coolers (TEC) and thermocouple Copper-Constantan as well as of Platinum resistance thermometers at high pressure and at low temperatures was studied. Design of optimized pressure probes, installation and performance evaluation on existing vessel units and on new HP vessels: the scope of the study was to develop new measuring possibilities and to unify techniques for temperature measurement at Partners laboratories. In order to equip high-pressure units used by the Project Partners, with multi-thermocouple probes, the following design works were made: - Design of three-thermocouple probe for Multivessel Apparatus U111 700 MPa: for Partner No 1 (TU Berlin), - Design of two-thermocouple probe for high pressure vessel Resato A100: for partner No 2 (VTT Helsinki), - Design of three-thermocouple probe for high pressure cell UNIPRESS HP-LT 700 MPa: for Partner No 3 (IF CSIC Madrid). Developing of the Microscopic Cell and the DTA Cell in the frame of this project, forced us to undertake works on new types of temperature probes. For this probes we develop new technology of feeds-though. In UNIPRESS, usually we use soldering for sheathed thermocouples feeds-though or Pyrophilite sealed cone type feeds-though for wire type thermocouple. In the new technology both sheathed and wire type thermocouples are glued-in using epoxy resin. This technology is also suitable for optical fibber feeds-thought. Two types of high-pressure probes were developed, manufactured and applied in the Microscopic and DTA cells: - Single and multi-thermocouple probes for connector 1/16, - Interchangeable single thermocouple probe for DTA cell plug. Development of experimental techniques for in-situ HP measurements based on optical and thermal methods: optical methods: manufacture of HPLT microscopic cell: the scope of this work was to develop a miniature, temperature-controlled and pressure-controlled high-pressure cell for in-situ observation of biological materials and ice crystals under optical microscope. The completely assembled prototype was tested in UNIPRESS at the assumed conditions of pressure and temperature at the stand shown in Figure ‎3.35. After equipping the test stand with inverted microscope (March 2005), the first microscopic observations were done. Finally, the prototype was installed and set up in TU Berlin (P1). The study of vegetal tissues under the high pressure low temperature microscope permits the visual observation of the tissues directly under pressure and during the different phase transition processes susceptible to occur in the working range of the facility, 0.1 to 700 MPa and -50 to +30°C.

Activities made in the field of quality parameters studies focused on the accumulation of product stability kinetics and optimisation related aspects data during SAFE ICE processing. The main objective was to identify, to evaluate and to optimise quantitatively critical product stability aspects of microbial models, model food systems and of real food systems subjected to a combination of high-pressure and low temperature. The following specific studies were carried out: - Quality related aspects (colour, drip loss and microstructure) of potatoes after PSF and PIT in the metastable region; - Effect of pressure-supported freezing processes on pore size and texture of starch gels; - Effect of pressure-supported freezing on baking quality of dough; - Influence of low-temperature blanching combined with high-pressure shift freezing on the texture of frozen carrots; - Infusion of pectinmethylesterase and calcium followed by high-pressure shift freezing to prevent texture loss of frozen strawberries; - Impact of process conditions on microstructure, texture and drip loss of salmon; - Effects of pressure shift freezing and pressure assisted thawing on sea bass (Dicentrarchus labrax L.) quality during the frozen storage; - Effect of PSF on pork muscle colour, drip loss and texture.

The decrease of temperature below its freezing/melting point leads to freezing of unfrozen samples at ambient and high pressure. However, to start nucleation of ice crystals supercooling is required depending on experimental conditions (cooling rate, physical properties, cooling system). Freezing of potato to higher ice modifications show a typical temperature plateau due to the exothermic thermal event when plotting temperature versus time. The degree of supercooling can be quantified by a sudden temperature increase from the nucleation to the initial freezing point. Therefore, temperatures of nucleation and phase transition are different. The freezing point of solutions is displaced by solvents towards lower temperatures at atmospheric pressure, but only few and disperse studies are available at HP. The effect on freezing point to ice I for a range of concentrations of NaCl and sucrose has been studied. High-Pressure Differential Scanning Calorimetry has been used to experimentally validate some of the computed results, obtain the experimental phase diagram of real products and calculate the latent heat of fusion and ice content. For tylose, partial phase diagrams were established. Singular points were obtained through pressure-assisted freezing profiles (at quasi-constant pressure), resulting in experimental points for phase transition and nucleation at different pressure levels.

Consumer attitudes towards low temperature high pressure processing of food were investigated through consumer survey in Spain, the Netherlands, Belgium and Finland (n = 936). Generally attitudes towards high pressure processing were neutral, even thought the term was unfamiliar for consumers. When given some information about pressure freezing technology, consumers considered it as appropriate, especially if it has advantageous consequences to the product. Conjoint analysis also indicated that when a new technology is applied technology as such is not very important for consumers, but it is important that the price does not go up and the use of technology has environmental benefits. Consumer acceptance is one of the key issues when products processed by a novel method are introduced to the market. Typically, consumers are not focused on processing method itself but rather to the benefits and risks associated with it. Benefits of a new method can be related to improved acceptability and convenience of new product. These benefits may even overcome negative consequences, like higher price. However, perceived risks may become barriers against use of a new product. To food manufacturing technological development enables various new processes which improve the quality of products in different ways. New products with better sensory and nutritional quality are available for consumers. When consumers purchase familiar products sensory quality is one of the main factors influencing purchase decision. Various extrinsic factors, such as contextual, cognitive, social, cultural and attitudinal variables related to both the product and the consumer of it, should be noted as factor influencing consumers' choices when products processed by new technologies are considered, and concerns of the nature of the processing method have to be taken to account. Also external quality of the product, like label, brand, price and other information, has an influence on product perception (Deliza et al., 2003). A new product needs to be accepted by consumers to be successful in the market. Novelty causes suspiciousness and consumers are often concerned about new processing technologies. Especially for women perceived unsafely of new foods influence attitudes towards the product. Consumers' evaluations of new technologies are often ambiguous. From the consumer's viewpoint new processing technologies are causing risks which are unobservable, unknown and out of their own control. Consumers by nature try to avoid risks, but some degree of risk may be tolerated if also benefits are gained. This is why benefits for consumers weigh heavily to the acceptance of new processing technologies. For example, acceptance of biotechnology has been found to depend on consumers perceiving some personal or societal benefit in it. To make new technologies more appropriate to consumers, technological innovations should be directed towards those outcomes, which consumers want, for example reduced environmental impacts, desirable quality or reduced price in food products.

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