L. Kulisiewicz

Basic aspects of phase changes under high pressure

Some substances of technological importance reveal phase change phenomena in the pressure and temperature range typically applied in biotechnology and food processing. For example, media with high molar volumes like edible oils and fats undergo liquid–solid phase transition at pressure increases up to several hundred megapascals. This article is concerned with theoretical considerations of the line of coexistence of solid and liquid phases in the pressure and temperature domain that corresponds to the phase boundary as a function of temperature and pressure. A universal model equation based on the equilibrium thermodynamics allowing prediction of the phase transition line of homogeneous media with high molar volume is presented. Approximate solutions of the model equation are discussed, which allow the phase boundary of substances with high molar volume to be described by a linear relation at least sequentially. The methods of experimental determination of the phase boundary under high pressure are presented and an attempt is made to validate the theoretical model with respect to the experimental data.

Observations of a high-pressure phase creation in oleic acid

Oleic acid is one of the unsaturated fatty acids which frequently appears in food products such as edible fats and oils. A molecule of oleic acid possesses a double carbon bond, C=C, which is responsible for a transition to a new phase when pressure is applied. This work presents the results of optical observations of such a transition. The observations were made in two cases, the first being static p–T conditions under 60 MPa at 20°C and the other the dynamic application of the pressure up to 350 MPa. The obtained visualization reveals differences in the creation of the phase and in its further appearance. Some crystal forms may be recognized. These results tend to be of interest for food engineers due to increasing interest in high-pressure food preservation among nutritionists and medical scientists concerned with fatty acids.

The particle image velocimetry method in the study of the dynamics of phase transitions induced by high pressures in triolein and oleic acid

Particle image velocimetry (PIV) is an optical measurement method capable of providing visualisation of velocity field of particle flow in fluids. After analysis of data acquired in the form of an image sequence, it is possible to retrieve information about flow parameters as mean values of velocity, vorticity, shear and normal strain. This paper presents the results of high pressure experiments using this method applied to triolein and oleic acid samples in their phase transition region. A high pressure optical chamber, He–Ne laser and light-sheet optics together with a digital camera and image acquisition computer allow us to study the motion of particles in high pressure conditions. The set-up was similar to that presented in Özmutlu et al. [Momentum and energy transfer during phase change of water under high hydrostatic pressure, Innov. Food Sci. Emerg. Technol. 7(3) (2006), pp. 161–168] and Kulisiewicz et al. [Visualization of pressure-shift freezing and thawing of concentrated aqueous sucrose solutions, High Press. Res. 27(2) (2007), pp. 291–297]. The analysis of phase transition dynamics in triolein and oleic acid is an extension to the work presented in Tefelski et al. [The investigation of the dynamics of the phase transformation in triolein and oleic acid under pressure, J. Phys.: Conf. Ser. 121(142004) (2008), pp. 1–6]. Oleic acid is a monounsaturated fatty acid and has a bent rod shape. Triolein is a triglyceride and has a “chair”-like shape. It is the base particle of many vegetable oils, especially olive oil. Triolein consists of three chains of oleic acid bound by a glycerol part. Information obtained by the study of phase transitions dynamics is important for food science and food technology processes which involve high pressure treatment. The PIV method shows differences in the solidification process of both substances in time, the existence of inhomogeneities (layers of different densities in the observed flow) and allows us to calculate the parameters of flow using the PIVview2C software from PIVTEC GmbH.

Visualization of pressure-shift freezing and thawing of concentrated aqueous sucrose solutions

A visualization of pressure-shift freezing of 0.7 w/w sucrose solutions was carried out at three temperatures 268, 253 and 235 K by release of pressure from 200 MPa to atmospheric value. Furthermore, pressure-shift freezing at 268 K and additionally pressure-shift thawing was carried out for a solution of 0.2 sucrose mass fraction. The solid phase observed at 268 K in the case of solution with 0.2 w/w sucrose fraction was ice I. The phase changes of 0.7 w/w sucrose solutions at 253 K and 235 K resulted in formation of spherical solids which are hypothesized to be eutectics, i.e. crystals with structure containing sucrose hydrates and water. The visual evaluation revealed differences in size and distribution of spherical crystals. The solids formed at 235 K were more numerous, more homogenously distributed and finally smaller than those observed at 253 K. It is explained by the higher supercooling of the solution in the former case, which provided higher driving force for nucleation and crystal growth.

Fluid Dynamics in Novel Thermal and Non-Thermal Processes

Publisher Summary This chapter deals with the novel treatment technologies applicable for food capable of flowing, which can be considered as fluid in a general sense. It discusses the major fluid-dynamical phenomena connected with novel food treatment concepts. It focuses on the nonthermal technologies such as high-pressure processing, pulsed electric fields, power ultrasound, and pulsed light and also thermal treatment making use of ohmic heating. Highly competitive markets in industrial nations and well-developed countries are generating an urgent need for developing novel food-processing technologies. Consumers demand food with high levels of organoleptic and nutritional quality but free of any health risks. The development of such kinds of products is usually connected with a reduction in the processing temperature, since thermal treatment often leads to loss of the desired organoleptic properties of fresh products and damage to temperature labile nutrients and vitamins. Therefore, the general tendency in development of novel food technologies consists in nonthermal processing or mild thermal processing, where the impact of temperature is aimed to be reduced by replacing pure thermal treatment by methods such as elevated pressure, electrical or acoustic fields, and light pulses or by combining methods. The state of the art concerning fluid dynamics in novel thermal and nonthermal technologies has achieved unequal levels. Fluid-mechanical effects regarding food treatment by means of pulsed light and ultrasound are far from being sufficiently understood. Overcoming the difficulties corresponding to the gaps in the available knowledge represents one of the most challenging tasks in literature.