Adriana Delgado

Heat and mass transfer models for predicting freezing processes - a review

Abstract A study of the literature published in recent years on freezing time prediction methods, with focus on simultaneous heat and mass transfer modelling is presented. Although models published vary from analytical simple equations to numerical methods, the most fruitful work is in finding simple equations for foods with regular shapes. It is difficult to develop more precise prediction methods for one-dimensional shapes as accurate experimental data are not easy to obtain. Heat transfer coefficients are hard to estimate with typical uncertainties of ±10–20% quoted and data on mass transfer coefficients are scarce. Research on the influence of thermal properties on the accuracy of models is useful. There are many models published for coupled heat and mass transfer for cooling, but very few models are available for freezing. Furthermore weight loss is an important factor for economic and quality reasons, therefore models should be developed in order to predict the weight loss during cooling and freezing.

Desorption isotherms and glass transition temperature for chicken meat

Abstract Desorption isotherms of chicken meat were obtained at different temperatures in the range 4–30 °C and at eight levels of salt solutions using the Novasina AWC203 multi-channel system. Several models were tested to describe the experimental sorption data; the Ferro Fontan model followed by the GAB equation gave the best fit for the whole range of water activity and temperatures, and the characteristic parameters that take into account the effect of temperature were determined. The BET and GAB models were comparable to predict the monolayer value, which was in agreement with the results obtained by using differential scanning calorimetry (DSC). The glass transition temperature of the maximum cryo-concentrated solution, Tg′, was measured by using DSC and an average value of −16.83 °C was determined.

Desorption isotherms for cooked and cured beef and pork

Abstract Desorption isotherms of cured and cooked beef and pork were obtained in a temperature range 10–50°C using the Novasina AWC203 multi-channel system. Beef and pork desorption curves show type III of the five general types of sorption isotherms due to the soluble components such as salts present in the samples. An inversion point was observed in pork while experimental beef curves indicate that an increase in temperature results in a decrease in equilibrium moisture content. Several sorption models were tested and the Iglesias and Chirife equation followed by Ferro Fontan, Peleg and GAB models was found to describe better pork and beef experimental data in the water activity range 0.10–0.94 and for the whole range of temperatures. Isosteric heats of desorption evaluated by applying the Clausius–Clapeyron equation to experimental isotherms are also reported.

Investigation of the effect of power ultrasound on the nucleation of water during freezing of agar gel samples in tubing vials.

Nucleation, as an important stage of freezing process, can be induced by the irradiation of power ultrasound. In this study, the effect of irradiation temperature (-2 °C, -3 °C, -4 °C and -5 °C), irradiation duration (0s, 1s, 3s, 5s, 10s or 15s) and ultrasound intensity (0.07 W cm(-2), 0.14 W cm(-2), 0.25 W cm(-2), 0.35 W cm(-2) and 0.42 W cm(-2)) on the dynamic nucleation of ice in agar gel samples was studied. The samples were frozen in an ethylene glycol-water mixture (-20 °C) in an ultrasonic bath system after putting them into tubing vials. Results indicated that ultrasound irradiation is able to initiate nucleation at different supercooled temperatures (from -5 °C to -2 °C) in agar gel if optimum intensity and duration of ultrasound were chosen. Evaluation of the effect of 0.25 W cm(-2) ultrasound intensity and different durations of ultrasound application on agar gels showed that 1s was not long enough to induce nucleation, 3s induced the nucleation repeatedly but longer irradiation durations resulted in the generation of heat and therefore nucleation was postponed. Investigation of the effect of ultrasound intensity revealed that higher intensities of ultrasound were effective when a shorter period of irradiation was used, while lower intensities only resulted in nucleation when a longer irradiation time was applied. In addition to this, higher intensities were not effective at longer irradiation times due to the heat generated in the samples by the heating effect of ultrasound. In conclusion, the use of ultrasound as a means to control the crystallization process offers promising application in freezing of solid foods, however, optimum conditions should be selected.

Ultrasound-Assisted Freezing

Freezing is a well-known preservation method widely used in the food industry. The advantages of freezing are to a certain degree counterbalanced by the risk of damage caused by the formation and size of ice crystals. Over recent years new approaches have been developed to improve and control the crystallization process, and among these approaches sonocrystallization has proved to be very useful, since it can enhance both the nucleation rate and the crystal growth rate. Although ultrasound has been successfully used for many years in the evaluation of various aspects of foods and in medical applications, the use of power ultrasound to directly improve processes and products is less popular in food manufacturing. Foodstuffs are very complex materials, and research is needed in order to define the specific sound parameters that aid the freezing process and that can later be used for the scale-up and production of commercial frozen food products.

One-dimensional finite difference modelling of heat and mass transfer during thawing of cooked cured meat

Abstract Failure to thaw frozen meat adequately is considered to be a contributory factor in sporadic outbreaks. Although much of the frozen products are thawed domestically, there are circumstances where frozen foods may be thawed for additional processing. Because the thawing process is generally assumed to be identical to that of freezing, any solution applied to freezing should also be valid for thawing. However, care must be taken when using these solutions, because the mass transfer at the product–air interface is of great importance in thawing. In this study heat and mass transfer during the thawing of ham was analysed. A computer program using the explicit finite difference scheme was implemented. The developed procedure was used to examine the influence on the thawing time of various operating conditions and physical properties. The results showed that the effect of the uncertainty of thermophysical properties on the prediction thawing time is important. The results of the simulation were compared with experimental measurements, and the predictive values were found to agree comparatively well with the experimental ones.

Multifractal Characterization of Apple Pore and Ham Fat-Connective Tissue Size Distributions Using Image Analysis

Pore size distribution (PSD) in apple tissue and fat-connective size distribution (FSD) in hams are the fundamental physical properties analyzed in assessing their quality. In apple tissue, PSD is related to the mass-transport phenomena characteristics and complexity of oxygen (O2) and carbon dioxide (CO2) diffusivity, and in the case of hams, FSD is related to sensory properties such as texture, taste, quality of raw meat, and visual appearance. In both food products, accurate representation of these microstructural properties is needed for an objective quality characterization and prediction during apple preservation and ham formulation.

Effective heat capacity for strawberry freezing and thawing calculations

Abstract Differential Scanning Calorimeter (DSC) measurements of the heat capacities of two strawberry cultivars have been correlated by means of a least-square curve fitting method to obtain accurate predicting equations of this variable in the range of −40 to 10°C. The values of the constants of the resulting equation for temperatures lower than the freezing point were used to calculate physical properties of the cultivars after experimentally evaluating the contents of the freezable and total water. The validity of the method was tested and the agreement between calculated and experimental values was found to be satisfactory. This is important because the method has the advantage of requiring only the water composition and the DSC measurement output to provide accurate specific properties of the product.

Spatial organization and correlation properties quantify structural changes on mesoscale of parenchymatous plant tissue

The study of plant tissue parenchymas intercellular air spaces contributes to the understanding of anatomy and physiology. This is challenging due to difficulty in making direct measurements of the pore space and the complex mosaic of parenchymatous tissue. The architectural complexity of pore space has shown that single geometrical measurements are not sufficient for characterization. The inhomogeneity of distribution depends not only on the percentage content of phase, but also on how the phase fills the space. The lacunarity morphometric, as multiscale measure, provides information about the distribution of gaps that correspond to degree of spatial organization in parenchyma. Additionally, modern theories have suggested strategies, where the focus has shifted from the study of averages and histograms to the study of patterns in data fluctuations. Detrended fluctuation analysis provides information on the correlation properties of the parenchyma at different spatial scales. The aim is to quantify (with...