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Abstract: Evaporative weight loss from food leads to both loss of saleable weight and quality deterioration so it need to be minimized. The effect of isothermal and fluctuating conditions on frozen dough weight loss was measured and compared with kinetic, physical and artificial neural network (ANN) models. Frozen dough samples were regularly weighed during storage for up to 112days in loose-fitting plastic bags. The storage temperatures were in the range of −8 to −25°C with fluctuations of ±0.1°C (isothermal), ±1, ±3 or ±5°C about the mean. For each combination of temperature and fluctuation amplitude, the rate of dough weight loss was constant. The rate of weight loss at constant temperature was nearly proportional to water vapour pressure consistent with standard theories for evaporative weight loss from packaged foods but was also accurately fitted by Arrhenius kinetics. Weight loss increased with amplitude of temperature fluctuations. The increase could not be fully explained by either the physic model based on water vapour pressure differences or the kinetic model alone. An ANN model with six neurons in the input layer, six neurons in hidden layers and one neuron in the output layer, achieved a good fit between experimental and predicted data for all trials. However, the ANN model may not be accurate for product, packaging and storage systems different to that studied. [Copyright &y& Elsevier]

Abstract: Butter is a water-in-oil emulsion so its behaviour during freezing is very different from that of most food products, for which water forms a continuous phase. The release of latent heat during freezing is controlled as much by the rate of crystallization of water in each of the water droplets as by the rate of heat transfer. Measurements of the freezing of butter show that the release of latent heat from the freezing water depends on the degree of supercooling, which, in turn, depends on the cooling medium temperature, the size of the butter item, the packaging and the type of butter. Four modelling approaches were tested against the experimental data collected for a 25kg block of butter. A “sensible heat only model” accurately predicted the butter temperature until temperatures at which water freezing becomes significant were reached. An “equilibrium thermal properties model” predicted a temperature plateau near the initial freezing point of the butter in a manner that was inconsistent with the measured data. A third model used a stochastic approach to ice nucleation based on supercooling using classical homogeneous nucleation theory. The predicted temperatures showed that supercooling-driven nucleation alone is not sufficient to predict the freezing behaviour of butter. A fourth approach took account of time-dependent nucleation and ice crystal growth kinetics using classical Avrami crystallization theory. The relationship between the ice crystal growth rate and the supersaturation was assumed to be linear. The model predicted the experimental data accurately, particularly by predicting the slow rebound in the temperature following supercooling that is found when freezing butter under some conditions. [Copyright &y& Elsevier]

Thermal conductivity data are important for food process modelling and design. Where reliable thermal conductivity data are not available, they need to be predicted. The most accurate ‘first approximation’ methodology for predicting the isotropic thermal conductivity of foods based only on data for composition, initial freezing temperature and temperature dependent thermal conductivity of the major food components was sought. A key feature of the methodology was that no experimental measurements were to be required. A multi-step prediction procedure employing the Parallel, Levy and Effective Medium Theory models sequentially for the components other than ice and air, ice and then air respectively is recommended. It was found to provide the most accurate predictions over the range of foods considered (both frozen and unfrozen, porous and non-porous). The Co-Continuous model applied in a single step also provided prediction accuracy within ±20% (on average), except for the porous frozen foods considered. For greater accuracy more rigorous modelling approaches based on knowledge of the foods structure would be required. [ABSTRACT FROM AUTHOR]

Abstract: Effective thermal conductivity measurements were performed for two ice–air systems (ice chips produced by a commercial ice-maker and synthetic snow) using a transient comparative technique. These systems were chosen because the ice fraction could be controlled, thereby de-coupling the problems of ice fraction and thermal conductivity prediction. None of the commonly used simple effective thermal conductivity models (specifically the Series, Parallel, two forms of the Maxwell–Eucken and Effective Medium Theory models) provided adequate prediction accuracy over the full range of ice fractions in the experimental data. The best predictions were provided by a composite model that combined the Effective Medium Theory structure and the recently developed co-continuous structure. [Copyright &y& Elsevier]

Abstract: A new structural model for a heterogeneous material with multiple continuous phases is proposed. The corresponding equation for effective thermal conductivity was derived using three methods. The new model is substantially different from the conventional five fundamental structural models (Series, Parallel, two forms of Maxwell–Eucken, Effective Medium Theory). The model has two applications. First, as a new fundamental structural model to produce composite models using the combinatory rules previously proposed by J.F. Wang, J.K. Carson, M.F. North, D. J. Cleland, A new approach to modelling the effective thermal conductivity of heterogeneous materials, International Journal of Heat and Mass Transfer, 49 (17–18) (2006) 3075–3083. Second, to narrow the bounds of the effective thermal conductivity for heterogeneous materials where the physical structure can be characterised into general classes. [Copyright &y& Elsevier]

Abstract: The effects of freezing and temperature fluctuations during frozen storage on frozen dough and bread quality were investigated. Storage regimes were selected and designed to mimic either good or poor practice likely to be experienced in the cold chain (±0.1°C, ±1°C, ±3°C or ±5°C). Quality changes and dough weight loss were measured for both constant and fluctuating frozen storage conditions. Quality was assessed as CO2 production rate, yeast viability, bread specific volume and bread crumb firmness relative to fresh dough. Both the freezing process and subsequent frozen storage had a significant effect on all quality parameters. Dough weight loss and bread crumb firmness increased with increasing storage time. CO2 production rate reduced with increased storage period, however, constant storage conditions (−18±0.1°C) and good temperature control (−18±1°C) gave no significant difference in CO2 production rate for up to 112 days after freezing. Large temperature fluctuations during frozen storage (−18±5°C) and storage at higher temperatures (a combination of −18°C, −13°C and −8°C) resulted in significantly more rapid loss of dough and bread quality than storage at constant and/or colder temperatures. [Copyright &y& Elsevier]

Abstract: A unifying equation for five fundamental effective thermal conductivity structural models (Series, Parallel, two forms of Maxwell–Eucken, Effective Medium Theory) was derived. A procedure for modelling complex materials as composites of these five basic structural models using simple combinatory rules based on structure volume fractions was proposed. The combined models have advantages over other generic models such as the semi-empirical Krischer model, in that each has a distinct physical basis, and that they are not dependent on any empirical parameter. As a by-product, a physical description has been identified for Levy’s model, which was previously used with reservation by some researchers because it was derived solely by mathematical reasoning without any explicit physical basis. [Copyright &y& Elsevier]