Process design data for prok chilling

In 1993 legislation is being enacted throughout the European Community (EC) that will require the maximum temperature of pork carcasses to be below 7°C before cutting or transportation. Results from a survey of UK abattoirs are summarized to highlight the problems and inadequacies of current commercial chilling systems. Design data are presented for conventional pork-chilling systems, which detail the effects of air temperature, air velocity and carcass weight on chilling times. Four novel or modified chilling systems are discussed: ultra-rapid chilling with air at −30°C; immersion chilling in brine at 0°C; ice-bank chilling in humid air at 2°C; and spray chilling in two stages, at 10°C for 2 h followed by 4°C for 21 h. The potential of such systems for the acceleration of chilling rates and for substantial weight savings is identified.     

Measuring the convective heat transfer coefficient while chilling carcasses

The importance of the heat transfer coefficient when chilling carcasses justifies its re-examination and work to its magnitude and variation. While the air velocity at the surface of carcass (with its rather irregular configuration) is an elusive quantity, the author demonstrates that the measurement of the transfer coefficient and its variation should be based on the rate of air flow over unit mass of carcass and on the rate of weight loss through evaporation.     

The effect of salt concentration on the freezing point of meat simulants

Accurate data on the initial freezing point of cured meat is required to predict freezing rates or identify optimal slicing temperatures. However, little data was found in the literature. Experiments were therefore carried out using the ‘Karlsruhe test substance’ (‘Tylose’) with varying salt concentrations as a cured meat substitute. Initial freezing points were −1.4, −3.1, −4.1, −5.2 and −6.3 °C at salt contents of 0.5, 2, 3, 4 and 5 kg salt/100 kg sample, respectively. These values were within ±0.5 °C of published values for cured pork and within ±0.9 °C of theoretical predictions. Modifying the salt content of Tylose is therefore a simple way of determining the initial freezing point of cured lean meats, and Tylose modified in this way can be used to simulate the freezing of cured meat.     

Influence of mist-chilling on post-harvest quality of fresh strawberries Cv. Mara des Bois and Gariguette

The aim of this study was to assess the impact of mist-chilling on high-grade strawberry post-harvest quality (Cultivars “Gariguette” and “Mara des Bois”). Strawberries were chilled at 2 °C using three processes: air blast chilling at 0.3 m s⁻¹ or 1 m s⁻¹ and mist-chilling at 1 m s⁻¹. After chilling, fruits were submitted to different distribution chains characterised by different handling conditions and storage temperatures (2 °C or 7 °C) and by a 12 h retailing step at 20 °C. Strawberry quality was assessed by measuring 7 parameters: weight loss, commercial loss, firmness, sugar content, acidity, colour and sensory quality. Compared to air-chilling, mist-chilling did not reduce chilling time but it reduced weight loss by 20–40%. Mist-chilling had no detrimental effect on commercial loss defined as the percentage of fruit more than 1/3 of surface affected. It did not induce any major changes on strawberry quality. Temperature fluctuations undergone during cold storage and retailing had a detrimental effect on weight loss. The beneficial effect of packaging on weight loss was confirmed.     

A computational fluid dynamic model of the heat and moisture transfer during beef chilling

The unsteady heat and mass transfer process during beef carcass chilling was modelled for a three-dimensional beef carcass geometry. A three-step method was used to simulate the simultaneous heat and mass transfer process in order to reduce the computational time. In the first step, a steady state simulation of the flow field was conducted. In the second step, the local heat and mass transfer coefficients were calculated. Finally, the third step consists of the simultaneous heat and mass transfer process simulation on the meat carcass only. A separate 1-D grid was used to calculate the moisture diffusion in the meat. The simulation of a 20-h chilling run takes 5 days on a 2.5 GHz Pentium 4 computer. The model allows calculating and predicting the heat load, temperatures, weight loss and water activity. Local variations in the heat and mass transfer coefficients, temperature and water activity were found around the beef carcass. The CFD model gives temperature predictions that agree with experimental data better than any previous model. The weight loss tends to be over-predicted probably due to neglecting the resistance caused by the fat cover.     

Interactions among cooling, fungicide and postharvest ripening temperature on peaches

Peach fruit (Prunus persica L. cv. ‘Miraflores’) harvested at the firm-ripe stage, treated or not with 2 g l⁻¹ iprodione, were cooled or not at 1°C and ripened at 15 or 20°C and 95% RH for 10 days. During ripening, weight loss, fungal development and changes in quality parameters (firmness, soluble solids content, titratable acidity, pH and ground and flesh color), and carbon dioxide and ethylene production were monitored. Cooling alone or combined with iprodione avoided Rhizopus nigricans decay during ripening at either ripening temperatures. A skin damage not previously reported on fungicide treated peach was observed at 20°C. Cooled fruit ripened at 15°C showed an anomalous respiration rate and ethylene production after the climacteric peak, a loss of firmness and a drop in titratable acidity after 7 days of storage, and reduced endo-polygalacturonase activity in presence of continuous pectinmethylesterase activity during the first week. Cooling before ripening at 20°C led to the best flavor without excessive total losses. These results helped in the optimization of warming cycles during cold storage used to avoid chilling injuries development on peaches.     

Analyse expérimentale et modélisation des cinétiques de température et de perte de poids de carcasses de lapin pendant la réfrigérationExperimental analysis and modelling of temperature and weight loss kinetics of rabbit carcasses during chilling

This study deals with the chilling of rabbit carcasses with constant or time-variable air properties, this latter situation being closer to industrial chilling conditions. The experiments have been carried out with carcasses with weights in the range of 0.8–2.0 kg (1) in constant chilling conditions, 55 trials using six carcasses each, covering the following range: air velocity 0.3–2 m s⁻¹, air temperature −3 to 10°C, relative humidity 70–90%, and (2) nine trials, each using two carcasses, have been performed with time-variable air velocity, between 0.5 and 2.0 m s⁻¹, or time-variable air temperature, between −5 and +5°C. The analysis of the kinetics was based on the slopes of the curves of the dimensionless temperature logarithm versus time. The influence of the carcass weight and of the air properties was assessed. A hidden variability of the cooling conditions was evidenced and explained by water evaporation. A model, based on analytical solutions relative to an infinite cylinder, was adapted to take into account the variation of the chilling conditions. A shape factor and an equivalent surface area have been fitted from the experiments performed with constant chilling condition. The calculated results were compared with the measurements for both constant and time-variable chilling conditions. The average difference between calculated and measured temperature kinetics was in any case lower than 1°C. The weight loss, calculated as a percentage of the initial weight, was predicted with an accuracy of 0.1%. The relative errors in chilling times were lower than 9%.     

Thermodynamic performance limit considerations for dual-evaporator, non-azeotropic refrigerant mixture-based domestic refrigerator-freezer systems

Non-azeotropic refrigerant mixtures (NARMs) are investigated for a two-temperature level heat exchange process found in a domestic refrigerator-freezer. Ideal (constant air temperature) heat exchange processes are assumed. The results allow the effects of intercooling between the evaporator refrigerant stream and the condenser outlet stream to be examined in a systematic manner. For the conditions studied, an idealized NARM system will have a limiting coefficient of performance (COP) that is less than that of the best performing pure refrigerant component. However, for non-ideal heat exchange processes (gliding air temperature), the NARM-based system can have a higher limiting COP than a system running on either pure NARM component. Intercooling significantly affects the COP of NARM-based systems; however, depending on the location of ‘pinch points’ in the heat exchangers, only one intercooling heat exchanger may be needed to obtain a NARM's maximum refrigerator COP. The results are presented for mixtures of R22–R142b, R22–R123 and R32–R142b.     

A generalized coefficient of performance for conditioning moist air

The psychrometric chart is a useful tool for calculating the energy change between any two states and visually identifying the contribution of ‘sensible’ and ‘latent’ effects. However, the amount of work needed to move from one state to another is not read directly from the chart. In this paper, temperature and vapor pressure changes are cast into terms of minimum work relative to a dead state, and work contours are plotted on the psychrometric chart. These contours show the minimum work needed to move the state of the air further from the ambient conditions or the maximum work produced by letting a state approach the ambient conditions. A dividing line between heat pump and refrigeration operation is set on the basis total energy of the conditioned space relative to the ambient. Finally, an example is used to illustrate the implementation of the analysis.     

Predicting local surface heat transfer coefficients by different turbulent k-ε models to simulate heat and moisture transfer during air-blast chilling

A numerical investigation using a computational fluid dynamics (CFD) code is carried out to predict the turbulent flow field, and heat and moisture transfer in a three-dimensional air-blast chiller with cooked meats of cylindrical and elliptical shapes. Three turbulent models [standard, low Reynolds number (LRN) and RNG k-ε model] have been used in these simulations. Based on local heat transfer coefficients on the surface of the meat calculated by CFD code, the unsteady heat and mass transfer were simulated which took into account of the effects of conduction within the meat, forced and natural convection, radiation and moisture evaporation on the surface of the cooked meat joint. The model allowed the simultaneous CFD prediction of both temperature distribution and weight loss of the meat throughout the chilling process. Good agreement with experimental results was obtained. The effect of using different models on the accuracy of the simulation of local heat transfer coefficient is presented.     

Low- and high-temperature vitrification as a new approach to biostabilization of reproductive and progenitor cells

Cells held in a liquid milieu undergo processes that cause progressive loss of viability. To prevent such degradation, cells need to be placed in conditions that essentially stop all chemical reactions for the duration of the time of storage. Because intracellular ice formation is lethal to most eukaryotic cells, stable storage of viable cells can be achieved only if intracellular vitrification without ice formation has occurred. This has been done by several methods, including equilibrium (slow) freezing, lyophilization (freeze-drying), and ice-free vitrification at subzero temperatures at moderate-to high cooling rates in the presence of cryoprotectants (‘conventional’ vitrification). In this paper, we discuss the mechanisms of vitrification, and specific aspects, advantages, and pitfalls of the different approaches. Particular emphasis is put on novel methods of cell preservation, such as cryoprotectant-free vitrification of sperm and high temperature vitrification by air/vacuum drying of progenitor and other nucleated cells.     

CFD evaluating the influence of airflow on the thermocouple-measured temperature data during air-blasting chilling

A numerical investigation using a computational fluid dynamics (CFD) code is carried out to examine the influence of airflow on the thermocouple-measured temperature during air-blast chilling. A CFD model is developed to predict the temperature distribution and weight loss of the foods with and without the presence of thermocouples; the simulated temperature data are compared with experimental results. The comparison shows significant differences in the temperature results, especially near the surface of the foods being chilled. The study demonstrates that the temperature error depends not only on the length of the thermocouple remaining outside the product, but also the depth of the thermocouple inserted into the product. This study reveals a hidden source of experimental error and may help process engineers to better understand the phenomenon, and to correctly evaluate and examine their experimental and/or simulated temperature results for the air-blast chilling process.     

Chilling injuries in peaches during conventional and intermittent warming storage

Firm-breaker and firm-mature peaches (Prunus persica L. Batsch cv. ‘Paraguayo’) were conventionally stored for 4 weeks at 2°C and 90–95% relative humidity or subjected to intermittent warming cycles of 1 day at 20°C every 6 days of storage at 2°C. Warming periods induced ripening (reduced flesh firmness, extractable juice and titratable acidity), while during continuous storage abnormal values of these parameters were found. After 2 weeks at 2°C and particularly after the subsequent 3 days at 20°C, woolliness and to a lesser extent, vitrescence and dryness of the cortical tissue were detected. Severe levels of these disorders were found more frequently in firm-breaker than in firm-mature fruits, which mainly developed vitrescence. Three cycles of intermittent warming prevented chilling injuries but increased weight loss and senescence symptoms. Compared with conventional storage, intermittent warming increased the shelf-life of firm-mature and firm-breaker peaches by 1 and 2 weeks, respectively. Some considerations on the commercial application of the intermittent warming technique are included.     

Experimental verification of a mathematical model for simulation of industrial refrigeration plants

A mathematical model, proposed for simulation of food refrigeration processes, was tested against experimental data collected in a New Zealand meat processing plant which had a total refrigeration capacity of 2.5 MW. Predicted air temperatures in freezing and chilling operations followed the same trends as measured data. Differences were shown to be more attributable to uncertainties in data than to deficiencies in the formulation of the mathematical model. The model is considered suitable for use in simulation of a wide range of food refrigeration processes. Such simulations provide useful information about plant performance that cannot be obtained by steady state analysis. Predictions were worst for short times immediately following step changes in plant operation; should more accurate simulation of these periods be required a different form of mathematical model is needed.     

Modeling the thawing of frozen foods using air impingement technology

With the continual growth in the use of frozen foods both in retail and in food service, there is a need to develop improved thawing methods. Current methods are often undesirably slow (still air) or are very expensive and cause uneven thawing (microwave). Air impingement technology is one possible method to improve the thawing of frozen foods. The objectives of this research were to develop a two-dimensional model for air impingement thawing frozen foods and to verify the model experimentally. Frozen products were thawed using a laboratory impingement system with a single impingement jet. A simulated meat product (Tylose gel) was used as the test material. Thawing of a Tylose disk (12.7 cm diameter, 1.98 cm thickness) with air at 6 °C without impingement required more than 12 h, while thawing under a single impingement jet took less than 3 h, over four times faster. Results from the finite difference model gave good agreement with experimental data. Moisture loss during thawing was typically over-predicted because moisture gain due to condensation was not modeled.     

An innovative air-conditioning load forecasting model based on RBF neural network and combined residual error correction

Accurate air-conditioning load forecasting is the precondition for the optimal control and energy saving operation of HVAC systems. They have developed many forecasting methods, such as multiple linear regression (MLR), autoregressive integrated moving average (ARIMA), grey model (GM) and artificial neural network (ANN), in the field of air-conditioning load prediction. However, none of them has enough accuracy to satisfy the practical demand. On the basis of these models existed, a novel forecasting method, called ‘RBF neural network (RBFNN) with combined residual error correction’, is developed in this paper. The new model adopts the advanced algorithm of neural network based on radial basis functions for the air-conditioning load forecasting, and uses the combined forecasting model, which is the combination of MLR, ARIMA and GM, to estimate the residual errors and correct the ultimate foresting results. A study case indicates that RBFNN with combined residual error correction has a much better forecasting accuracy than RBFNN itself and RBFNN with single-model correction.     

Analysis of the gas compressibility effects on the constant-entropy reversible processes of refrigerants and refrigerant mixtures

Thermally and calorically real gas modelling based on the Martin–Hou equation of state is assumed for pure and mixed refrigerants in the superheated vapour phase. It allows the constant-entropy reversible processes which take place within the work transfer components of ideal vapour compression cycles to be properly analysed. These processes, in fact, occur in a region of the Mollier diagram close to the saturated vapour curve where covolume and molecular forces alter the equation of state of an ideal gas. Thus, real gas effects are significant and cannot be ignored. They give a more accurate indication of the refrigerant end temperature associated with an isentropic compression as well as of the corresponding work exchanged and volumetric efficiency. In particular, it is shown that the gas compressibility effects play a ‘favourable’ role during the isentropic compression processes since they allow the work transferred to be reduced up to 10% for HFC-refrigerant 134a, and HFC-refrigerant mixtures 407C and 410A. But, at the same time, they play an ‘unfavourable’ role since they can reduce the compressor volumetric efficiency (i.e. refrigerant mass flow rate) and, consequently, the cooling (or heating) capacity of the vapour compression system up to 7%.     

Modelling the chilling of the leg, loin and shoulder of beef carcasses using an evolutionary method

An evolutionary algorithm was used to adjust unknown parameters during the beef cooling process. These parameters are the equivalent diameter and the initial temperature profile, which are difficult to estimate given the irregular geometry, the elapsed time after slaughter and variations in both the air temperature and velocity. The adjusted parameters produced accurate predictions of the center and surface temperature profiles of the leg, loin and shoulder. The adjusted dimensions agreed very well with the measured carcass dimensions. Empirical equations were obtained to correlate this diameter with the weight and fat grade of beef carcasses.     

Thawing of frozen strawberries

Frozen strawberries were thawed under different controlled conditions (natural thawing at room temperature, thawing in circulating air, thawing in a refrigerator, thawing in water and thawing in a convection oven). The temperature rise at the geometric centre of the strawberries was monitored until the temperature reached 6°C and thawing diagrams were drawn. The effects of thawing method on the weight loss in strawberries were determined. Strawberries thawed at higher temperatures showed greater weight loss. During thawing in circulating air, thawing time decreased with increasing air velocity.