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    Significant factors affecting the forced-air cooling process of polylined horticultural produce : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Food Technology at Massey University, Palmerston North, New Zealand
    (Massey University, 2016) O'Sullivan, Justin
    New Zealand is the world’s third biggest producer of kiwifruit, with 94 % of the kiwifruit produced exported (NZ $ 1.0 bn in 2014). Forced-air cooling of the produce (from the harvest temperature of about 20 °C to near storage temperature of 0 °C) immediately after harvest improves storage potential and maintains produce quality before transportation to market. The design of the kiwifruit packaging system influences the rate of cooling and temperature achieved, mainly by affecting the airflow within and throughout the package. The typical kiwifruit package contains 10.5 kg of fruit and consists of a cardboard box and polyliner bag to prevent the loss of moisture and fruit shrivelling. Individual boxes are assembled onto pallets (10 boxes to a pallet layer, 10 layers high) Open areas or vents (in the box) facilitate cooling by allowing cool air to enter and circulate throughout the package. In forced-air cooling pallets are assembled into double rows with an aisle between the rows. Cool air is sucked through the pallets by a fan in the aisle, cooling the fruit and warming the air. The air is then either blown or ducted to the refrigeration system to be re-cooled. The polyliner keeps the local humidity high near the fruit, preventing weight loss due to evaporative cooling, but, as a barrier to direct fruit to air contact, slows the cooling rate. This project investigated the impact of operating conditions and package design on the cooling performance in such systems. A numerical model was developed (a CFD model implemented using the Fluent CFD software) that describes and predicts the temperature profiles of palletised kiwifruit packages undergoing forced-air cooling. The capability of the model to predict the fruit temperatures in each package was quantitatively validated against experimental data. The numerical model was able to predict temperature profiles within experimental error bars over 14 h of cooling. The numerical model was used to determine the operating point (in terms of pressure drop and flowrate across the pallet) to ensure rapid cooling of the produce without incurring excessive operational costs due to the power requirements. Results from both experimental work and the numerical model informed that there was an effective limit to the volumetric flowrate of 0.243 L kg-1 s-1: flowrates in excess of the limit had no or little effective benefit. This threshold flowrate is below the typical range recommended in industry for the forced-air cooling of non-polylined horticultural produce, which is 0.5 – 2.0 L kg-1 s-1. The numerical model demonstrated that the overall cooling performance (cooling rate, uniformity, power consumption and pallet throughput per week) can be improved by controlling the airflow distribution between the fastest and slowest cooling kiwifruit packages. An alternative design that channels cool air through the pallet towards the slowest cooling packages, located at the back of the pallet, by using two package designs in the same pallet, was presented. At 0.243 L kg-1 s-1 it was found that the pressure drop and power required to achieve equivalent cooling rates with the new design was reduced (by 24 % each) compared to the conventional design. Additionally, at the half-cooling time the cooling uniformity was improved by 19 %. The key features of the new design can be expected to be applicable for the cooling of horticultural produce involving an inner packaging liner.
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    Prediction of chilling times of foods : a thesis presented in partial fulfilment of the requirement for the degree of Doctor of Philosophy in Process and Environmental Technology at Massey University
    (Massey University, 1994) Lin, Zhang
    Chilling is one of the most important branches of food preservation under low temperature as it retains, more closely than any other means, the "fresh" quality and appearance of the food. No simple method to predict chilling times for a wide range of geometric shapes without major disadvantages was found in the literature. Investigation via a set of test problems showed that for each available method, there were ranges of conditions under which accuracy of prediction reduced significantly. This justified development and testing of a new method. A theoretical and experimental study of methods for predicting the chilling times of both regularly and irregularly shaped foods was carried out. Using the sphere as a reference shape and based on the first term of the series analytical solution, empirical mathematical expressions for extending the existing concept of equivalent heat transfer dimensionality E to take account of the effect of geometry on unsteady state heat conduction processes, which has been successfully applied in freezing time prediction, were derived. These cover a wide range of heat transfer environmental conditions and multi-dimensional regular and irregular geometries. Empirical formulae for the lag factor, L, as a function of object geometric shape were also developed for the thermal centre and mass-average temperatures. Guidelines to define object geometry for irregular shapes were established. The recommended dimensional measurement approach uses actual measurements of the three dimensions of an irregular geometry to define the dimensional ratios for an equivalent ellipsoid. The neglect of sharp protrusions and of hollows in taking measurements is recommended. Experimental chilling data for foods found in the literature, were limited in usefulness for model testing because the experimental conditions were usually not sufficiently accurately measured, described or controlled. Therefore, a comprehensive set of 3879 analytical solution simulations, 351 finite element method procedures and 165 experiments of chilling processes were made over a wide range of conditions. The chilling experiments were carried out using sixteen different two- and three-dimensional irregularly shaped objects made of Tylose, a food analogue, or of Cheddar cheese. Experiments for bricks of Cheddar cheese with uniformly distributed voids were also conducted because of the scarcity of published experimental data for chilling of products with voids. For regular geometries (short cylinder, squat cylinder, infinite rectangular rod, rectangular brick, oblate and prolate spheroids), and across a wide range of conditions and product shape ratios the methodology predicted chilling times to within -7.6 to 5.6% of the theoretical solutions for the thermal centre position and ±9.4% for the mass-average condition. For many commonly encountered conditions the lack of fit was considered acceptably low when likely data uncertainties are taken into account. When the guidelines for defining an equivalent ellipsoid and the simple method were tested, the 95% confidence interval of the percentage difference comparing predictions with the experimentally measured chilling times for thermal centre temperatures of two-dimensional irregular geometries was -3.1 to 14.4%. For three-dimensional irregular geometries, the equivalent interval was -6.4 to 11.6%. No experimental testing of predicted mass-average temperatures was carried out. The simple prediction method failed to match the experimental data in a similar manner to the finite element method. Lack of fit was probably more due to experimental error than errors in the form of geometric approximation and the prediction method itself. In situations where the product contains significant uniformly distributed voids, either the simple empirical prediction method or any relevant analytical solution can be applied if an "equivalent thermal conductivity" is defined. Keey's method, with a distribution factor dependent on voidage fraction, is recommended for calculating the equivalent thermal conductivity on the basis of best fit to experimental data but ranges of applicability require further investigation. It could not be established whether natural convection in the voids was a significant enhancer of the cooling rate. In many industrial applications, data such as heat transfer coefficients and thermal properties are difficult to estimate, and non-uniformity of chilling conditions is difficult to avoid. The overall accuracy of predictions in such applications is unlikely to be significantly increased through further reduction in the inherent inaccuracy of the proposed methodologies. The methodologies are therefore suitable for routine industrial use.
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    Prediction of chilling times of foods subject to both convective and evaporative cooling at the product surface : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology and Bioprocess Engineering at Massey University
    (Massey University, 1995) Chuntranuluck, Sawitri
    No published chilling time prediction method which covers a wide range of practical conditions, and which can be applied using only simple algebraic calculations for chilling with evaporation at the product surface has been proven accurate. The objective of the present work was to develop and test a simple chilling time prediction method with wide application for situations where significant evaporation as well as convective cooling occurs from the product surface. A numerical method (finite differences) was used to simulate convection and evaporation at the product surface in cooling of solid products of simple shape (infinite slab, infinite cylinder, and sphere) with constant surface water activity. Semi-log plots relating temperature change to be accomplished to time were linearised by appropriate scale transformations based on die Lewis relationship. The effect of evaporation on cooling rate was measured by considering the slope and intercept of such plots, and comparing these to the slope and intercept that would arise in convection-only cooling. The enhancement of cooling rate due to the evaporative effect depended on six parameters; initial product temperature, cooling medium temperature, Biot number, relative humidity, product shape factor, and surface water activity. Four simple algebraic equations were curved-fitted to the numerically simulated data for predicting temperature-time profiles at centre and mass average positions in the product. The numerically generated results and the simple algebraic equations agreed well with a mean difference close to 0% for all three shapes, and 95% confidence bounds of about ±3% for the infinite cylinder, and ±5% for the infinite slab and the sphere geometries. To test the simple models, chilling experim ents were conducted in a controlled air flow tunnel across a range of conditions likely to occur in industrial practice. Trials were conducted using infinite cylinders of a food analogue as an idealised product (with saturated salt solutions percolating over a wet cloth on the product surface to maintain constant surface water activity), and carrots (both peeled and unpeeled) as examples of real food products. Measured centre temperatures for both the idealised products and peeled carrots were predicted by the proposed method, assuming a constant surface water activity, within a range of differences which was almost totally explainable by experimental uncertainty. For unpeeled carrots, predictions mode using three different surface water activities in the model (one to represent the initial condition, one to represent the active chilling phase, and one to represent the quasi-equilibrium state at the end of chilling) agreed sufficiently well with experimental centre temperature data for the lack of fit to be largely attributable to experimental uncertainty. No experimental verification for prediction of mass-average temperatures was attempted. The proposed method is recommended for predicting chilling times of food products of infinite slab, infinite cylinder or sphere shapes, across a wide range of commonly occurring chilling conditions provided the product has constant surface water activity. The establishment of bounds on a theoretical basis for limiting the ranges in which surface water activity values are selected for making predictions for products with non-constant surface water activity is proposed, and some guidance on application of these bounds established. Further work to refine the use of these bounds for a range of food products, to consider a wider range of shapes, to test the ability of the proposed method to predict mass-average temperatures is recommended.