Massey Documents by Type

Permanent URI for this communityhttps://mro.massey.ac.nz/handle/10179/294

Browse

Has files: YesSubject: search.filters.subject.Frozen foods

Search Results

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Heat transfer during freezing of foods and prediction of freezing times : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University
    (Massey University, 1977) Cleland, Andrew Charles
    A study of methods for predicting the freezing time of foods was made. Four shapes - finite slabs, cylinders, spheres and rectangular bricks were considered. For each shape experimental measurements of the freezing time were made over a wide range of conditions using Karlsruhe test substance, a defined analogue material. Experiments with slabs of minced lean beef and mashed potato were also conducted. Practical food freezing problems, where the material is initially superheated above its freezing point and the third kind of boundary condition (convective cooling) is applied, have not been solved analytically because of the non-linear boundary conditions. Food materials when freezing release latent heat over a range of temperature which further complicates any attempt at solution. The accuracy of the various solutions to the freezing problem proposed in the literature was evaluated by comparison of the various calculated freezing times with the experimentally determined values over a total of 187 freezing experiments. For those solutions requiring numerical evaluation, the best was found to be a three-level finite difference scheme which generally gave a prediction of the freezing time to within ±9% of the experimental values with 95% confidence. With the regular geometric shapes investigated finite elements have no advantage over finite differences and were not considered. For the existing exact and approximate analytical solutions, it is shown that these do not give accurate prediction of the freezing time for any of the four shapes, mainly because all but two of these solutions do not take account of initial superheat in the material to be frozen, and these two solutions are for initial superheat in a semi-infinite slab, not a finite slab. For the existing empirically modified solutions and empirical relationships, it is shown that they do not give accurate prediction of freezing time; at best the 95% confidence limits of the percentage difference between the calculated and experimental freezing times are 0% to +20% for slabs, cylinders and spheres. All solutions for freezing of rectangular bricks with the third kind of boundary condition use the geometric factors derived by Plank. These factors are shown to be subject to error, the error increasing as the ratios of the two larger dimensions to the smallest increase. A group of formulae is proposed which are simple to use and give accurate prediction of freezing time. They modify the geometric factors in Plank's equation, taking initial superheat into account, and in the case of rectangular bricks correct the errors inherent in these geometric factors. This group of simple formulae are shown to predict the freezing time with 95% confidence to within 5% of the experimental values for slabs, to within 7% for cylinders and spheres, and to within 10% for rectangular bricks. The prediction accuracy of the simple formulae and the three level finite difference scheme are similar but the simple formulae can be calculated quickly without the use of a computer which is a big advantage. In addition to simplicity and accuracy the simple formulae are also versatile, and by use of suitable approximations can handle some practical problems in which conditions change with time.
  • Thumbnail Image
    Item
    Prediction of freezing and thawing times for foods : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Biotechnology at Massey University
    (Massey University, 1985) Cleland, Donald John
    A study of methods to predict the freezing and thawing times of both regular and irregular shaped foods was made. Experimental thawing data for foods found in the literature, were limited in value because the experimental conditions were not sufficiently accurately measured, described and controlled to allow meaningful testing of thawing time prediction methods to be made. A comprehensive set of 182 experimental measurements of thawing time were made over a wide range of conditions using regular shapes made of Tylose, a food analogue, and of minced lean beef. Freezing and thawing experiments for irregular shapes were also carried out because of the paucity of published experimental data. Using twelve different two-and three-dimensional irregular shaped objects 115 experimental freezing and thawing runs were conducted. Combining experimental results with reliable published experimental data for freezing, a data set comprising 593 experiments was established against which prediction methods were tested. The partial differential equations that model the actual physical process of heat conduction during freezing and thawing can be solved by the finite difference and finite element methods. Testing of the finite element method has not been extensive, particularly for three-dimensional shapes. Therefore a general formulation of the finite element method for one-, two- and three-dimensional shapes was made and implemented. Both numerical methods accurately predicted freezing and thawing times for regular shapes. Sufficiently small spatial and time step intervals could be used so that errors arising from the implementation of the methods were negligible compared with experimental and thermal property data uncertainties. Guidelines were established to choose space and time grids in application of the finite element method for irregular shapes. Adherence to these guidelines ensured that prediction method error was insignificant. A simplified finite element method was formulated and implemented. It had lower computation costs but was less accurate than the general formulation. No accurate, general, but simple method for predicting thawing times was found in the literature. Four possible approaches for a generally applicable, empirical prediction formula were investigated. Each could be used to predict experimental data for simple shapes to within ±11.0% at the 95% level of confidence. This accuracy was equivalent to that displayed by similar formulae for freezing time prediction, and was only slightly inferior to the accuracy of the best numerical methods. All four methods are recommended as accurate predictors. For multi-dimensional shapes there were two existing geometric factors used to modify slab prediction methods - the equivalent heat transfer dimensionality (EHTD) and the mean conducting path length (MCP). New empirical expressions to calculate these factors for regular shapes were developed that were both more accurate and more widely applicable than the previous versions. Principles by which EHTD and MCP could be determined accurately for any two- or three-dimensional shapes were established. The effect of the first and second dimension were accurately predicted but lack of sufficient data (due to high data collection costs) prevented accurate modelling of the effect of the third dimension for some irregular shapes.
  • Thumbnail Image
    Item
    Modeling heat transfer in butter products : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy (Ph. D.) in Bioprocess Engineering, Institute of Technology and Engineering, Massey University, Palmerston North, New Zealand
    (Massey University, 2007) Nahid, Amsha
    Butter keeping quality and pallet physical stability during transport and storage are dependent on the temperature distribution through the product. Understanding these temperature changes are of vital importance for the dairy industry with regard to butter manufacture, storage and shipping. Three dimensional mathematical models of heat transfer were developed to predict thawing and freezing in butter products. These models require accurate thermophysical data as an input. Specific heat capacity and enthalpy of butter with different composition was measured using Differential Scanning Calorimetry. The specific heat capacity of butter differs for cooling and heating operations due to significant supercooling and delayed crystallization of the fat fraction of butter at temperatures well below the equilibrium phase change temperature during cooling. This reduces the heat capacity for cooling relative to that for heating. Thawing of individual blocks of butter was accurately predicted by the conduction only model (no mass transfer limitations) with equilibrium thermal properties giving accurate predictions when the butter was completely frozen before thawing. For partially frozen butter the conduction model with the measured temperature dependent specific heat capacity data for unfrozen butter including melting of some of the fat fraction gave accurate predictions. For freezing it was observed that water in the butter supercools many degrees below its initial freezing point before freezing due to its water in oil structure. Experiments suggested that during freezing release of latent heat observed as a temperature rebound is controlled as much by the rate of crystallisation of water in each of the water droplets as by the rate of heat transfer. A conduction only model including water crystallization kinetics based on the Avrami Model predicted freezing in butter successfully. Simple models with equilibrium thermal properties and nucleation only kinetics (based on homogenous nucleation theory) or the sensible heat only model (no release of latent heat) gave poor predictions. The models for individual blocks were extended to predict heat transfer in butter pallets. A butter pallet contains product, packaging material and the air entrapped between the packaging and butter cartons. Measurements were made for freezing and thawing of full and half pallets at a commercial storage facility and in the University laboratory. Thawing and freezing in wrapped tightly stacked pallets was predicted accurately by the conduction only model with effective thermal properties (incorporating butter, packaging and air) estimated by the parallel model. For unwrapped tightly stacked or loosely stacked pallets there is potential for air flow between the adjacent cartons of butter. An alternative approach was developed which consisted of modeling the pallet on block by block basis using effective heat transfer coefficients for each surface. Different heat transfer coefficients were used on different faces of the blocks depending on the location of the block in the pallet. This approach gave good predictions for both unwrapped tightly stacked and loosely stacked pallets using the estimated effective heat transfer coefficients from the measured data. Further experimental and/or modelling work is required in order to develop guidelines for estimating effective heat transfer coefficient values for internal block face for industrial scenarios.
  • Thumbnail Image
    Item
    Yeast metabolism in fresh and frozen dough : 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. Institute of Technology and Engineering, 2006) Miller, Simon Derek; Loveday, SM
    Fresh bakery products have a very short shelf life, which limits the extent to which manufacturing can be centralised. Frozen doughs are relatively stable and can be manufactured in large volumes, distributed and baked on-demand at the point of sale or consumption. With appropriate formulation and processing a shelf life of several months can be achieved.Shelf life is limited by a decline in proofing rate after thawing, which is attributed to a) the dough losing its ability to retain gas and b) insufficient gas production, i.e. yeast activity. The loss of shelf life is accelerated by delays between mixing and freezing, which allow yeast cells the chance to ferment carbohydrates.This work examined the reasons for insufficient gas production after thawing frozen dough and the effect of pre-freezing fermentation on shelf life. Literature data on yeast metabolite dynamics in fermenting dough were incomplete. In particular there were few data on the accumulation of ethanol, a major fermentation end product which can be injurious to yeast.Doughs were prepared in a domestic breadmaker using compressed yeast from a local manufacturer and analysed for glucose, fructose, sucrose, maltose and ethanol. Gas production after thawing declined within 48 hours of frozen storage. This was accelerated by 30 or 90 minutes of fermentation at 30;C prior to freezing.Sucrose was rapidly hydrolysed and yeast consumed glucose in preference to fructose. Maltose was not consumed while other sugars remained. Ethanol, accumulated from consumption of glucose and fructose, was produced in approximately equal amounts to CO2, indicating that yeast cells metabolised reductively.Glucose uptake in fermenting dough followed simple hyperbolic kinetics and fructose uptake was competitively inhibited by glucose. Mathematical modelling indicated that diffusion of sugars and ethanol in dough occurred quickly enough to eliminate solute gradients brought about by yeast metabolism.