Research Projects in 1999-2000

Effect of Vacuum Cooling on Processing Time, Mass Loss, Physical Structure and Quality of Large Cooked Beef Products

Karl McDonald and Da-Wen Sun

The objective of this project was to investigate the application of vacuum cooling to the cooked meats industry as a practical technique of rapid cooling and gain a better understanding of the physical, chemical, microbiological and sensory effects this cooling technique has on a cooked beef product. Previous research on vacuum cooling has indicated that it has adverse effects on meat quality and product yield. Results from this project initially indicated that vacuum cooling was very rapid in comparison to conventional cooling methods such as air-blast cooling. Product yields for vacuum cooled products were up to 10% lower than other cooling methods. However, sensory and instrumental analysis indicated that vacuum cooled samples suffered only marginal losses in quality. Microbiological analysis indicated a better product in terms of overall microbial counts and a safer product due to rapid cooling times of less than 90 min in comparison to cooling times of up to 525 min for air-blast cooling.

Further trials indicated that by manipulation of processing techniques such as product injection level product yield could be improved while maintaining a high level of product quality. In trials it was ascertained that injection levels of 135% resulted in vacuum cooled beef products with similar or better product quality and yield than high quality water immersion cooled beef products. However, concerns were emphasised about the careful control of injection ingredients such as salt, which at higher injection levels caused adverse sensory results. It was further observed that increasing the injection level from 120% to 145% increased cooling times.

Further research investigated the application of pressure regulation and pre-wetting of beef samples to further improve yield at injection levels of 135%. Pressure regulation from an initial vacuum chamber pressure of 100 mbar resulted in slower cooling times of up to 155 min at slower regulation rates but also increased yield by up to 1.8%. Pre-wetting of samples resulted in nonsignificant decreases in cooling times and increases in product yield. However the results did indicate the positive application of pre-wetting of samples. But the technique was difficult to investigate due to limitations in the vacuum cooling equipment and there are concerns about the safety of spraying cooked meats with water.

Throughout the first three trials it was observed that sample mass did not have a direct effect on vacuum cooling times. It was hypothesised that other factors such as internal porosity may have an influence. Analysis of samples indicated that porosity had a significant effect on cooling times and that method of processing samples received prior to vacuum cooling influenced its development during vacuum cooling. Increasing Injection level and tumbling beef samples without vacuum resulted in lower porosity and slower cooling times. Minced samples had porosities up to 12%, which resulted in cooling times of less than 20 min. Subsequent study of the pore structure of vacuum cooled samples revealed that the cooling technique resulted in the development of large void volume spaces rather than discrete pores which contributed towards increasing porosity and reducing cooling times. Samples which demonstrated the highest porosity and largest pore size distribution cooled quickest when all other variables such as surface area were equal. The development of porosity invacuum cooled samples and its effects on the composition of the beef samples particularly moisture content resulted in significant effects on the thermophysical properties of the samples. Vacuum cooled samples have lower thermal conductivity, diffusivity and specific heat capacity than air-blast, water immersion and cold storage cooling techniques.

The extension of vacuum cooling technology from more traditional practices such as rapid cooling of mushrooms and lettuce to the prepared consumer food industry has a strong theoretical background. The project has demonstrated the capability of vacuum cooling as an enhanced process for cooked meats such as beef. The findings will be of significant interest to commercial meat producers and catering services where cold rooms for air-blast chilling are needed on a 24-hour basis. Rapid vacuum cooling techniques would allow for same day dispatch operations, which would greatly reduce energy, manpower, maintenance and runningcosts. The scientific evidence provided by this project will enable the food industry to research this new technology and encourage equipment manufacturers to provide appropriate cost-effective vacuum coolers. The findings of this project also support further research into its application to other products, including cooked foods and perishable crops.

Sponsors: EU Food Research Programme, Teagasc

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Statistical Models for Growth and Digestibility of a Permanent Grassland Silage Sward

Dong Han, Da-Wen Sun and P. O'Kiely*

The thesis focuses on analysing and modelling of herbage dry matter yield and digestibility of a permanent grassland pasture sward managed for silage production during its primary and regrowth stages.

The analysing and modelling is based on raw data obtained from field experiments conducted in 6 successive years at Teagasc’s Grange Research Centre on a typical indigenous permanent grassland pasture. Statistical modelling approaches are adopted mainly because of the project’s strong practical application background.

The work develops prediction systems based on meteorological variables, duration of growth and time of year, for both herbage harvestable dry matter yield and in vitro dry matter digestibility at various stages of the permanent grassland silage sward growth. All the recommended models have been tested with independent data and show reasonably good accuracy under varying weather conditions.

The analyses conducted also found that under Irish weather conditions, water shortage may restrict grass growth cultivated during mid growing season in some years. A method is proposed and applied to take account of the adverse influences of moisture deficit and herbage dry matter yield models are developed for herbage growth under varying degrees of moisture deficit. Herbage dry matter losses caused by moisture deficit are assessed for each growth and for the whole growing season when moisture deficit prevails. The dry matter yield and digestible dry matter yield obtained under the two different silage harvesting schemes adopted in this experiment are also compared and recommendations on silage sward harvesting scheduling are made.

A Web-based silage grassland management decision support system has been developed based on the models obtained in this project. The system has been put into operation for trial use and demonstration purpose. Farmers with Internet access can use the decision support system with a web browser.

* Teagasc Grange Research Centre

Sponsor: Teagasc Walsh Fellowship

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Numerical Simulation of Coupled Heat and Mass Transfer during Food Cooling Process by Using Computational Fluid Dynamics Code

Zehua Hu and Da-Wen Sun

In order to preserve the quality and improve the safety of cooked meats, rapid cooling technique, such as air-blast chilling and vacuum cooling, has been commonly used in the food industry to remove the heat of foods after cooking. The overall aim of this research project was to develop the validated CFD models, to provide a better understanding of the cooling mechanism to improve the knowledge of heat and moisture transfer of cooked meats during chilling processes, so that the chilling performances could be predicted and equipment design and operating conditions could be optimised.

The work consisted of two main parts. The first part involved the development of a mathematical model and CFD simulation for the air-blast chilling. Several models were developed which were focused on calculating the local heat transfer coefficient on the meat surface based on a mathematical analytical solution model of unsteady heat and mass transfer. The calculation took into account the effects of forced convection, radiation and moisture evaporation on the ham surface. Different turbulent models have been used in the simulations of cooked meats with different shapes. Analysis on the inaccuracy sources of the simulation and experiment were explored, by means of CFD simulation. The effect of thermocouple insert into the meat on its measured temperature data was revealed as a hidden source of experimental temperature error.

The second part was to research into the vacuum cooling process. A new model based on Fourier’s, Fick’s and Darcy’s laws, was developed for water and vapour diffusion within the porous medium, which takes into account the coupling effects of conduction heat transfer, water evaporation and moisture diffusion within the meat and radiation on the surface. The CFD simulation is also capable of accounting for the effects of the dependent variables such as pressure, temperature, density and water content, viscosity of vapour, thermal shrinkage, permeability, and the anisotropy property of the food so that a comprehensive simulation of coupled heat and moisture transfer at low saturation pressure was conducted simultaneously.

These simulations allow simultaneous predictions of chilling time, temperature distribution and weight loss of cooked meats throughout the chilling process. Experiments were carried out and comparisons were made. The predictions showed good agreements with the experimental results. The simulation can be easily applied to other food thermal process under different operating conditions. The success of the CFD modelling has great potential for the application of CFD in the food processing industry.

Sponsors: EU Food Research Programme, Teagasc

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Evaluation of Functional Properties of Cheese with a Computer Vision System

Hai-Hong Wang and Da-Wen Sun

Functional properties including melting and browning etc. are crucial for the quality of cheese products, especially in the applications of cheese as toppings or ingredients for some prepared consumer foods. Many attempts have been made to control the functional properties of cheese. However, measurement of these properties has always been a problem because previous methods either are not accurate enough or have limited suitability.

The research project involves developing a protocol for the analysis of functional properties of various cheeses by image analysis techniques using computer vision systems. A computer vision system was set up and successfully applied to determine the melting and browning properties of cheese. The influence of cooking conditions and dimensions of cheese etc. on the functional properties of cheddar and mozzarella cheese was investigated using the developed method. These investigations are almost impossible with previous methods due to their limitations in scales and heating conditions. Therefore the developed method provides a promising solution to the evaluation of functional properties of cheese.

Sponsors: EU Food Research Programme

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Extension of Vacuum Cooling to Cut Flowers

Tadhg Brosnan and Da-Wen Sun

Cut flowers deteriorate rapidly at filed temperatures due to their high rates of respiration and low tolerance to heat, hence in order to maintain their quality and freshness cut flowers are pre-cooled after harvest prior to cold storage or transport. Forced-air cooling is the most commonly used pre-cooling method for cut flower. However, research into other pre-cooling methods and the effects of pre-cooling on cut flower quality is very limited. Therefore the objective of this project was to assess vacuum cooling as a practical and viable pre-cooling method for different cut flowers. Vacuum cooling is used commercially for a variety of vegetables. It is achieved by the evaporation of moisture off the product surface. The heat of vaporization required to evaporate the moisture is furnished by the product, which is cooled accordingly.

Initial trials focused on the vacuum cooling of cut daffodil flowers. In this research, freshly cut daffodil flowers were vacuum cooled to 2^oC and their cooling process monitored. The average cooling time for the flowers was 65 s with the longest cooling time being 100 s. Further studies examined the vase life of the vacuum cooled cut flowers with and without cold storage. The vase life was determined subjectively by various criteria. Results indicated that vacuum cooling gave an increase in vase life of 33% (from 3 to 4 days) in the case of cooled flowers stood in water at room temperature. The vase life of the cut flowers vacuum cooled and stored at 2^oC for 7 days was doubled, from 2 days for controls to 4 days for cooled flowers.

In subsequent research different treatments were assessed to determine if the inherent weight loss associated with vacuum cooling could be reduced. The techniques examined were vacuum cooling dry flowers, water sprayed flowers, and flowers adsorbing water through the stems for cut lilies flowers. Cold room cooled and uncooled flowers were used as controls. The cold room technique gave the slowest cooling and the greatest weight loss of the methods tested with means of 18.9 min and 5.3% (s.e. 0.11) respectively. The dry vacuum cooled flowers gave the highest weight loss of the vacuum cooling techniques examined with a mean value of 5.1% (s.e. 0.33). Initial cooling rates for all the vacuum cooling techniques were extremely quick with a temperature of 10ºC reached in less than 60 s. However for the dry vacuum cooled and covered absorption methods the final stem temperature reached was high at 5-7ºC. Vacuum cooling water sprayed flowers gave the best results with a cooling time of 62-87 s to reach the storage temperature of 2-3ºC. This method gave no weight loss unless the entire surface of the plant was not wetted. The water sprayed technique also yielded the best results as regards to vase life extension with a vase life of 11 d (s.e. 0.37) for room held flowers, with the cold stored samples having a vase life of 13.3 d (s.e. 0.26). This was an increase in vase life of 16% and 18% respectively over the controls. The results establish that water spraying cut lilies prior to vacuum cooling gives the most effective and beneficial results.

Further research investigated the use of computer vision for the objective assessment of cut flower vase life in comparison to the subjective criteria used in the above trials. Initial tests focused on the vase life of cut flowers that were not subjected to any cooling treatment. Results show that the computer vision was well able to map the colour, size and shape changes in the flowers examined over their life. There is a gradual decrease in grey value for each flower which indicates that it is fading. However due to differences between individual flowers no set grey value can indicate when the flower has reached the end of its vase life.

The extension of vacuum cooling to the floricultural industry is favorable to the cut flowers examined. Its benefits are that the cut flowers will have a longer vase life and hence provide the consumer with a better product. The different treatments examined indicate that the technique and its advantages can be easily enhanced. The most significant influences on the vase life of the flowers are seen especially for the stored situation. This is important because the majority of cut flowers are transported long distances to their markets in cold storage containers, and vacuum cooling would maintain their quality during storage and thus extend their subsequent vase life. However it remains to be seen if the grower, retail and consumer are willing to pay for the added advantages this techniques offers.

Sponsors: UCD Research Demonstrationship

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Evaluation of Performance of Slow Air, Air Blast and Water Immersion Cooling Methods in Cooked Meat Industry by Finite Element Method

Lijun Wang and Da-Wen Sun

The performance of slow air, air blast and water immersion cooling methods in the cooked meat industry is evaluated by a validated finite element model. The cooked meat joints in commercial and controlled geometries are cooled by these three traditional cooling methods under different operating conditions. Simulations show that it is difficult for traditional cooling methods to cool the large commercial cooked meat joints because the poor thermal conductivity of meats controls the cooling rate. It takes about 8.5, 6.5 and 3.5 h for air blast cooling with air velocity of 3 m/s and air temperature of 0^oC to reduce the cooked meat joints in a typical ellipsoid shape with the weights of 8, 5 and 2 kg from the core temperature of 74^oC to 10^oC, respectively. For water immersion cooling with water velocity of 0.15 m/s and water temperature of 0^oC, the cooling times can be reduced to 7, 5, 2.8 h, respectively. A rapid cooling can be achieved by controlling the shortest dimension. For cooling a cooked meat joint with the weight more than 5 kg by air blast cooling to meet the cooling requirements specified in cook-chill guidelines, the shapes of infinite slab with the shortest dimension of 5 cm and infinite cylinder with the diameter of 10 cm can be chosen. For water immersion cooling, the shortest dimension of the brick shaped meat joints can be increased to near 10 cm.

Sponsors: EU Food Research Programme

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Modeling Three Dimensional Transient Simultaneous Mass and Heat Transfer of Cooked Meat during Vacuum Cooling Process by Finite Element Method

Lijun Wang and Da-Wen Sun

The three dimensional transient mass and heat transfer of cooked meat during vacuum cooling process is modelled by the finite element method. The model mainly includes three sub-models, describing mass transfer with the inner vapour generation, heat transfer with the inner heat generation and vacuum cooling system, respectively. The variations in physical properties and the shrinkage of the cooked meat during vacuum cooling process are covered in this model. For vacuum cooling a block of cooked meat with the weight of 5.3 kg from the core temperature of around 74^oC to below 10^oC, the maximum deviation between the predicted and experimental core temperature is within 2.5^oC. The deviation between the predicted and experimental total weight losses is 7.8%. The simulation indicates that it takes only 17-18 min for vacuum cooling to reduce the core temperature to 10^oC, compared with 6.5 h for air blast cooling with air temperature of 0^oC and air velocity of 3 m/s. The predicted total weight loss is 9.12% of the original weight before vacuum cooling, which is higher than 6.57% for air blast cooling.

Sponsors: EU Food Research Programme

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Desorption Isotherms for Cooked and Cured Beef and Pork

Adriana E. Delgado and Da-Wen Sun

Desorption isotherms of cured and cooked beef and pork were obtained in a temperature range of 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. 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 Iglesias and Chirife equation followed by Ferro Fontan, Peleg and GAB models were found to describe better pork and beef experimental data in the water activity range of 0.10 – 0.94 and for the whole range of temperatures. Isosteric heats of desorption evaluated by applying Clausius-Clapeyron equation to experimental isotherms are also reported.

Sponsor: National Council of Technological and Scientific Research of Argentina, EU Food Research Programme

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Quality evaluation of pizzas and other consumer foods using computer vision

Lu Wang and Da-Wen Sun

The visual features are part of the whole quality of consumer foods. Traditional, the appearance inspections are fulfilled by human-contact in the food factories. The inherited instability and lack of common sense of the human inspection by employing few inspecting personnel along the producing lines make it necessary to look for better ways to overcome these drawbacks.

The research project aims to using computer vision systems to replace human inspectors. Computer vision is a promising technology involving image capture and image analysis. By studying the image aided by computer, the appearance defects can be automatically found.

Pizza products are the main part of the project. Pizza bases, pizzaa topped with sauce, and some final pizza products were investigated by computer vision, separately. Other consumer foods such as ice cream, biscuit, chocolate and sliced bread were also studied. Apart from the image analysis techniques, fuzzy mathematics were introduced as well. The results showed that computer vision technique provided more steady and accurate evaluation results than general human sensory inspection.

Sponsors: EU Food Research Programme, Green Isle Foods Ltd.