Ice Recrystallization in Frozen Beef

Temperature fluctuations produced during storage and transport of frozen food lead to activation of the recrystallization processes, causing enlargement of the ice crystals and diminishing advantages obtained in quick freezing of products. A model for interpreting recrystallization of ice in frozen beef was established considering that in this phenomenon the driving force is the tendency to a decrease in surface energy of the crystalline phase and that rate of movement of the grain boundary is proportional to its curvature. Meat samples were frozen and submitted to higher storage temperatures. Histological analysis of tissues immediately after freezing and of samples stored at different times made it possible to measure evolution of ice crystal size and obtain kinetic constants for proposed recrystallization model. Mechanisms for interpreting the phenomena were discussed, determining that transition of molecules occurs preferently through the liquid phase.     

Ice morphology in frozen beef

A histological study was made of pieces of meat which had been frozen from one side, in conditions where the direction of the heat flux was approximately unidirectional and perpendicular to the muscle fibres. The morphology of the ice crystals in tissue frozen under various conditions was studied, and the range of freezing velocities over which no intracellular ice existed, was established. The average diameter D , of ice crystals, measured in different experiments and at different levels in the same piece of meat was related to the characteristic freezing time t _c, by the equation:
D = a + b log t _c     

Antimicrobial Activity of Undissociated Sorbic Acid in Vacuum Packaged Beef

Inhibitory effectiveness of sorbic acid was analyzed in vacuum packaged refrigerated beef by studying the modifications of bacterial growth parameters. Total counts, Lactobacillus sp., Pseudomonas sp., Enrerobacreriaceae, Brochorhrix thermosphacta and yeasts in samples of different pH values (5.70–6.20) which had been treated with potassium sorbate (27–3125 ppm) showed variations in the lag phase, growth rate, stationary phase and inhibition index. For refrigerated beef the contribution of sorbic acid dissociated fraction to the inhibitory effect was negligible, Considering growth parameters’dependence on undissociated sorbic acid concentration a mathematical model of microbial growth in treated beef was derived, which could be used to predict product shelf-life.     

MATHEMATICAL SIMULATION OF THE THERMAL BEHAVIOR OF FROZEN MEAT DURING ITS STORAGE AND DISTRIBUTION

During the various stages of marketing, frozen food products are frequently subjected to changes in temperature which affect their quality. In the present work, a mathematical model is developed to analyze the thermal behaviour of frozen meat subjected, under different insulation conditions, to thermal step functions and periodic fluctuations in temperature varying in amplitude and frequency. The heat transfer equation has been numerically solved, since it is a system with physical properties that vary with the temperature, and fluctuating boundary conditions. At the same time, laboratory experiments were performed to verify that the mathematical model is applicable to one-dimensional and three-dimensional heat transfer problems. The obtained results were compared with analytical solutions for the heat transfer equation, with constant coefficients, determining the temperature range for which it is possible to use average values in the physical properties.     

Ice Crystal Size Modifications during Frozen Beef Storage

Studies of ice recrystallization in frozen stored bovine meat were performed at ?5°, ?10°, ?15°, and ?20°C over a 5-month period. A histological technique of isothermal freeze-substitution was used. Experimental results showed the existence of a limit crystal diameter independent of temperature and initial diameter. A kinetic model for ice recrystallization based on the difference between the mean and the limit curvature of the system satisfactorily fitted the obtained data. An equation to predict theoretically the limit diameter—also applicable to other food systems—was proposed. Significant effects of thermal oscillation on ice crystal sizes were observed. Activation energy of recrystallization was related to protein denaturation and exudate production.     

Thermal Conductivity of Frozen Beef Liver

The thermal conductivity of frozen beef liver has been determined, using a transient method. Since phase change in liver takes place within a wide temperature range, there are limitations in the application of the probe method which has been developed for a system without phase change. For that reason, the experiments have been performed setting limited values for the operational parameters (maximum heating power and measurement time range) in order to minimize errors of measurements. Thermal conductivity data were expressed as a function of temperature and a mathematical model based on Maxwell Eucken's equation has been included for the interpretation of the results.     

Influence of packaging film permeability and residual sulphur dioxide on the quality of pre-peeled potatoes

The effects of sodium bisulphite concentration and gas permeability of the packaging film on the surface colour, texture, exudate production and microbial growth of pre-peeled refrigerated potatoes were evaluated to determine shelf-life and the lowest level of preservative which could be used.
The product was packaged in polyethylene film, or in vacuo , using EVA-SARAN-EVA laminate and stored refrigerated. Polyethylene was not adequate in several aspects; packaging in oxygen barrier film allowed reduction of bisulphite levels, and vacuum packaging of 50 ppm bisulphite treated samples doubled the shelf-life of the product compared with aerobic packaging.     

HEAT TREATMENT EFFECT ON TEXTURE CHANGES and THERMAL DENATURATION of PROTEINS IN BEEF MUSCLE

Meat tenderness is one of the most important quality criteria when evaluating results of cooking conditions. Changes in tenderness and weight losses of heat-treated meat (semitendinosus muscle) for different time-temperature combinations were analyzed; the relationship between protein denaturation and textural changes was studied.
Heat treatments of meat samples (1.5 cm in diameter, 2 cm long) were performed in a thermostatic bath in the 60–90C range. Maximum heating times were 180 min. Meat hardness was determined by Warner-Bratzler measurements using an Instron testing machine. Protein denaturation was followed by Differential Scanning Calorimetry (DSC) analyzing peaks for myosin (I and II), sarcoplasmatic proteins and collagen (II) and actin (III).
Between 60 and 64C, hardness decreased with cooking time until reaching the lowest asymptotic values. This was related to protein denaturation of peak I and II. Between 66 and 68C, hardness decreased at first but increased later due to actin denaturation; at the temperatures 81 and 90C no modifications were observed and hardness remained at its higher values.
The kinetic model proposed fit the experimental results satisfactorily. Activation energies of tenderizing and toughening processes are similar to those of protein denaturation of peak II and III. Weight losses due to cooking were also modelled increasing through the entire temperature range.     

A NUMERICAL METHOD FOR SIMULATING HEAT TRANSFER IN HETEROGENEOUS and IRREGULARLY SHAPED FOODSTUFFS1

Heat conduction during cooking of foodstuffs with heterogeneous thermal properties was simulated, mainly for irregularly shaped geometries. A code employing boundary-fitted grids was implemented in a numerically conservative form (control volume formulation). This technique consists of finding a numerical coordinate transformation which fits a regular grid to the domain of integration. With the grid as data, coefficients for a transformed differential equation are then obtained. The proposed model was experimentally verified on irregularly shaped pieces of beef containing a nonuniform distribution of fat. Relative errors between predicted and measured temperatures were less than 2%. The effects of heat transfer coefficient and temperature of the cooking medium on the temperature distribution during the process were analyzed. the influence of different fat patterns was also considered.     

Potassium Sorbate Inhibition of Microorganisms Growing on Refrigerated Packaged Beef

The influence of potassium sorbate concentration (27–1325 ppm of sorbic acid) on the growth of natural flora in raw beef samples wrapped in plastic films of different oxygen permeability was studied at 0° and 4°C. Changes in aerobic plate counts, Pseudomonas spp., Brochothrix thermosphacta, Lactobacillus spp., Enterobacteriaceae and yeasts were monitored. Sorbate treatment effect on percentage increment of lag phase and reduction of growth rate during exponential phase for the different microorganisms were analyzed and antimicrobial action was evaluated by the Inhibition Index. A significant increase in the time to reach aerobic counts of 10^6.5 CFU/cm² was observed at low storage temperatures and pH values in vacuum packaged treated samples.