REHYDRATION KINETICS OF MICROWAVE-DRIED OKRAS AS AFFECTED BY DRYING CONDITIONS

The aim of the present work was to understand the effect of drying conditions (microwave output powers and sample amounts) and rehydration temperature on rehydration kinetics of microwave-dried okras. Four different models, Peleg's, Weibull, first order and exponential association, were applied to experimental data, and the corresponding parameters of the models were obtained. In addition, the kinetic parameters of the models were correlated with the ratio of microwave output power to sample amount and temperature. Although all the models applied provided a good agreement with the experimental data with high values of the coefficient of determination of R ², the first-order kinetic model was chosen as the appropriate model because of its simplicity. By using the kinetic parameters of this model, the activation energy for rehydration kinetics was estimated as 1.15 kJ/mol.

PRACTICAL APPLICATIONS

Drying, in general, is a means of removal of water from the material. The purpose of drying food products is to allow longer periods of storage with minimized packaging requirements, reduce shipping weights, and preserve seasonal plants and make them available to consumers during the whole year. Besides these advantages, some important changes take place, as structural and physicochemical modifications, which affect the final product quality during dehydration. Rehydration is a complex process aimed at the restoration of raw material properties when dried material contacted with water, and can be considered as a measure of the injury to the material caused by drying. The objective of rehydration study is to attain as many products in their original characteristics as fast as possible. An improved knowledge of rehydration kinetics would significantly enhance the feasibility of this process.     

TEXTURE PROPERTIES OF GELLAN GELS AS AFFECTED BY TEMPERATURE

Gellan gels can be made very brittle, similar to agar gels, or very flexible, like gelatin gels. The entropy or enthalpy nature governing those gellan gel behaviors was studied by mechanical testing at temperatures varying from 2 to 62C. Both failure stress and strain for 1% low acyl and low acyl/high acyl mixed gellan gels decreased with increasing temperature, indicating that the hydrogen bonding contributed significantly to the stabilization of gellan gels in addition to the polyanion-calcium-polyanion bonding. Hydrophobic interactions were less important. The initial Young's modulus for two mixed high and low acyl gellan gels containing 2 mM Ca⁺⁺ increased with temperature from 2–42C, indicating entropy elasticity. Average molecular weight between adjacent crosslinks for these two mixed gels was larger than 10⁴. For other gels, the entropy elasticity was not a dominant mechanism for elastic force because of molecular weights between crosslinks and from the observation of negative temperature dependence of the modulus.     

THERMAL PROPERTIES OF PASSION FRUIT JUICE AS AFFECTED BY TEMPERATURE AND WATER CONTENT

The specific heat, thermal conductivity and density of passion fruit juice were experimentally determined from 0.506 to 0.902 (wet basis) water content and temperatures from 0.4 to 68.8C. The experimental results were compared with existing and widely used models for the thermal properties. In addition, based on empiric equations from literature, new simple models were parameterized with a subset of the total experimental data. The specific heat and thermal conductivity showed linear dependency on water content and temperature, while the density was nonlinearly related to water content. The generalized predictive models were considerably good for this product but the empiric, product‐specific models developed in the present work yield better predictions. Even though the existing models showed a moderate accuracy, the new simple ones would be preferred, because they constitute an easier and direct way of evaluating the thermal properties of passion fruit juice, requiring no information about the chemical composition of the product, and a reduced time of the estimation procedure, as the new empiric models are described in terms of only two physical parameters, the water content and the temperature.     

LIPID OXIDATION OF FISH LIVER OIL AS AFFECTED BY LIGHT, ANTIOXIDANTS AND TEMPERATURE

Crude and refined Mackerel liver oil with or without antioxidants were stored in artificial light or in darkness to assess the effects of photo-oxidative stress on the stability of oils and the antioxidant activity of butylated hydroxy-anisole (BHA) and tert-butylhydroquinone (TBHQ) on those oils. Changes in peroxide value (PV) and thiobarbituric acid value (TBA) were monitored over 18 days at 5C and 40C. The results showed that visible light exposure played the most critical role in the acceleration of fish liver oil oxidation. The addition of antioxidants showed a significant effect in retarding oxidation with TBHQ more effective than BHA. The oxidation rate of the oils decreased in the following order: control with light > control + BHA with light > control without light > control + TBHQ with light > control + BHA without light > control + TBHQ without light. Higher PV and TBA values were observed in refined oils versus crude oils. Increasing storage temperature accelerated the oxidation of only refined oils in light.     

TENSILE PROPERTIES OF BEEF SEMITENDINOSUS MUSCLE AS AFFECTED BY HEATING RATE AND END POINT TEMPERATURE

The effects of heating rate and end point temperature on tensile properties of beef semi-tendinosus muscle were studied. Samples of meat heated in glass tubes in a water bath to simulate oven roasting at 93 °C had lower (P < 0.01) shear values than those heated to simulate oven roasting at 149 °C. Samples heated to 60 and 70°C had lower (P < 0.001) shear values than samples heated to 50°C. Instron breaking strength values varied (P < 0.001) with end point temperature, but were not significantly affected by the rate of heating. Significant (P < 0.001) interactions between the end point temperature and heating rate, as well as between the end point temperature and fiber direction suggested that Instron breaking strength measurements may be more sensitive to changes in tenderness than Warner Bratzler shear measurements.     

THERMAL BROWNING OF TOMATO SOLIDS AS AFFECTED BY CONCENTRATION AND INHIBITORS

SUMMARY– The amount of brown color developed was always determined by reconstituting the sample to 5.6% total solids and measuring optical density (O.D.) of an acetone extract at 420 mμ. Tomato solids at concentrations of 5.6 to 98% were heated for 15 min at 95°C. The O.D. increased slowly with increasing concentration to about 50% solids. Thereafter, the O.D. increased rapidly to a peak at 95% and decreased to half this value at 98%. The effect of heating time at 55, 75 and 95°C and at 5.6 and 45% solids was studied. The browning rate was slight at the lower 2 temperatures relative to that at 95°C, and was much more rapid at the higher concentration than at single strength. Under each temperature and solids condition, the rate of browning was highest at the earlier times. The inhibition effects of pH and SO₂ were studied. Browning was at a minimum at pH 2.5. Browning could be further reduced at each pH by increasing levels of SO₂. The protective effect of acidification and SO₂ addition increased with increasing solids concentration.     

CHEMICAL COMPOSITION OF TOMATOES AS AFFECTED BY MATURITY AND FERTIGATION PRACTICES

The objective of this study was to determine the effect of various fertigation practices and maturity stages on tomato fruit composition. Soluble solids, reducing sugar content, and sugar/acid ratio of tomatoes increased with maturity. Fertigation treatments had less of an effect on the chemical composition of the fruit than the stage of maturity. Mineral (P, K, Ca, Mg) levels of tomato fruits in fertigated treatments were higher than in the control plot treatments receiving no supplemental water during the production season. Ascorbic acid content in whole mature-red fresh tomatoes was 13.24% and 25.71% higher than that in mature-pink and mature-light pink tomatoes, respectively. The locule section contained more ascorbic acid than other parts of the fruit at maturity. Intensive mineral fertigation and harvesting at the ripe stage contributes toward higher nutritional quality in fresh tomatoes.     

VISCOSITY AND QUALITY OF TOMATO JUICE AS AFFECTED BY PROCESSING METHODS

ABSTRACT

Quality and viscosity of tomato juice is strongly governed by mechanical and thermal abuse during processing. The effect of processing and storage duration on the viscosity and quality parameters of tomato juice was evaluated in the present work. Tomato juice was obtained by two different methods, normal method (NM) and variable method (VM), and was heated by both conventional and microwave hot break treatments. Tomato juice was evaluated for various quality characteristics including precipitate weight ratio, degree of serum separation (DOSS), Brookfield viscosity, lycopene levels, vitamin C, phenols and antioxidant activity. Changes observed in several quality parameters during storage were statistically insignificant. The methods and processing had the greatest contribution toward viscosity, whereas storage duration had no significant effects. Viscosity parameters viz precipitate weight, DOSS and Bostwick consistency revealed that juice processed by VM was more viscous than that of NM, irrespective of the hot break processing used. Ascorbic acid and lycopene content varied significantly with mechanical methods and processing conditions. However, phenolic content and antioxidant activity remained stable. The variable process represents an improvement over the conventional by enabling tomato processors to improve the consistency of the tomato product.

PRACTICAL APPLICATIONS

Quartering or slicing tomatoes followed by hot break processing techniques can significantly improve the quality of tomato juice in terms of reduced degree of serum separation and precipitate weight ratio. The modified technology could be considered as a viable and economical method for small‐ and medium‐scale tomato processors with limited resources and facilities at their disposal.

     

COLOR AND VITAMIN C CONTENT IN MANDARIN ORANGE JUICE AS AFFECTED BY PACKAGING MATERIAL AND STORAGE TEMPERATURE

Mandarin orange (Citrus reticulata) juice was packed in two different non-transparent cartons made of polyethylene, cardboard and (1) aluminum foil (Tetrapak ^® ) or (2) ethylene vinyl alcohol copolymers (Pure-pak ^® ) and was stored at 4C or room temperature (25  ±  3C). During the shelf life of these juices (35 days), changes in the headspace gas composition, vitamin C and CIEL*a*b* color coordinates were evaluated. In addition, a consumer panel evaluated the sensory color of the juices. Mandarin orange juices darkened and ascorbic acid content decreased with storage time. Type of carton and storage temperature played a predominant role in determining the final juice quality, with Tetrapak ^® carton providing the juice with the best quality (intense orange color and high vitamin C content and consumer preference) throughout their shelf life.

PRACTICAL APPLICATIONS

This study was a clear example of how research can help real food companies in selecting the right materials for their production lines. In this particular case, the results presented show how Tetrapak^® packaging led to a final mandarin orange juice of higher quality compared to Pure-pak^® packaging. Therefore, citrus processing companies have some scientific data comparing the behavior of these two types of non-transparent cartons. Besides, the companies manufacturing these cartons also have some information about their materials and can use it to improve them.     

THERMOPHYSICAL PROPERTIES OF COCONUT WATER AFFECTED BY TEMPERATURE

The effect of temperature on the thermophysical properties of coconut water was investigated from 5 to 80C, taking density, Newtonian viscosity, thermal diffusivity and thermal conductivity into consideration, besides specific heat. The validation of existent models of other foodstuffs, as well as the proposal of new models, was also investigated. It was verified that temperature significantly affected these properties, presenting in general a linear behavior, except for Newtonian viscosity, which presented an exponential behavior. Among the existent equations, results from the multiple linear model were appropriated to predict the density, thermal diffusivity and thermal conductivity values, while the model, which depended on solids fraction, was the best one for specific heat value, and the exponential model was the best in relation to Newtonian viscosity values. The proposed models fitted well, and were simple and easy to use.

PRACTICAL APPLICATIONS

Most of the known values of thermophysical properties of fluid foods typically concern products from subtropical and cold areas. Until today, there was little information concerning tropical products such as coconut water. Food composition and temperature are important factors that affect thermal properties. In order to create and adapt specific equipment, optimize processes that involve pumping and heat transfer, contributing to an effective design of food-processing equipment for tropical fluid foods, it is important to know the factors. Besides, the evaluation of existent models and equations, and the introduction of new equations is important to allow an adequate prediction of studied properties, thus saving time, material and money.     

BUTTER STABILITY AS AFFECTED BY EXTRACTS OF SAGE, ROSEMARY AND OREGANO

Effects of methanolic extracts of sage ( Salvia fruticosa L.), rosemary ( Rosmarinus officinalis L.) and oregano ( Origanum vulgare L.) on butter stability were investigated. All extracts were individually added to butter at 0.02 or 0.05%. For comparison, 0.02% butylated hydroxyanisole (BHA) and control group were also prepared and tested. The samples were stored at 25 or 5C. The peroxide value, free fatty acids and thiobarbituric acid value were determined at regular intervals for all samples. Spice extracts and their combinations exhibited a better effect than that of BHA in stabilizing butter against oxidation. The most effective extract was that of sage. The extracts significantly stabilized butter (P<0.05). Butter samples stored at 5C were more stable (P<0.05) than those kept at 25C.     

RHEOLOGY OF APPLE AND POTATO TISSUE AS AFFECTED BY CELL TURGOR PRESSURE

Experiments were performed on two varieties of apple tissue and one variety of potato tissue, in which cell turgor pressure was varied and the concomitant mechanical properties of the tissue were tested. Turgor pressure was controlled by immersion in solutions of various mannitol concentrations, and the degree of swelling or contraction of the tissue samples was measured. Under constant-strain-rate loading, mode of failure varied among the different commodities, and underwent a transition as the turgor pressure was varied. Rate of loading also caused a transition in failure mode. The stress and strain at failure and tissue stiffness were related to mode of failure, turgor pressure, and strain rate. From the osmotic swelling data, estimates were made of the cell wall stress-strain behavior and of the probability distribution for cell-wall strength in Ida Red apple tissue.