Heat transfer studies of steam/air mixtures for food processing in retort pouches

Heat transfer characteristics associated with steam/air mixtures were studied in two pilot scale batch type steam/air retorts: a vertical positive flow retort and a horizontal forced circulation Lagarde retort. A method employing transient heat conduction into rectangular bricks of aluminum and stainless steel was developed to evaluate the surface heat transfer coefficient (h) of steam/air mixtures. A system was designed to facilitate an instantaneous drop of the test brick, from an insulated box inside the retort, into a specified steam/air medium after the come-up period. The influences of steam content, temperature, flow rate and flow direction of the heating medium and orientation of test bricks on the associated h values, temperature distribution and pressure stability in the retorts were studied. In addition, thermal processing efficacy was evaluated by measuring the rate of heat penetration into bricks of silicone rubber and rigid nylon which have thermal diffusivities in the range common for foods. In both retorts, steam content (S) of the mixture was found to be the major factor influencing h (p<0.05); however, temperature had no significant effect (p>0.05). Further, the flow direction and flow rate of the heating media in the positive flow retort, and brick orientation in the Lagarde retort also influenced h (p<0.05). The general relationship between h and S was exponential: h = a exp(bS). In the positive flow retort with the test brick in the vertical orientation, the values of a and b were 153 W/m²C and 0.0421 respectively, for steam/air media flowing in an upward direction, and were 337 W/m²C and 0.0355 respectively, for the media flowing downward. The surface heat transfer coefficient was also found to increase linearly with the medium flow rate. With the Lagarde retort, steam/air flow was always horizontal and flow rate was not adjustable. In this case, h was influenced by the test brick orientation. For bricks in the vertical orientation, the exponential parameters, a and b, were 1011 W/m²c and 0.0226 respectively, whereas in the horizontal orientation, these were 1669 W/m²C and 0.0132 Temperature distribution studies in the positive flow retort indicated that the overall standard deviation of the medium temperature at several locations during the cook period (excluding come-up) increased (p<0.05) with a decrease in the steam content and flow rate of the heating media. The effects of temperature and flow direction were nonsignificant (p>0.05). In the Lagarde retort, the temperature distribution was not influenced either by steam content or temperature of the steam/air medium. Pressure stability studies indicated that the air content and temperature of the medium increased (p<0.05) the standard deviations of retort pressure during the cook period. Based on the temperature and pressure deviations in the two retorts, steam/air mixtures with 86-90% steam contents were considered to provide satisfactory overriding air pressures for processing of retort pouches at 105-120°C. Heat penetration studies in the positive flow retort using nonpackaged test bricks of silicone rubber and nylon revealed an increase of up to 11% (p<0.05) in the heating rate index (f) of test bricks when the steam content of the media decreased from 100% to 50%. Heating of bricks at 120°C resulted in f values that were 5.5% larger (p<0.05) than those for bricks heated at 105°C. In the Lagarde retort, the effects of temperature and steam content of the media on f values were not significant. Heating bricks in the vertical orientation resulted in higher f values than in horizontal orientation in some tests, while a reverse trend was observed in others. The influence of entrapped air (15-30 mL per pouch) in retort pouches containing the bricks on f values was small when using a vertical rack that tightly constrained the bricks, whereas up to 260% higher values of f resulted when using an unconstraining horizontal rack while processing at 105-120°C in media of steam contents above 65%. These increases in f value could be prevented by using overriding air pressures of 70-100 kPa during the retort operation. The lag factor, j, was generally in the range of 0.5-1.0 for test bricks, with or without packaging, in the positive flow retort, and 0.8-1.1 in the Lagarde retort, when evaluated at 42% effectiveness for the come-up time. It was observed that in order for the j values to match the theoretical value of 1.27 for an infinite plate, the effectiveness was in the range of 60-90%.Land and Food Systems, Faculty ofGraduat

Optimization of ultrasound-assisted osmotic treatment of Aleo vera gel impregnated with grape pomace phenolic compounds using response surface methodology

This study was focused on investigating the use of pulsed ultrasound-assisted osmotic dehydration (UAOD) for enhancing the infusion of phenolic compounds extracted from grape pomace (GPx) to Aloe vera (Aloe barbadensis Miller,) gel (AVG) using response surface methodology (RSM) and produce a dehydrated product with high antioxidant content. Fresh AVG were initially immersed in a sucrose solution of 50° B with three levels of GPx (10, 20, and 30 v/v) and subjected to pulsed ultrasound (US) treatment with a probe of 2 cm diameter, two levels of ultrasound amplitude (UA) (50 and 100%), and three ultrasound treatment times (UT) (30, 135 and 210 min). The pulsed on and off times were set at 15 and 1 min, respectively. A multiple regression analysis was carried out to develop second-order polynomial models with high coefficients of determination value (R2 &gt; 0.81). The optimal conditions were obtained using the Design Expert software. The best condition obtained for the UAOD optimization was UA = 59%, 20% grape pomace extract in 50% sucrose, UT = 173 min to result in a moisture loss of 51.3 % (predicted 49.9%), solid gain of 4.3 % (5.0%), total phenolic contents of 30.5 mg/g (31.8%), and DPPH radical scavenging activity of 15.3% (15.0%). UAOD therefore appears to be a promising process for antioxidant incorporation and preparing added value product.

Thermo-physical properties of apples and prediction of freezing times

In two varieties of apples, Golden Delicious and Granny Smith, the temperature dependence of different thermo-physical properties has been investigated. Detailed regression equations are given to cover the variations of thermal conductivity, apparent specific heat and thermal diffusivity of apples with temperature both above and below the freezing point. Tissue density has been studied at four different temperatures. The thermo-physical properties determined in this study have been employed to predict the freezing times of apples under various conditions of freezing using different models reported in the literature. The freezing conditions included for both variety of apples were: five freezing systems viz., freezing in air at -21 to -25°C and at -28 to -30°C, freezing by immersion in ethylene glycol at -18 to -20°C and -20 to -24°C and by immersion in liquid nitrogen at -197°C; three container sizes, viz., cans of size 300x407, 307x409 and 401x411; two initial product temperatures, 16-25°C and 1-7°C; and two target temperatures, -10° and -18°C. Two types of prediction methods were used, the analytical methods of : Plank (194]) , Nagaoka et al. (1955), I. I. R. (1972), Mellor (1976), Cleland and Earle (1979b), and the numerical methods with constant as well as varying thermal properties. The predicted values of freezing times by the different models were compared with experimental values and the relative merits of each model discussed. Based on an analysis of the prediction errors, a modification of Plank's equation to give the least error was suggested as follows [equation omitted]. The mean overall prediction error of the suggested model was 6.64% which was less than 5% beyond the experimental error of 2.38%.Land and Food Systems, Faculty ofGraduat

Characterization of Caseinate–Carboxymethyl Chitosan-Based Edible Films Formulated with and without Transglutaminase Enzyme

Edible composite packaging has the advantage of complementary functional properties over its individual bio-components. However, film composites made from caseinate (CA) and carboxymethyl chitosan (CMCH) have not yet been well explored. In this study, four types of CA-CMCH composite films were prepared and evaluated with and without transglutaminase (TGase) supplement. Aqueous CA (8%, w/v) and CMCH solutions (2%, w/v) were mixed in different volume ratios of CA: CMCH as 100:0, 75:25, 50:50, 75:25, and 25:75. Those to be supplemented with TGase were incorporated at 10 U/g of caseinate protein level. Results revealed that CMCH incorporation to CA facilitated a smooth and uniform surface microstructure on films and markedly improved the transparency, water barrier properties, mechanical properties, and solubility of the composite film. Furthermore, addition of TGase resulted in an improvement in the water vapor permeability. TGase successfully enforced the formation of CA-CMCH composites with some enhanced functional properties. The resulting composite film offers potential for applications as an alternative edible film or in the preparation of edible packaging films

Optimization of Microwave-Osmotic Pretreatment of Apples with Subsequent Air-Drying for Preparing High-Quality Dried Product

Prepared apple (Red Gala) cylinders were subjected to microwave-osmotic dehydration treatment under continuous flow medium spray (MWODS) conditions and then air-dried to a final 20% moisture content. The dried samples were evaluated for color and textural properties, and rehydration capacity. The MWODS pretreatments were based on a central composite rotatable design and a response surface methodology using five levels of sucrose concentration, temperature, and contact time at a constant flow rate of 2800 mL/min. The air-drying was carried out at 60°C, 15±1% relative humidity, and 0.64±0.02 m/s air velocity. The results were compared to untreated air-dried (AD) (worst-case scenario) and freeze-dried (FD) (best-case scenario) apples without the MWODS treatment. Color properties were affected regardless of the type of treatment. Conventional AD apples were darker in color, whereas MWODS-treated samples were lighter with higher L∗ and b∗ values, higher Hue and Chroma values but lower a∗ value and ΔE. Further the color parameters of MWODS-treated samples were closer or equal to the FD apples. The texture properties were also affected by the osmotic variables with MWODS treatment resulting in softer and chewier products. The AD samples were hard, and FD apples were brittle

Effect of Soluble Solids and High Pressure Treatment on Rheological Properties of Protein Enriched Mango Puree

The effects of high pressure treatment on the rheological properties of protein enriched mango puree were evaluated. In the first part, the original soluble solids in mango puree (28&#176; Brix) was lowered to 26, 23 and 20&#176; Brix, and each was supplemented with 2, 5 and 8% of whey protein to assess the influence of added protein. The samples were then evaluated by dynamic rheology. Pressure treatment resulted in a progressive increase in elastic (G&#8242;) and viscous modulii (G&#8243;) as well as complex viscosity (&#951;*). Values of G&#8242; were higher than G&#8243; demonstrating that the product behaved more like a gel. Additional tests were carried out by simultaneously varying protein and soluble solids contents, and keeping the total solids at 28%. In this case, the effects of pressure levels and holding times were similar to previous results. However, the positive effects of higher protein were negated by the negative effects of lower soluble solids, resulting in an overall decreasing effect on rheology. The developed models effectively predicted the combined influence of protein and soluble solid concentrations on rheological parameters (R2 &gt; 0.85). Sensory evaluation of 2% and 5% protein supplemented and pressure treated (500 MPa/3 min) mango puree yielded acceptable sensory qualities, resulting in a product with enriched protein content