The food processing industry has matured over the years with an impressive record on safety and a vibrant marketplace for new product development. Consumer demands for high-quality products has inspired researchers and the food industry to explore alternative methods as replacement for traditional processing methods. The food industry is poised to adopt cost effective technologies that offer better quality and safe products. Given the impressive safety record associated with traditional systems, one may be tempted to conclude that there is little room for advancement and innovation to meet current consumer demands. Process optimization will continue to evolve to enhance quality and overall energy utilization either in traditional or novel systems. The need for efficient operations will certainly drive system automation, control and monitoring systems that can handle complex mathematical routines in real-time. Such systems would certainly require vigorous validation and verification for industry to embrace them. It truly sounds illogical for industry to re-evaluate existing process schedules based on studies that demonstrate non-linearity of survival curves. However, the need to optimize quality and operating costs could potentially prompt re-evaluating existing systems to capture additional benefits. New processing concepts such as the application of variable retort temperature have received attention from processing experts and promises to improve both the economy and quality of thermally processed foods.

Thermal processing and quality: Principles and overview

Evaluation of snack foods from barley-tomato pomace blends by extrusion processing

Microwave radiation in Escherichia coli and Bacillus subtilis cell suspensions resulted in a dramatic reduction of the viable counts as well as increases in the amounts of DNA and protein released from the cells according to the increase of the final temperature of the cell suspensions. However, no significant reduction of cell density was observed in either cell suspension. It is believed that this is due to the fact that most of the bacterial cells inactivated by microwave radiation remained unlysed. Scanning electron microscopy of the microwave-heated cells revealed severe damage on the surface of most E. coli cells, yet there was no significant change observed in the B. subtilis cells. Microwave-injured E. coli cells were easily lysed in the presence of sodium dodecyl sulfate (SDS), yet B. subtilis cells were resistant to SDS.

Differential damage in bacterial cells by microwave radiation on the basis of cell wall structure.

Summary Extrusion at higher moisture contents (> 40%), also known as wet extrusion, is relatively less investigated compared to low and intermediate moisture extrusion. Literature on high moisture food extrusion has been reviewed. Wet extrusion applications utilise twin screw extruders due to their efficient conveying capabilities. Extruders can be used as bioreactors for starch hydrolysis using thermally stable enzymes. This process is usually followed by saccharification inside or outside an extruder to produce a high DE (dextrose equivalent) syrup. Starch-based high moisture extrusion research also reports a few modelling studies. The rheological properties, torque and energy requirements of high moisture extrusion systems are different from those of low and intermediate systems. Other research reviewed includes the extrusion of low-cost plant and animal proteins to manufacture nutritious food products that imitate the texture, flavour, and mouthfeel of meat.

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High moisture food extrusion

In the past, research and development in drying has focused on the process and technology and food drying was performed mainly to extended the shelf life without much importance on retaining quality attributes. Recently, however, efforts have been made to develop high-quality dried foods. This is achieved by utilizing novel drying technologies, by improving and optimizing existing drying methods, and by maximizing quality attributes such as structure, color, flavor, nutrition, etc. In an effort to highlight quality aspect of dried foods and biomaterials, a special issue of Drying Technology [2005, 23(4)] was published. The objective of this article is to present an overview of quality attributes normally considered in the drying of food and biomaterials and highlight the recent advances in drying methods for the retention of nutritional and functional properties of fruits and vegetables.

Drying of Fruits and Vegetables: Retention of Nutritional/Functional Quality

Surface response modeling was used to characterize the effect of extrusion processing on the functional properties of corn fiber/corn starch formulations. Process parameters evaluated included screw speed (200-500 rpm), temperature (90-150°C) and pH (3-11). Generally, the water holding capacity of starch increased with increasing extrusion temperature while the water holding capacity of the fiber decreased with increasing extrusion temperature. The foam stability decreased, while the residual moisture increased, with an increase in fiber concentration. X-ray diffraction profiles indicated that extrusion did not affect fiber crystallinity. No significant changes in the ratio of soluble to insoluble fiber were found as a result of extrusion.

Twin-Screw Extrusion Modification of a Corn Fiber and Corn Starch Extruded Blend

Chemical kinetics encompasses the study of the rates at which chemical reactions proceed. The area of kinetics in food systems has received a great deal of attention in past years, primarily due to efforts to optimize or at least maximize the quality of food products during processing and storage. In formulated systems the presence of breakdown products may strongly influence the order of the reaction; the reaction may follow apparent first-order kinetics for only a given value of initial concentration. The degradation of ascorbic acid has been primarily found to follow first-order kinetics in food systems. Pyridoxamine appeared to be the vitamer with the highest complexity in its degradation mechanisms and kinetics. Myoglobin is the major heme-protein pigment that has been often overlooked in terms of kinetic stability either during thermal processing or during storage. The general principle of chemical kinetics may apply to enzymatic reactions, the phenomenon of saturation with substrate is unique to enzymatic reactions.

Reaction Kinetics in Food Systems

Comparative study on injury and recovery of Staphylococcus aureus using microwaves and conventional heating

A mathematical model defining the relation of moisture, sucrose and salt on the degree of starch gelatinization obtained from DSC, was developed. As moisture increased to 1.0g water, gelatinization rose to 85% and was complete at 1.5g water. Addition of 5% to 15% sucrose had no effect between 0.5 and about 0.8g water, but at higher moistures degree of gelatinization decreased with increasing sucrose. Salt showed a similar effect. The model covered 30% to 80% water, 0% to 15% sucrose, and 0% to 7.5% salt (R2=0.94). A test application of the model extended to rice and wheat starch, showed some correlation, but not conclusively.

Ricardo Villota

Papers

Twin-Screw Extrusion Modification of a Corn Fiber and Corn Starch Extruded Blend

Reaction Kinetics in Food Systems

Comparative study on injury and recovery of Staphylococcus aureus using microwaves and conventional heating

A Model for Quantitating Energy and Degree of Starch Gelatinization Based on Water, Sugar and Salt Contents

The Effect of Microwave Sublethal Heating on the Ribonucleic Acids of Staphylococcus aureus.