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Food dehydration is still one of the most relevant and challenging unit operations in food processing, although the art of food preservation through the partial removal of water content dates back several centuries. This article provides essential information on the fundamental, including psychrometry, and applied engineering aspects of food dehydration with up-to-date available commercial applications. The evolution of drying technology, divided into four generations, is thoroughly reviewed, from tray drying to the combination of some drying technologies (the hurdle technology approach in drying) in order to optimize the process in terms of final food quality and energy consumption. The study of each generation covered numerous examples of different dryers, including their principles of operation, basic configurations and most common applications, as well as their main advantages and disadvantages.

Every food manufacturing and processing operation has inherent risks affecting the safety of food products. Non-thermally processed foods are not exempt of those risks. This chapter provides the reader with an overview of various non-thermal technologies (e.g. irradiation, pulsed electric fields, high hydrostatic pressure, intense pulsed lights, membrane filtration and hurdle technology). Each of these technologies has specific critical process parameters that must be monitored as part of critical control points. In-depth understanding of these technologies is the key while considering their implementation. The main challenge on non-thermal processes is standardization when compared to thermal treatments. Non-thermal processes seem to be product specific resulting in additional research work to define process parameters. Nevertheless, a significant amount of research data are available and specific process conditions can be found in the literature.

Liquid egg inoculated with Escherichia coli was exposed to a 26 kV/cm pulsed electric field with 2 and 4 μs pulse duration, 1.25 and 2.50 Hz pulsing rates, up to 100 pulses/unit volume, and stepwise and continuous recirculation treatment schemes while maintaining a bulk temperature below 37C. The inactivation of E. coli was a function of the pulse duration and the number of pulses. The destruction of Escherichia coli in liquid egg followed a first order kinetic and the treatment was more effective when the applied pulses were of a 4 its pulse duration. A 6D reduction was obtained for viable E. coli using both pulsing rates and treatment schemes with no protein coagulation.

The use of electric discharges to inactivate microorganisms and enzymes in food products has evolved since the 1920s from the ‘ElectroPure process’ (ohmic heating process) to the use of high-intensity pulsed electric fields in the 1990s. The non-thermal inactivation of microorganisms and enzymes using electric fields was demonstrated in the 1960s with a variety of microorganisms suspended in simulated food systems. A variety of liquid foods and beverages, including orange, apple and peach juices, pea soup, beaten eggs and skim milk, has been successfully processed during the 1980s and 1990s by several research groups. Little by little, the food industry is demonstrating increasing interest in this promising emerging technology; furthermore, it is expected that it will soon be adopted to process several liquid food products.

Pea soup inoculated with 10 7 CFU/mL of Escherichia coli, Bacillus subtilis, or a mixture of the two organisms was treated with pulsed electric fields (PEF) of selected intensities of 25, 28, 30, and 33 kV/cm. The pulsing rate was adjusted to 2.9, 4.3, or 6.7 Hz to achieve 10 or 15 pulses per pass while using either 0.5 L/min or 0. 75 L/min flow rate in a continuous treatment chamber. Two passes were selected to reach 20 or 30 pulses under selected conditions. Inactivation of Escherichia coli and Bacillus subtilis suspended in pea soup increased with increases in intensity of the electric field, number of pulses, and pulsing rate. A reduction of 6.5D was obtained at 33 kV/cm, 0. 5 L/min, 4.3 Hz, and 30 pulses with E. coli and 5.3D for B. subtilis, each microorganism alone. Meanwhile, reductions of up to 4. 8D were observed when the pea soup containing a mixture of the microorganisms was treated with PEF of 30 kV/cm at 6.7 Hz and 0. 75 L/min. In general, pea soup inoculated with a mixture of the microorganisms and exposed to PEF greater than 30 kV/cm and bulk temperatures over 53 C exhibited inactivation greater than 2.0D while less than 1.6D was observed when the temperature was below 53C.

The use of pulsed electric fields is reported as a nonthermal process in the inactivation of bacteria and yeast. The inactivation of microorganisms is caused mainly by an increase in their membrane permeability due to compression and poration. Up to 2.2 log reductions in plate counts are observed when both pH and electric field are modified: pH from 6.8 to 5.7 and electric field from 20 to 55 kV/cm. Similar results are obtained when the ionic strength is reduced from 168 mM to 28 mM. The electric field and ionic strength are more likely related to the poration rate and physical damage of the cell membranes, while pH is more likely related to changes in the cytoplasmic conditions due to the osmotic imbalance caused by the poration. In this context, pulsed electric fields can be considered a hurdle which, combined with additional parameters such as pH, ionic strength, temperature and antimicrobial agents, can be effectively used in the inactivation of microorganisms.

Dos propiedades termodinámicas, el calor de sorción (Qs) y el cambio en energía libre de Gibbs (∆G) fueron evaluadas a diferentes temperaturas (5, 25, 30, 35 y 55°C) en pulpa de piña liofilizada. Se encontró que: 1) el calor de sorbición sigue la relación de Clausius-Clapeyron; 2) la forma de la curva que representa la relación de calor contra humedad es sigmoide; 3) los calores de sorbición obtenidos de los datos de adsorción son más altos que los obtenidos de los datos de desorción; 4) los cambios en energía libre están linealmente relacionados con el contenido de humedad en equilibrio. Estos dos parámetros son muy importantes cuando se estudia la estabilidad de un alimento, ya que conociendo el valor de cualquiera de ellos es posible predecir la actividad de agua a cualquier temperatura.

Se estudiaron dos procesos de deshidratación osmótica para piña y papaya: inmersión en sirop de sacarosa a 70° Brix y en sirop de sacarosa a 70° Brix con 1,000 p.p.m. de sorbato de potasio y 150 p.p.m. de metabisulfito de sodio. Los aditivos no sólo actuaron como preservativos sino que, de alguna manera, ayudaron a aumentar la fuerza motriz de las soluciones osmóticos, disminuyendo la actividad del agua de los productos finales a base de fruta y aumentando el pH final de las soluciones sin afectar el de las frutas. La piña mostró una tendencia mayor que la papaya hacia la absorción del azúcar y la papaya indujo menos cambios químicos en las soluciones osmóticos que la piña. Por lo tanto, las soluciones en que se sumergen las frutas pueden volverse a usar más veces con papaya que con piña.

EFECTIVIDAD DE PRESERVATIVOS DURANTE EL ALMACENAMIENTO EN PRODUCTOS DE PINA DE HUMEDAD INTERMEDIA

Se estudió el efecto de la escaldadura sobre la vida útil y las características químicas y físicas de dos productos de humedad intermedia de piña de la variedad Española Roja. La escaldadura se efectuó antes de sumergir las rodajas de piña en un almíbar de sacarosa con sorbata de potasio y bisulfito de sodio. Los productos se empacaron en bolsas de polipropileno transparentes y almacenaron a la temperatura ambiente (28° C a 30° C.) fuera del alcance de la luz solar. Los productos escaldados presentaron una mayor estabilidad en cambios químicos y físicos que los no escaldados. La vida útil de los productos escaldados fué más de 30 días.

There are other drying techniques in addition to the ones already discussed in Chapters 5, 6, 7, and 8 based on similar concepts and used in the food industry. This chapter covers drying techniques such as sun drying, vacuum drying, drum drying, microwave drying, extrusion cooking, fluidized bed drying, and pneumatic drying. In addition, this chapter includes some important aspects related to the packaging of dried foods: mechanical damage, permeability, temperature effect, light transmission and, compatibility of packaging materials.

Air—water mixtures are used in most drying operations. The thermodynamic properties of this type of mixture are reported elsewhere (Himmelblau, 1982), but we consider it important to include in this chapter fundamental aspects of an air—water mixture before entering into the details of drying operations such as spray drying, fluidized bed drying, or atmospheric drying.

The main purpose of food dehydration is to extend the shelf life of the final product. The process meets this objective by reducing the moisture content of the product to a level that limits microbial growth and chemical reactions. Hot air is used in most drying operations and air dryers have been in use for several years around the world.

The preservation of the initial cellular structure of the material being dried is, in most cases, a function of the process applied to the food. This consideration applies not only to drying but also to blanching and freezing. In the removal of water from high-moisture containing prod­ucts such as fruits and vegetables, it is important to remember that cell membranes may or may not been dam­aged during processing (Le Maguer, 1987).

CANNING AND QUALITY EVALUATION OF SWEET POTATO DESSERT

Una manera objetiva para determinar la madurez de las frutas es en función de la textura de la cáscara. Con un texturómetro se midió el esfuerzo de penetración en función del tiempo de maduración para la piña (variedad Española roja). Las frutas se maduraron bajo condiciones controladas de temperatura (65° F) y humedad relativa (85%). Se encontró una relación entre la dureza de la cáscara de la fruta y la madurez. En el caso de la piña, además hay diferencias significativas a lo largo del fruto que coinciden con la manera tradicional de clasificar la madurez de la piña en términos del color de la cáscara y su proporción en la fruta.