M. Marcela Góngora-Nieto

Advances in dehydration of foods

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.

Non-thermal food preservation: Pulsed electric fields

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.

Pulsed electric fields inactivation of attached and free-living Escherichia coli and Listeria innocua under several conditions

The use of pulsed electric fields (PEF) is considered as a mild process in the inactivation of microorganisms present in liquid food products. PEF treatments of Escherichia coli and Listeria innocua suspended in milk and phosphate buffer, with same pH and same conductivities, yielded to similar inactivation. Reduction rates obtained in distilled water indicated that conductivity of the food product is a main parameter in bacterial inactivation. Bacteria attached to polystyrene beads were inactivated by PEF at a greater (E. coli) or equal rate (L. innocua) than free-living bacteria. Base on the use of selective and non-selective enumeration media, no clear indications were obtained for sublethal damage of microorganisms surviving the PEF treatment. E. coli cells subjected to 60 pulses at 41 kV/cm were examined by transmission and scanning electron microscopy. Changes in the cytoplasm were observed and the cell surface appeared rough. The cells outer membranes were partially destroyed allowing leaking of cell cytoplasm.

Influence of several environmental factors on the initiation of germination and inactivation of Bacillus cereus by high hydrostatic pressure.

The influence of pH, aw, L-alanine, and fat concentration of milk on the initiation of germination and inactivation by high hydrostatic pressure (HHP) (250 mPa at 25 degrees C for 15 min and 690 mPa at 40 degrees C for 2 min) of Bacillus cereus sporulated at 20, 30 and 37 degrees C was investigated. B. cereus sporulated at the lowest temperature was found to be the most resistant to the initiation of germination and inactivation by HHP. At ambient pressure, the rate and extension of germination induced by L-alanine were also lower in B. cereus sporulated at 20 than 30 or 37 degrees C. The optimum pH for the germination and inactivation of B. cereus depended on the sporulation temperature. At 250 mPa the extent of germination for the three suspensions increased with higher pH. At 690 mPa, the pH barely affected the germination of B. cereus sporulated at 20 degrees C (3 log cycles), but the inactivation increased as the pH of the medium was lowered. After the same treatment, pressure optimally initiated the germination of B. cereus sporulated at 30 and 37 degrees C (6-7 log cycles) around neutral pH. Higher inactivation was obtained at pH 6. High concentrations of sucrose protected the spores from the germinating and inactivating effect of HHP. At aw 0.92, no germination was detected when the spores were pressurized at 250 mPa, and only 1 log cycle of B. cereus sporulated at 20 and 30 degrees C and 2 log cycles of B. cereus sporulated at 37 degrees C were germinated at 690 mPa. In addition, no inactivation was observed at aW 0.92 even at 690 mPa. The presence of L-alanine in the medium of pressurization increased the germination initiated by HHP at 250 mPa, but not at 690 mPa. A combination of 250 mPa at 25 degrees C with L-alanine (100 mM) was found to give an additive response. The initiation of germination and inactivation by HHP were not affected by the fat concentration of the milk.

Fundamentals of High-Intensity Pulsed Electric Fields (PEF)

This chapter reviews the key aspects of pulsed electric field (PEF) technology as a suitable means to pasteurize food products, and finds it a significant innovation that may be implemented in the near future for the purpose of food preservation. It involves the application of a short burst of high voltage to a food placed between two electrodes, which destroys the bacterial cell membrane by mechanical effects with no significant heating of the food. PEF technology has the potential to economically and efficiently improve energy usage, as well as provide consumers with microbiologically safe, minimally processed, nutritious, and fresh-like foods. This chapter discusses the action mechanisms of the technology by describing the important components of the PEF system and how the energy from a high-voltage power supply is stored in a capacitor and discharged through a food material contained, or flowing through a treatment chamber. A technical drawback in PEF processing—of particular relevance—is the dielectric breakdown of foods, which is characterized by a spark and evolution of gas bubbles. This makes the technology unsuitable for the pasteurization of liquid foods containing particles.

PEF-Induced Biological Changes

This chapter discusses the different biological changes induced by pulsed electric field (PEF), and how these changes take place in the cell membrane of the treated microorganisms. To analyze the membrane response to PEF, a capacitor charged by electric field pulses is considered, and an explanation is provided as to how after the membrane has reached a certain potential, it undergoes an electrical breakdown or major perturbation in its structure that leads to a permeability increase in the membrane. Some of the biological changes induced by electric fields include electropermeabilization, electrofusion, motility alteration, and microorganism inactivation. External electric fields induce transmembrane potential across cell membranes and cause electroporation and/or damage of cell organelles that lead to cell inactivation. The subsequent inactivation of microorganisms by PEF is affected by treatment conditions, treatment time, electric field strength, temperature, pulse waveshape, and pulse width; the type, concentration, and growth stage of the microbial entity; and the physical and electrical properties of the treatment media. Conductivity, ionic strength, pH, antimicrobials, the presence of particles or gas bubbles, and the dielectric properties of a medium are all important characteristics that alter the biological changes produced during PEF treatment.

PEF Inactivation of Vegetative Cells, Spores, and Enzymes in Foods

This chapter discusses how pulsed electric field (PEF) inactivates microbes, enzymes, and spores in model and real foods. Microbial challenge tests are conducted to determine the effect of electric fields on the inactivation kinetics of selected microorganisms inoculated in real or model foods. The tests are conducted by applying an electric field, which causes the inactivation of a maximum number of microorganisms without an electrical breakdown of the food. The results show that the intensity of the electric field, treatment time, and number of pulses affect the inactivation of Saccharomyces cerevisiae suspended in different treatment media. The direct relationship between the effects of the pulse treatment on cell inactivity and membrane damage—as measured by poor to no spheroplast formation—demonstrates that the death of Staphylococcus aureus spp. is a result of membrane damage. The high inactivation levels obtained over enteric and pathogenic bacteria such as E. coli and Salmonella spp. give confidence in the pasteurization abilities of PEF. This chapter presents PEF studies of many other microorganisms and describes in detail the enzymatic activity reduction and spore inactivation phenomena. It also demonstrates the usefulness of kinetic models to compare the sensitivity of different microorganisms and species under the same treatment conditions.

Chapter 3 – Biological Principles for Microbial Inactivation in Electric Fields

This chapter reviews microbial inactivation mechanisms in electrical fields—applicable to pulsed electric field (PEF) technology—and provides experimental evidence that support some of these theories. The primary effect of biological cell exposure to electric fields is an increase in the transmembrane potential, which results in electroporation. The formation of pores in the cell membrane leads to reversible (electrical) or irreversible (mechanical) breakdown and depends on the magnitude of the transmembrane potential. Pore formation involves a two-step mechanism of initial perforation followed by pore expansion; the entire process depends on the electric field intensity and pulse duration. If the electric field is stronger than the critical transmembrane potential, membrane cracks may develop and a large piece of the membrane may even get ripped off from the cell. The cause of hemolysis or rupture of the membrane is caused by osmotic imbalance generated by the leakage of ions and small molecules. An alternative theory suggests that pores are formed due to electro-conformational changes in lipid or protein molecules. Based on recorded observations of electric field-induced structural changes in microbial cells and membranes of different microorganisms, this chapter lends support to some of the reviewed theories although the microbial inactivation principle seems to vary from microorganism to microorganism and species to species.

Design of PEF Processing Equipment

This chapter reviews the key elements for designing a pulsed electric field (PEF) system and describes the main components that are involved in the production of high-voltage pulses. It addresses the importance of high-voltage repetitive pulser, switches, treatment chamber(s), cooling system(s), voltage and current measuring devices, control units, data acquisition system, and packaging system. A pulsed power supply is used to obtain high voltage from low-utility level voltage, and the former is used to charge a capacitor bank and switch to discharge energy from the capacitor across the food in the treatment chamber. For larger scale operations, continuous chambers are more efficient as compared to static ones. This chapter describes the designs of various continuous chambers such as those with ion-conductive membranes separating the electrodes and food, those with baffles, continuous cofield flow PEF chambers, and the modified coaxial treatment chambers. The food temperature is maintained by circulating constant temperature water through the cooling jackets built into the electrodes. Electrical parameters such as voltage and current pulse waveforms applied during PEF treatments are recorded via a digital acquisition system. After PEF treatment, food is aseptically packaged using a range of materials and stored. The effectiveness of aseptic packaging has been demonstrated by the extended shelf-life of several PEF products.

Food Processing by PEF

This chapter describes the different stages involved in the pasteurization of food products by pulsed electric field (PEF) technology. For each food product, PEF processing design includes four stages: microbial challenge tests to establish the amount of treatment needed; treatment of the native microbial population in the food; microbial, chemical, and physical analyses of the food after processing; and sensory and shelf-life studies of the processed food product. Because PEF is a non-thermal process, products such as fruit juices, milk, and beaten eggs have been under extensive research to implement the process at an industrial level. Microbial analyses consist of aerobic plate counts, detection of yeasts and molds, acidic bacteria, Salmonella and coliform count, the Moseley quality test, and the Listeria test. Chemical analyses of processed foods include estimation of their protein, carbohydrate, fat, and ash contents. Physical analyses of foods include determination of pH, acidity, water activity, moisture content, color changes, and viscosity. Sensory evaluation and shelf-life studies are conducted to determine the relative acceptance of foods processed by electric field treatment as compared to foods available at the supermarket processed by other methods. Stored foods are tested periodically for their microbiological quality and physical and chemical changes.