Usha R. Pothakamury

Fundamental aspects of controlled release in foods

Abstract The fundamental equations governing the controlled release of active ingredients and the application of controlled-release technology in food systems are reviewed in this article. The method of microencapsulation, among others, can be applied to achieve controlled release in foods. Some of the release mechanisms employed in the food industry involve one or a combination of the following stimuli: a change in temperature, moisture or pH; the application of pressure or shear; and the addition of surfactants. Encapsulation is a method of protecting food ingredients that are sensitive to temperature, moisture, microorganisms or other components of the food system. Such food ingredients include flavors, sweeteners, enzymes, food preservatives and antioxidants, and are encapsulated using carbohydrates, gums, lipids and/or proteins. With a properly designed controlled-release delivery system, the food ingredient is released at the desired site and time at a desired rate.

Inactivation of Escherichia coli by combining pH, ionic strength and pulsed electric fields hurdles

Abstract 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.

Nonthermal pasteurization of liquid foods using high‐intensity pulsed electric fields

Processing foods with high-intensity pulsed electric fields (PEF) is a new technology to inactivate microorganisms and enzymes with only a small increase in food temperature. The appearance and quality of fresh foods are not altered by the application of PEF, while microbial inactivation is caused by irreversible pore formation and destruction of the semipermeable barrier of the cell membrane. High-intensity PEF provides an excellent alternative to conventional thermal methods, where the inactivation of the microorganisms implies the loss of valuable nutrients and sensory attributes. This article presents recent advances in the PEF technology, including microbial and enzyme inactivation, generation of pulsed high voltage, processing chambers, and batch and continuous systems, as well as the theory and its application to food pasteurization. PEF technology has the potential to improve economical and efficient use of energy, as well as provide consumers with minimally processed, microbiologically safe, nutritious and freshlike food products.

Effect of Growth Stage and Processing Temperature on the Inactivation of E. coli by Pulsed Electric Fields

The effect of growth stage and processing temperature on the inactivation of Escherichia coli subjected to pulsed electric fields was studied. Simulated milk ultrafiltrate (SMUF) inoculated with E. coli was subjected to high-intensity exponentially decaying or square-wave pulses with a field strength of 36 kV/cm and pulse duration of 2 μs at selected temperatures ranging between 3 and 40°C. The rate of inactivation increased with an increase in the processing temperature. Furthermore, square-wave pulses were more lethal than exponentially decaying pulses. At 7°C after 100 μs, square-wave pulses produced a 99% decrease while exponential decaying pulses produced a 93% decrease in bacterial cell population. Cells harvested at lag, log, and stationary phases were subjected to 2 and 4 pulses with an electric field intensity of 36 kV/cm at 7°C. Logarithmic-phase cells were more sensitive than stationary- and lag-phase cells to the pulsed electric field treatment.

Inactivation of Escherichia coli and Staphylococcus aureus in model foods by pulsed electric field technology

Abstract Inactivation of microorganisms exposed to high-voltage pulsed electric fields is a promising non-thermal food preservation technology. This paper demonstrates and validates the inactivation of Escherichia coli , a Gram-negative bacterium and Staphylococcus aureus , a Gram-positive bacterium, subjected to high-voltage electric field pulses. A four-log cycle reduction in microbial population is achieved in model foods such as simulated milk ultrafiltrate (SMUF) with a peak electric field strength of 16 kV/cm and 60 pulses with a pulse width ranging between 200 and 300 μs. The temperature of the cell suspension was kept below the lethal temperature, demonstrating that inactivation is not due to thermal effects induced by the pulses of high-voltage electricity. Thermal food preservation causes undesirable changes in the physical character, quality and nutrient content of foods. Non-thermal preservation techniques minimize the undesirable changes in foods. A comparison between the inactivation of microorganisms by high-voltage pulsed electric fields and thermal methods of food preservation is also discussed.

Ultrastructural changes in Staphylococcus aureus treated with pulsed electric fields / Cambios ultraestructurales en Staphylococcus aureus sometida a campos eléctricos pulsantes

Early stationary phase cells of Staphylococcus aureus were inoculated into a model food, simulated milk ultrafiltrate (SMUF) and subjected to 16, 32, and 64 pulses at electric field intensities of 20, 40 and 60 kV/cm at 13 °C. In addition temperatures of 20, 25 and 30 °C were also tested with 32 pulses and an electric field of 60 kV/cm. The temperature of the SMUF increased by 1-2 ° C at the end of the 64 pulses. Cells subjected to 64 pulses at 20, 40 and 60 kV/cm were observed for ultrastructural changes using scanning and transmission electron microscopy techniques. The cell surface was rough after treatment with electric field when observed by scanning electron microscopy (SEM). The cell wall was broken, and the cytoplasmic contents were leaking out of the cell after exposure to 64 pulses at 60 kV/cm when observed by transmission electron microscopy (TEM). The breaking of the cell wall is an indication of electro-mechanical breakdown of the cell. The increase in inactivation with an increase in the electric field strength can be related to the increase in the damage to the cells. Cells subjected to 32 pulses at 60 kV/cm and 13, 20 or 25 °C were compared microscopically with the untreated control cells. Cells subjected to heat treat ment (10 min, at 66 °C) were compared with electric field-treated and untreated control cells. Although important changes were observed in the protoplast, no cell wall breakdown was observed in heat-treated cells when compared to the electric field-treated cells. This result indi cates a different mechanism of inactivation of cells with heat treatment.

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.