Dietrich Knorr

Antibacterial action of chitosan

Abstract The antibacterial action of chitosan hydroglutamate (CH), chitosan lactate (CL) and chitosan derived from fungal mycelia was examined against both gram‐negative and gram‐positive bacteria. Plate counts indicated inactivation rates of one‐ to five‐log‐cycles within one hour. Fungal chitosan had significantly less antibiotic effect than CH and CL. The antibacterial action of CH and CL was very similar and shown to be concentration dependent with 0.1 mg/mL more effective than 2.0 and 5.0 mg/mL. When CH (or CL) and polygalacturonate were added to cell suspensions, death was prevented, possibly indicating that chitosan complexed with polygalacturonate could not penetrate the cell or disrupt the membrane. Leakage of intracellular components caused by chitosan was determined by exposing lactose‐induced Escherichia coli to chitosan with assay for s‐galactosidase activity indicating that cell permeabilization occurred more extensively at the low chitosan concentrations. Microscopic examination showed that...

Opportunities and Challenges in High Pressure Processing of Foods

Consumers increasingly demand convenience foods of the highest quality in terms of natural flavor and taste, and which are free from additives and preservatives. This demand has triggered the need for the development of a number of nonthermal approaches to food processing, of which high-pressure technology has proven to be very valuable. A number of recent publications have demonstrated novel and diverse uses of this technology. Its novel features, which include destruction of microorganisms at room temperature or lower, have made the technology commercially attractive. Enzymes and even spore forming bacteria can be inactivated by the application of pressure-thermal combinations, This review aims to identify the opportunities and challenges associated with this technology. In addition to discussing the effects of high pressure on food components, this review covers the combined effects of high pressure processing with: gamma irradiation, alternating current, ultrasound, and carbon dioxide or anti-microbial treatment. Further, the applications of this technology in various sectors—fruits and vegetables, dairy, and meat processing—have been dealt with extensively. The integration of high-pressure with other matured processing operations such as blanching, dehydration, osmotic dehydration, rehydration, frying, freezing / thawing and solid-liquid extraction has been shown to open up new processing options. The key challenges identified include: heat transfer problems and resulting non-uniformity in processing, obtaining reliable and reproducible data for process validation, lack of detailed knowledge about the interaction between high pressure, and a number of food constituents, packaging and statutory issues.

Review: Potential of High Hydrostatic Pressure and Pulsed Electric Fields for Energy Efficient and Environmentally Friendly Food Processing

The application of emerging, novel processing techniques such as high hydrostatic pressure or pulsed electric fields can be utilized to replace, enhance or modify conventional techniques of food production. In addition to quality improvements and consumer benefits by gentle microbial inactivation and improvement of mass transfer processes, their potential to improve energy efficiency and sustainability of food production will be discussed within this review.

Strategies for the improvement of secondary metabolite production in plant cell cultures

Plant cell and tissue cultures can be established routinely under sterile conditions from explants, such as plant leaves or stems. Strain improvement, methods for the selection of high-producing cell lines, and medium optimizations can lead to an enhancement in secondary metabolite production. However, most often trials with plant cell cultures fail to produce the desired products. In such cases, strategies to improve the production of secondary metabolites must be considered. One of the main problems encountered is the lack of basic knowledge of the biosynthetic routes, and mechanisms responsible for the production of plant metabolites. Where the productivity of the desired metabolites is limited by the lack of particular precursors, biotransformation using an exogenous supply of biosynthetic precursors may improve the accumulation of compounds. Feedback inhibition of metabolic enzymes as well as inhibition of membrane transport can be eliminated by the accumulation of synthesized products in a second phase introduced into the aqueous medium. Organ cultures often have sites of synthesis and storage of secondary metabolites in separate compartments. Elicitors, compounds triggering the formation of secondary metabolites, can be abiotic or biotic. Natural elicitors include polysaccharides such as pectin and chitosan, which are also used in the immobilization and permeabilization of plant cells. Immobilization provides several advantages, such as continuous process operation, but for the development of an immobilized plant cell culture process natural or artifically induced secretion of the accumulated product into the surrounding medium is necessary.

Recent developments in osmotic dehydration: methods to enhance mass transfer

Abstract Osmotic dehydration, due to its energy and quality related advantages, is gaining popularity as a complimentary processing step in the chain of integrated food processing. Generally, osmotic dehydration being a slow process, there has been a need for additional ways to increase the mass transfer without adversely affecting the quality. This gave the required motivation for many recent advances in this area. However, certain constraints still exist for the wide industrial adoption of osmotic dehydration, which need to be addressed in current and future research in the area. In order to compare the results of various investigators, there is a need to express research results in terms of more fundamental parameters like diffusion coefficient. Consequently, suitable methods to estimate such parameters in various foods of different size and geometry are discussed. The mechanism of osmotic dehydration proposed recently is described. Various methods to increase the rate of mass transfer, such as application of high hydrostatic pressure, high electrical field pulses, ultrasound, vacuum and centrifugal force are also presented.

Preservation of liquid foods by high intensity pulsed electric fields—basic concepts for process design

Abstract In excess of a critical transmembrane potential ΔϕM of −1 V produced by high intensity pulsed electric fields a rapid electrical breakdown and local conformational changes of cell membranes occur which result in a drastic increase in permeability and an equilibration of the electrochemical and electrical potential differences of the cell plasma and the extracellular medium. As irreverible membrane permeabilization impairs most vital physiological control systems, high intensity pulsed electric fields may be applied as a highly effective process for the microbial decontamination of liquid foods. The efficiency of the treatment is largely influenced by the inherent properties of the foods and of the spoiling microorganisms. In addition a number of technical limitations have to be considered. In this review an approach is presented which reduces the diversity of parameters that affect microbial inactivation during pulsed power treatment. In particular, the required total specific energy input is discussed.

Food application of high electric field pulses

Abstract Non-thermal physical processes are evolving as potential alternatives to thermal and chemical unit operations in food processing. The use of high electric field pulses (HEFP) is such an emerging process and consequently deserves attention. Its instant distribution throughout a conductive food system, its short treatment times and its use in continuous processes with very little heating of the medium make HEFP treatment an attractive candidate for applications in cell disruption, metabolite release and food preservation.

Impact of temperature on lethality and energy efficiency of apple juice pasteurization by pulsed electric fields treatment

The applicability of pulsed electric fields as a non-thermal preservation process for liquid food decontamination has been shown in several studies. However, high costs of operation due to the occurrence of a high amount of dissipated electrical energy inhibited an industrial exploitation so far. In this study the focus was put on improving energy efficiency of this process for pasteurization of apple juice inoculated with Escherichia coli by investigating the relation between achieved reduction in survivor count and electric field strength and treatment temperature. An empirical mathematical model was derived to predict the required input of electrical energy for a given inactivation. Using synergistic effects of elevated treatment temperature of 35–65 °C on microbial inactivation the energy consumption could be reduced from above 100 to less than 40 kJ kg−1 for a reduction of 6 log cycles and the need to preheat the juice before treatment provided a possibility to recover the dissipated electrical energy after treatment, leading to a drastic reduction in operation costs. To evaluate the thermal load of the product the pasteurization unit (PU) and the cook value, key benchmarks for the thermal load, were used to compare PEF and conventional heat treatment.

Potential food applications of high-pressure effects on ice-water transitions

Pressure depresses the freezing point of water and the melting point of ice, as well as enabling various high-density forms of ice to be obtained. These effects of pressure on the solid-liquid phase diagram of water have several potential applications in food technology, including pressure-assisted freezing, pressure-assisted thawing and non-frozen storage at low temperature (under pressure). Studies that have been published in these and in related areas are reviewed, and the potential applications and limitations are highlighted.

Antimicrobial effect of water‐soluble chitosans with high hydrostatic pressure

Abstract Two commercially available water‐soluble chitosan salts, chitosan lactate and chitosan hydroglutamate, were examined for antagonistic effect against Escherichia coli V517, Staphylococcus aureus MF‐31 and Saccharomyces cerevisiae 15. Significant inactivation of each population was evident within 2 min of incubation with Chitosan. S. cerevisiae was the most sensitive of the microorganisms examined. Concentration effects varied but chitosan hydroglutamate was usually the more effective of the chitosans for inactivation of these microorganisms. Application of high hydrostatic pressure (2,380 atmospheres) to chitosan‐treated cultures of E. coli V517 or S. aureus MF‐31 resulted in additional inactivation but an amplified or synergistic effect was not found.