Topic 3. Non-thermal and combined preservation (HURDLE) technologies
Key concepts Overview of non-thermal preservation technologies Irradiation Principles and practice of high pressure processing Principles and practice of pulsed electric field processing Methods of surface decontamination Combining treatments for improved process and quality outcomes
Readings for this section:
Food Processing Faraday (date unknown). Extension of Product Shelf-life for the Food Processor, pp. 28 36. Gould, G.W. (2001). New processing technologies: an overview. Proceedings of the Nutrition Society 60 , 463474 [O/L; provided as a reading for module 1] Ross, A.I.V, Griffiths, M.W., Mittal, G.S. and Deeth, H. (2003). Combining nonthermal technologies to control foodborne microorganisms. International Journal of Food Microbiology 89 , 125 138 [O/L] Scott, J.S. and Pillau, S. (2004). IFT scientific status summary: irradiation and food safety. Food Technology 58 , 48 55. [O/L]
5.1 Overview of non-thermal preservation processes
Heat treatment of foods inevitably brings with it some changes in the texture, aroma, flavour or appearance of the product. To better meet consumers demands for fresher products with more natural taste and appearance, and with fewer additives, food researchers and manufacturers have investigated the potential of many non-thermal treatments for assuring adequate shelf life and safety of foods. Note non-thermal is put in quotation marks here to signal some of the physical processes we will be discussing do in fact have a thermal component and this may contribute to the preservation effect obtained.
Non-thermal processes may form part of a minimal processing strategy for preserving the food and extending its shelf life. As noted in module 1, minimal processing describes a process or combination of processes designed to minimally influence the quality characteristics of a food whilst, at the same time, giving the food sufficient shelf-life during storage and distribution(Ohlsson 2002). Minimal processing systems can also generally be expected to better preserve the nutritional and bioactive components in the food product, especially where these have been deliberately increased through fortification or introduced in the development of a functional food product (Huis int Veld, 1996; Ohlsson, 2002).
There are a large number of non-thermal processes with potential applications in food preservation and processing (e.g. see Gould, 2001, Ohlsson & Bengtsson 2002). These include treatments whose basis is primarily physical, chemical or biochemical, advanced packaging technologies, and the combination of these.
This module focuses on those technologies that primarily have a physical or chemical basis and introduces the use of combined treatments for food preservation. Advanced packaging
systems and their combined use with other minimal processing techniques are considered in a later module.
Of those processes with a physical basis, relatively few have been commercialised and none has yet achieved widespread use (Gould 2001; Ohlsson & Bengtsson 2002, Karel & Lund 2003). Karel and Lund (2003) also note that in many physical treatments, some temperature rise also occurs, but this is (generally) not the principle mechanism of action of the process. Similarly, chemical changes (e.g. change in pH, or generation of charged species) may also occur as a result of physical processing and it is more difficult to separate the consequences for safety and quality of these chemical effects from the physical effects for many of these alternative processes.
Some of the available non-thermal technologies and examples of their application are summarised in Table 5.1.
Table 5.1: Non-thermal technologies (adapted from Ohlsson & Bengtsson 2002; Karel & Lund 2003)
Process Example applications Status
Physical Irradiation Sterilization of spices, ground meats or poultry; microbial reduction, pest control and increasing storage life of horticultural products
High pressure processing (HPP/ UHP)
Inactivation of microorganisms and enzymes in juice, processed meats, shellfish and fruit and vegetable products. Note that it is possible some enzyme systems may be activated by HPP.
Pulsed electric fields (PEF)
Inactivation of microorganisms and enzymes in juice and other food liquids
Ultrasound Inactivation of microorganisms and enzymes in liquid foods; improving oil yields
Ultraviolet light (UV-C)
Surface decontamination and sterilisation of packages; sterilisation of water
Pulsed white light Surface decontamination Pilot scale Oscillating magnetic fields
Inactivation of vegetative flora in liquid and packaged foods
Removal of cells from liquid foods by membrane filtration
Bactofugation Removal of cells from liquid foods by centrifugation
Bio/chemical Natural antimicrobials Partial inactivation of vegetative flora, can be linked with active packaging systems
Antioxidants Partial inactivation of vegetative flora, can be linked with active packaging systems
Chemical decontaminants Surface decontamination of meats, fish, dairy products, and fresh produce
- Commercial for other applications (e.g. degassing, foam control, laboratory-scale homogenisation)
A few of these processes are considered in more detail in the following sections.
5.2.1 Radiation sources and doses
The preservation opportunity offered by ionizing radiation has been one of the most intensively studied and controversial aspects of modern food process technology. Irradiation has been intensively researched for over 50 years and the physical and chemical changes that occur in irradiated foods are well documented. The scientific consensus is that the technology is safe (e.g. see Scott & Pillau 2004) but consumers and consumer organisations have usually been less convinced of its safety, except for those situations where there are no alternative acceptable preservation options. This has long been so for satisfactory disinfection of spices and irradiation is now increasingly accepted, especially in the USA, to address the specific challenge posed by new or serious pathogens, e.g. E. coli OH157 in ground beef.
Three types of ionizing radiation can potentially be applied to food materials (Karel & Lund 2003, Scott & Pillau 2004): gamma () rays emitted by radioactive isotopes of cobalt-60 and caesium-137, electron beams (or rays) emitted from a hot cathode and accelerated to high velocities by an accelerator (E-beam) machine, and X-rays from machine sources
Gamma ray machines offer the benefits of deeper penetration compared to the other sources, enabling pallet loads of product to be treated, but the radioactive materials required are lethal to humans and therefore installations require special construction and security measures. E-beam (and X-ray) machines do not contain radioactive material but outputs of ozone from these still present an environmental concern and are regulated. The penetration depth of electron beams is limited to ~ 3 cm (Ohlsson & Bengtsson 2002), however irradiation from both sides and beam scanning permit wider packages to be processed, and more uniformly (Karel & Lund 2003).
Energy is carried in the form of photons which interact with food materials in various ways that result in the adsorption of energy by the food system. The energy input associated with the radiation dose is measured in rads or Grays: 1 rad = 100 ergs.g- 1 Gray (Gy) = 100 rads (this is equivalent to an energy input of 1 Joule.kg-1)
The required radiation doses for food preservation are in the kGy range; as a point of reference, lethal doses for humans are only in the range of 2 10Gy (microorganisms and spores will rule the world after a nuclear war, with cockroaches!).
5.2.2 Effects of radiation
The major effect of energy input by radiation is the splitting of water and other chemical bonds this is called radiolysis. The energy input does not cause the food to itself become radioactive in any sense. Radiolysis gives rise to a range of reactive chemical species; as water is the most prevalent component in most food systems, these include (Karel & Lund 2003): excited water (H 2 O*) free radicals OH. and H.
aqueous electron eaq and H 2 O+
These react with each other and other components of the food system. Common products include molecular hydrogen, oxygen, hydrogen peroxide (H 2 O 2 ) and hydrogen and hydroxyl radicals. Hydrogen peroxide and hydroxyl radicals are particularly reactive and can promote: extensive disruption of bonds within nucleic acid chains, contributing to cell death, scission (cutting) of CH, -SH and NH bonds in food polymers, leading to further radical formation, which promote: o lipid oxidation reactions, o inactivation of enzymes, o cross-linking of proteins, o losses of nutrients, particularly among vitamins B 1 , A, C and E, and o some changes in texture and colour.
The extent of these effects on the preservative action and other quality changes depends on the (Karel & Lund 2003): (1) Water content: the lethal effects on microorganisms are greater in more dilute media. (2) Water state: lowering the amount of unfrozen water when treating frozen foods has a protective effect. (3) Temperature: the effects are greater at higher temperatures. (4) Composition: the pH, oxygen content and some chemical components affect the sensitivity of cells to radiation. (5) The extent of any pretreatments to promote spore germination, which increases their sensitivity to radiation.
In common with the range of heat treatment options available, various process outcomes can be achieved by ionizing radiation as summarised in Table 5.2.
Table 5.2: Uses of ionizing radiation in food preservation
Process Effect Typical dose (kGy) Radappertization Sterilization of foods in hermetically sealed containers, i.e. killing vegetative microorganisms and spores as an alternative to retorting, especially for military and space travel uses
10 - 50
Radurization Reduction in the concentration of spoilage organisms, i.e. a process equivalent to pasteurisation
Radicidation Inactivation of non-spore forming pathogens. A milder treatment than radurization.
0.1 - 8
Disinfestation Death of insect pests associated with storage of grains and fresh produce
0.1 - 2
Postharvest control Inhibition of sprouting, control of rots and slowing of senescent changes in horticultural products
- Typically to 1 kGy for vegetables and 2 kGy for fruit
5.2.3 Lethal dose
The lethal effect of ionizing radiation on microorganisms can be described using a simple first order function of the radiation dose (D ), analogous to thermal processing (Fig. 5.1):
D 0 is a dose lethality constant, which depends on the organism and the irradiation
conditions. This is more commonly expressed asD 10 , the dose required to reduce the
Typical values of D 0 for vegetative cells are in the range 0.1 2 kGy and for spore
formers, 0.3 > 6 kGy (Fig.5.1; note 1 kGy = 1 x 10^4 rads).
Fig. 5.1 Examples of microbial dose response curves (Karel & Lund 2003)
Approved applications and permitted dosages, and some of the possible safety and quality issues associated with irradiation, are summarised in Scott & Pillau (2004)
5.3 High pressure processing (HPP)
High pressure processing, or ultra-high pressure processing (UHP), is an emerging commercial process for food preservation and texture modification.
In HPP, food materials are packaged in flexible (usually high barrier) pouches and batch processed in cylindrical pressure vessels of up to 215 L. These are water filled and pressures of 400 700 MPa (i.e. ~ 4,000 7,000 atmospheres) are applied using a high pressure pump. Compression to such high pressures has a wide range of physical, chemical and biological effects, which can be positive or negative with respect to food quality (Table 5.1)
Some of the key characteristics of the process and changes which occur include (Knorr 1999): (1) Pressure acts instantaneously and uniformly throughout the food material and the time to reach the final process condition is much less than that in thermal processing. (2) There is an inevitable temperature rise associated with HPP, typically about 2.5 3C per 100 MPa (depending on the foods composition). (3) A significant reduction in volume results (typically about 15%): this compaction may disrupt some metabolic processes and structures leading to cell death or enzyme inactivation, but can also activate some (enzyme) systems. Residual or enhanced enzyme activity can result in undesirable changes post HPP treatment. (4) Retention of flavour and colour is generally better than for thermal treatment; both desirable (or minimal) and adverse texture changes can occur, depending on the food material processed. Irreversible water loss can occur from fresh fruits and vegetables and generally quality is best maintained for processed foods (e.g guacamole, ham). (5) The pH generally is reduced during processing due to increased solubility of CO 2 at high pressures.
The preservative action of HPP is attributed primarily to disruption of enzyme or membrane structures and the change in pH.
Inactivation of enzymes and cells is a function of time and pressure, analogous to thermal processing. This is often modelled as a first order process:
t k t N
where kP is the rate constant for pressure inactivation (s-1) and D 0 is the pressure-induced
decimal reduction time. Lines of equal effects can be drawn on a t-P diagram, in a similar fashion to those for thermal preservation (Fig. 5.2).
Fig. 5.2: Lines of equal process effect for HPP of a milk product (from Rademacher & Hinrichs 2001).
The foods pH has an important influence on the outcome of HPP and sterility can be better assured when the pH is less than 4.5, as for thermal treatment. Verification of the preservation effect is a key issue for HPP that is currently being addressed by various research and regulatory agencies.
HPP is commercially applied to preservation of: guacamole (based on avocado puree), fruit and vegetable juices, processed meats (e.g. hams), jams and related products, and shucking of oysters.
Typical commercial conditions are 400 – 600 MPa for 5 30 min, at a nominal (starting) temperature of 20C (Ohlsson & Bengtsson 2003).
An excellent source of information on HPP is the website run by the HPP research group at Ohio State University: http://grad.fst.ohio-state.edu/hpp/
5.4 Pulsed electric field (PEF) processing
PEF processing involves exposing a liquid food product in contact with two electrodes to pulses of high voltage for short durations. Typically, the electric field strength will be 15 40 kV/cm and the pulse duration < 10 ms. The process effect depends on the field strength and number and duration of the pulses (Ohlsson & Bengtsson 2003). Different pulse wave shapes are possible and may influence the preservation effect achieved.
PEF processing may inactivate cells by a variety of mechanisms. The most important is thought to be electroporation, where the electric field pulse creates pores in the cell membrane, thus disrupting regulatory processes and possibly causing swelling and rupturing of the cells. Other mechanisms may include (Ohlsson & Bengtsson 2003): formation of reactive electrolysis products, induced oxidation and reduction reactions in the cell that disrupt metabolic processes, and heat produced from transformation of the supplied electrical energy.
The method has shown promise for treatment of a range of particle-free liquid foods, especially fruit juices and other beverages. Papers by Prof H. Zhang will provide further information.
5.5 Other non-thermal preservation processes
Many other possibilities exist for preserving foods via the application of energy in forms other than heat, pressure or electricity, as indicated in Table 5.1 and the readings. In many cases these have yet to be commercialised and may never be. Two of these alternative processes, ultrasound and microfiltration, are briefly introduced below.
Low power ultrasound is already widely used in the food industry for non-destructive measurement of physical, chemical and rheological properties of materials, e.g. the % fat in a carcass or to follow the progress of starch or protein gelation.
High power ultrasound (HPU) has many potential applications in both food preservation and food processing. HPU operates with power inputs (up to 10 kW/m^3 ) many orders of magnitude higher than the low power applications and has already proved equally versatile in the mechanical and chemical manufacturing industries, where it is used, e.g., for machining, for welding of metals and plastics, for cutting, for cleaning, and for modifying chemical reactions.
HPU delivers acoustic energy to a material in the frequency range of 20 100 kHz (the limit for human hearing is about 16 kHz, but dogs, among other animals, can detect ultrasound). Depending on the power intensity this will lead to marked increases in local heat and mass transfer and, through cavitation (the formation and violent collapse of voids) cause the disruption of material structures, intense localised (at the microscopic scale) mixing and shearing, and generation of (potentially very) high localised temperatures and pressures. These may chemically influence material structures and reactions, and disrupt
cell and enzyme structures causing microbial death and enzyme inactivation. These various mechanisms can be controlled by the rate and level of acoustic power input, and the system temperature and pressure.
High energy inputs are required to achieve a satisfactory preservative effect and this often causes adverse quality changes in the products texture, flavour or other quality attributes. Ultrasound may therefore be best used in combined preservation processes (see section 5.7).
Microfiltration is a membrane separation process suitable for processing liquid foods. A feed stream is introduced into a module containing the polymeric (a thermoplastic) or ceramic microporous membrane and, under the influence of a pressure driving force, part of the material passes through the membrane and is removed as a permeate or filtrate stream. The rate of permeation per unit of membrane area is called the flux and has SI units of kg.m-2.s-1, but is commonly reported as l.m-2.h-1 (litres per square metre per hour, or LMH). Suspended particles, including micro-organisms, are retained by a sieving mechanism, the effectiveness of which will depend on the nominal pore size of the membrane; this is typically in the range 0.4 1.4 m for most commercial applications.
The retained fraction of the feed may be removed from the module, in which case it is termed the retentate or concentrate stream. When a retentate stream is removed the process is operating in crossflow (or tangential flow) mode, i.e. the feed flows parallel to the membrane surface and only a portion of this passes through the membrane. This is in contrast to dead-end operation, typical of conventional filtration, where the feed is essentially perpendicular to the membrane or filter surface.
Dead-end operation results in the continual accumulation of a solid cake on the membrane surface, whereas in crossflow operation shear forces generated by the flow over the membrane minimize cake formation. This effect increases with increasing crossflow velocity and enables the flux to be maintained at a higher value for longer periods.
Alfa-Laval has patented the so-called "Bactocatch" process to remove bacteria from skim milk. This uses ceramic membranes in which permeate is recirculated through the outer part of the housing to ensure a constant, small pressure drop across the membrane. A large pore size (1.4 m) is used to minimize protein retention by the membrane and high crossflow velocities promote high fluxes (Fig. 5.3). Flux values of > 500 LMH are reported for runs of 6 hours or more. Protein transmission values of > 95 % are claimed with reductions of 10^2 – 10^3 in the numbers of bacteria. Sometimes the process is referred to as cold pasteurisation but this is not appropriate, since the bacterial removal efficiencies are very low compared to the reductions of 10^6 10^9 typically achieved (and in some cases, legally required) for thermal pasteurisation.
Microfiltration is also used extensively for the sterile filtration of fruit juices, wines and beer. Most of these applications operate in dead-end, rather than crossflow, mode because of the low solids removal rate required and use pleated cartridge filters. The pore size is also smaller at ~ 0.45 m, but flux values can be high as there is less fouling compared to the case above. Good shelf life is obtained due to extremely high removal efficiencies of microorganisms achieved in the process.
Fig. 5.3: The Bactocatch MF process for improving the microbiological qulaity of milk (from Mistry, 2003)
5.6 Methods of surface decontamination
A variety of methods are available for surface decontamination applications. These can be variously applied to reduce spoilage bacteria and pathogens on the surface of: meat and fish, cheese and other dairy products, and fresh fruit and vegetables.
Steam contact, possibly under vacuum to reduce the temperature, is one obvious mechanism that can be employed as mentioned in module 3. In this case the principles of conventional heat treatment apply with respect to calculating the killing and cooking effects, respectively. Usually, to minimise the effects on heat on the food surface, the extent of the thermal effect on the microorganisms must be limited.
A range of other chemical (e.g. acid solutions) and physical (e.g. UV light) surface treatments have been investigated and these are summarised in the Food Processing Faraday reading.
5.7 Combining treatments for improved process and quality outcomes
In many cases, non-thermal processes are more effective when combined or jointly applied with a mild heat treatment. Such systems are examples of hurdle technology, where several barriers to cell growth are combined within a food system. As identified by Leistner & Gorriss (1995), each hurdle of itself may be not be of sufficient intensity to limit growth and has only a limited effect on product quality (so satisfying the requirements for minimal processing) but the combination of hurdles is sufficient to provide the required degree of preservation.
Hurdle technologies are often applied in traditional foods, e.g. the combination of added salt or nitrites, lowered water activity, lowered redox potential and organic acids that provides extended shelf life in cheeses and fermented meats.
Combined physical treatments that appear to offer particular benefits with respect to minimal processing include: heat and HP processing, heat and ultrasound, as thermosonication, and possibly with elevated pressure(but at much lower levels than in HPP), as mannothermosonication.
These and other combined processes are discussed in Ross et al. (2003).
Huis int Veld, J.H.J. (1996). Microbial and biochemical spoilage of foods; an overview. International Journal of Food Microbiology 33 , 1 18. Karel, M. and Lund, D.B. (2003). Physical Principles of Food Preservation. 2nd ed. New York: Marcel Dekker. Knoor, D. (1999). Process assessment of high-pressure processing of foods: an overview. In: Processing Foods: Quality Optimization and Process Assessment, Oliveira, F.A.R & Oliveira, J.C. (eds), pp. 249 – 267. Boca Raton: CRC Press Leistner, L. and Gorriss, L.G.M (1995). Food preservation by hurdle technology. Trends in Food Science and Technology Ohlsson, T. (2002). Minimal Processing Technologies in the Food Industry. Cambridge (England): Woodhead Publishing Limited. Ohlsson, T. and Bengtsson, N. (2002). Minimal processing of foods with non-thermal methods. In: Minimal Processing Technologies in the Food Industry, Ohlsson, T. (ed.), pp. 34 – 60. Cambridge (England): Woodhead Publishing Limited. Rademacher, B. and Hinrichs, J. (2001). Ultra high pressure technology for dairy products. Bulletin of the IDF 374 , 12 18. Scott, J.S. and Pillau, S. (2004). IFT scientific status summary: irradiation and food safety. Food Technology 58 , 48 55.