Joseph H.Hulse Trends in Food Science Technology 15(2004)3-18 11 kiloGrays (kGy),I Gray being equivalent to 1 Joule tems.Quartz crystals facilitate controlled outputs ran- per kg. ging from 500 W to 50 kW with 80-90%energy In the USA,irradiation is permitted for microbial efficiency.Computer modelling programmes determine control in dehydrated enzymes (10 kGy),spices (30 optimum conditions for different purposes.Main con- kGyJ),poultry (3 kGy),various pharmaceuticals and straints include relatively high capital costs and need for other biological materials.Data in parentheses are highly skilled engineers for operational control. maximum permitted doses.The higher the dose,the greater the inactivation of microorganisms.High doses Ultra-high hydrostatic pressure(UHP) can induce molecular disruption and generate highly Lethal effects on microorganisms of isostatic pres- reactive free radicals which in turn cause unpredictable sures between 500 and 10 k bar(50 kPa-1 MPa)were biochemical modifications.In general,higher doses are discovered over a century ago.UHP food processing permitted in biologicals consumed in small quantities has been applied mainly to fruit juices and jams.Industrial (e.g.spices)or in prescribed pharmaceuticals.A WHO equipment maintains pressures from 400 to 800 MPa. 1997 report states that at legally permitted doses,irra- Biomaterials in flexible or semi-rigid packages,evac- diation of foods will not cause toxicological difficulty or uated before sealing,are immersed in a fluid in a high significant nutrient loss. pressure vessel.The UHP is transmitted through the Apart from consumers'suspicions,still evident,the fluid to the biomaterial.In acidic products vegetative main constraints to food irradiation are economic. cells are inactivated at 400 MPa,bacterial spores after Capital costs are high,emissions from radioactive iso- 30 min at 600 MPa.UHP minimizes loss of nutritional topes cannot be switched off,so to derive maximum and functional properties.Constraints include high benefit there must be a constant supply,24 h every day, capital cost,precise engineering and skilled operational 365 days every year,of high value material to be pro- control cessed.Irradiation processes call for skilled bioengineers and physicists to ensure safety of all workers who must Pulsed energy come close to the equipment.In most cases,irradiation Three forms of pulsed energy for microbial inactiva- is uneconomic for grain disinfestation even at the rela- tion are under study:(1)Pulsed electric fields(PEF):(2) tive low doses required. Pulsed light (PL):(3)Pulsed magnetic fields (PMF) With PEF,induced electric potential causes lethal irre- Ohmic heating versible polarization of cell membranes.Critical poten- When an electric current flows through a substance of tial varies with species,cell morphology and ambient suitable conductivity,heat is uniformly generated.Ohmic conditions.Vegetative cells are inactivated at field heating is effective for fluids and particles suspended in strengths between 15 and 30 kV/cm,alternating polarity fluid media.The fluid is pumped through a column pulses being more effective than constant polarity. between two electrodes between which the current pas- Pulsed energy is not yet effective against spores or ses.The sterilised product is rapidly cooled and passes degradative enzymes. aseptically into sterile containers.Heating is uniform Pulsed light activates an inert gas lamp to generate and of short duration.Commercial models range from a broad band light flashes.20000 times the intensity of 10 kW pilot scale that processes 100 kg/h,to 300 kW sunlight at the earth's surface.PL is effective against machines to process 3 t/h.Capital costs range between surface vegetative organisms. 375,000 and two million pounds sterling.Operating Pulsed energy systems bear high capital costs and costs depend on the power consumed and properties of need precise operational control. the products processed. Ultrasonics (US) Microwave(MW)and radio frequency (RF)heating Ultrasonics use sound waves at frequencies higher MW and RF depend on electromagnetic energy gen- than detected by human ears (20 kHz).Microbes in erated from a magnetron to produce an electric field liquid suspension are inactivated by alternating pres- that alternates at radio or microwave frequencies.Heat sures and cavitation.With mild heat,US inactivates is generated in biological materials by rapid reversal of vegetative cells and can remove dirt inaccessible to con- molecular polarization.MW and RF provide uniform. ventional cleaning.US is used industrially to accelerate short-time heating with high internal temperatures. or control crystallization,filtration,hydrogenation of Most widely known are domestic MW ovens,indust- lipids and aging of alcoholic beverages. rially MW and RF processes are used in dehydration, microbial inactivation and cooking. Process and product quality control (QC) International electromagnetic compatibility regula- Simply defined,QC objectives are to ensure (1)the tions limit industrial processes to specific frequency properties of raw materials and final products comply bands that do not interfere with communication sys- with defined specifications;(2)consistency of essentialkiloGrays (kGy), 1 Gray being equivalent to 1 Joule per kg. In the USA, irradiation is permitted for microbial control in dehydrated enzymes (10 kGy), spices (30 kGyJ), poultry (3 kGy), various pharmaceuticals and other biological materials. Data in parentheses are maximum permitted doses. The higher the dose, the greater the inactivation of microorganisms. High doses can induce molecular disruption and generate highly reactive free radicals which in turn cause unpredictable biochemical modifications. In general, higher doses are permitted in biologicals consumed in small quantities (e.g. spices) or in prescribed pharmaceuticals. A WHO 1997 report states that at legally permitted doses, irra￾diation of foods will not cause toxicological difficulty or significant nutrient loss. Apart from consumers’ suspicions, still evident, the main constraints to food irradiation are economic. Capital costs are high, emissions from radioactive iso￾topes cannot be switched off, so to derive maximum benefit there must be a constant supply, 24 h every day, 365 days every year, of high value material to be pro￾cessed. Irradiation processes call for skilled bioengineers and physicists to ensure safety of all workers who must come close to the equipment. In most cases, irradiation is uneconomic for grain disinfestation even at the rela￾tive low doses required. Ohmic heating When an electric current flows through a substance of suitable conductivity, heat is uniformly generated. Ohmic heating is effective for fluids and particles suspended in fluid media. The fluid is pumped through a column between two electrodes between which the current pas￾ses. The sterilised product is rapidly cooled and passes aseptically into sterile containers. Heating is uniform and of short duration. Commercial models range from a 10 kW pilot scale that processes 100 kg/h, to 300 kW machines to process 3 t/h. Capital costs range between 375,000 and two million pounds sterling. Operating costs depend on the power consumed and properties of the products processed. Microwave (MW) and radio frequency (RF) heating MW and RF depend on electromagnetic energy gen￾erated from a magnetron to produce an electric field that alternates at radio or microwave frequencies. Heat is generated in biological materials by rapid reversal of molecular polarization. MW and RF provide uniform, short-time heating with high internal temperatures. Most widely known are domestic MW ovens, indust￾rially MW and RF processes are used in dehydration, microbial inactivation and cooking. International electromagnetic compatibility regula￾tions limit industrial processes to specific frequency bands that do not interfere with communication sys￾tems. Quartz crystals facilitate controlled outputs ran￾ging from 500 W to 50 kW with 80–90% energy efficiency. Computer modelling programmes determine optimum conditions for different purposes. Main con￾straints include relatively high capital costs and need for highly skilled engineers for operational control. Ultra-high hydrostatic pressure (UHP) Lethal effects on microorganisms of isostatic pres￾sures between 500 and 10 k bar (50 kPa–1 MPa) were discovered over a century ago. UHP food processing has been applied mainly to fruit juices and jams. Industrial equipment maintains pressures from 400 to 800 MPa. Biomaterials in flexible or semi-rigid packages, evac￾uated before sealing, are immersed in a fluid in a high pressure vessel. The UHP is transmitted through the fluid to the biomaterial. In acidic products vegetative cells are inactivated at 400 MPa, bacterial spores after 30 min at 600 MPa. UHP minimizes loss of nutritional and functional properties. Constraints include high capital cost, precise engineering and skilled operational control. Pulsed energy Three forms of pulsed energy for microbial inactiva￾tion are under study: (1) Pulsed electric fields (PEF); (2) Pulsed light (PL); (3) Pulsed magnetic fields (PMF). With PEF, induced electric potential causes lethal irre￾versible polarization of cell membranes. Critical poten￾tial varies with species, cell morphology and ambient conditions. Vegetative cells are inactivated at field strengths between 15 and 30 kV/cm, alternating polarity pulses being more effective than constant polarity. Pulsed energy is not yet effective against spores or degradative enzymes. Pulsed light activates an inert gas lamp to generate broad band light flashes, 20 000 times the intensity of sunlight at the earth’s surface. PL is effective against surface vegetative organisms. Pulsed energy systems bear high capital costs and need precise operational control. Ultrasonics (US) Ultrasonics use sound waves at frequencies higher than detected by human ears (20 kHz). Microbes in liquid suspension are inactivated by alternating pres￾sures and cavitation. With mild heat, US inactivates vegetative cells and can remove dirt inaccessible to con￾ventional cleaning. US is used industrially to accelerate or control crystallization, filtration, hydrogenation of lipids and aging of alcoholic beverages. Process and product quality control (QC) Simply defined, QC objectives are to ensure (1) the properties of raw materials and final products comply with defined specifications; (2) consistency of essential Joseph H. Hulse / Trends in Food Science & Technology 15 (2004) 3–18 11