Pulsed Electric Fields for Food Processing Technology-ppt.pptx
- 1. Pulsed Electric Fields for Food Processing Technology Presented by Paththuwe Arachchige Maduni Jayahansi Wijepala PhD Food Science & Technology Department of Food Science & Technology College of Agriculture & Veterinary Medicine UAEU
- 2. Outline • Introduction • The Evolution of PEF • How Pulsed Electric Field Works? • Microbial Inactivation by PEF • System Components • Power Supply • High-Power Capacitors • Switches • High Voltage Pulse Generator • Treatment Chamber • Applications of Pulsed Electric Field Technology • Factors Affecting The Outcome of Pulsed Electric Fields Treatments • Technological Factors • Biological Factors • Media Factors • Consumer Acceptance of PEF-treated Products • Conclusion • References
- 3. Introduction • One of the non-thermal methods of food preservation • Uses short (µs), high-voltage (10-80 KV/cm) pulses of electricity between two electrodes for microbial inactivation • Less effect on food quality attributes (Physical and Sensory Properties) • Maintain color, flavor, texture, and nutritional value of unprocessed foods • Used for pasteurization and sometimes for sterilization combined with other preservation techniques, such as reduction of pH, temperature elevation, and high-pressure processing • Commonly used for liquid or semi-solid foods • Commonly used for milk and dairy products, egg products, juices and other liquid foods
- 4. • Major focus of researchers: Aspect of food preservation regarding the inactivation of microbes • Recent Investigations: • Potential of PEF in • Improving food processing efficiency (Enhancement of juice extraction from fruits) • Speeding-up of food dehydration or drying (Taiwo et al., 2002) • PEF: • PEF causes penneabilization or depolarization of microbial membranes • Inactivates bacterial and yeast vegetative cells in various food commodities • Bacterial spores cannot be killed as they are resistant to PEF • Major focus is on pathogenic and spoilage microorganisms, especially for low pH food products • Extends shelf-life without using heat • Preserve sensorial and nutritional quality attributes • Improve energy usage in an efficient and economical way (A study by Heinz et al., 2003 on apple juice reported that “the energy consumption could be reduced from above 100 to less than 40 kJ.kg−1)
- 5. Flow chart of PEF system with major components
- 6. The Evolution of PEF • In the 1920s and 1930s, the electropure process was introduced to pasteurize milk using electricity (USA). This was the first attempt to use electric fields for microbial inactivation. But the electric field used here was a small, 220V alternating current and not pulsed. This was similar to ohmic heating as heat generated in the food due to the resistance to electricity caused lethal effects on microorganisms. • In 1950s electrohydraulic treatment was developed to kill microorganisms. This involved a discharge of high-voltage electricity rapidly across the electrodes. The electrodes were submerged in the liquid medium containing the microorganisms. In this process, UV light, electrochemical reactions, and shock waves were responsible for microbial inactivation. This was not used at the industrial level since food can be contaminated due to the corrosion of electrodes and as there is a possibility of particulate disintegration of food matter due to shock waves. • In 1967, Sale and Hamilton conducted systematic studies on the effect of PEF on microbial inactivation. They found that the number of pulses and the pulse width are the two most significant factors involved in microbial inactivation. They proved that the microbial inactivation was not due to the outcomes of electrolysis. They proposed that the microbial inactivation was due to the irreversible damage that happened to the microbial cell membranes leading to the loss of its ability to act as a semipermeable barrier between the cell and its environment.
- 7. How Pulsed Electric Field Works? • Basic Principle: Short pulses of high electric (Voltage) fields are involved for duration of micro to milliseconds with intensity in the range of 10-80KV/cm • The process depends on, • How much of pulses delivered to the product which is held between two electrodes • During PEF: High-voltage is applied to inactivate the microorganisms • PEF is applied in different forms • Exponentially decaying waves • Bipolar waves • Oscillatory pulses • Can be carried at various temperature ranges • Ambient • Sub-ambient • Above-ambient • After PEF Stored under refrigerated conditions
- 8. The shapes of commonly used pulse waves in PEF technology: (a) Exponentially decaying pulse, (b) Square pulse, (c) Bipolar exponential, (d) Bipolar square Rashida, A. & Divya, B. & Joy, P.P.. (2016). NON-THERMAL PROCESSING OF FOODS. • Square wave pulses are far more efficient than the exponential pulses • Efficiency of bipolar pulses are higher than mono-polar • A sudden reversal of the used electric field orientation will change the movement direction of the charged groups of the cell membrane and leads to breakdown • No effect of pulse frequency on inactivation ratio
- 9. • Food Contains several ions that provide a definite level of electrical conductivity to the product • Therefore, electrical pulses can transfer from one point to another in liquids • PEF involves delivering pulsing power to the product that is placed between two electrodes confining the treatment gap of the PEF chamber • The food material kept between the electrodes experiences a force per unit charge that leads to a burst of bacterial cell membranes
- 10. Mechanism of Microbial Inactivation The microbial cell membranes: • Protects the microorganism from the surrounding environmental conditions • Acts as a semi-permeable barrier (Controls the passage of nutrients into the cell and the passage of excretory products of metabolic activities out of the cell) • Maintains an effective osmotic margin between the cell and its environment • When high-voltage PEFs are applied, • The cell membrane is disrupted • This results in the leakage of intracellular contents • Leads to loss of cell’s metabolic activities • Leads to loss the ability of cells to grow and divide in a food medium
- 11. Steps of Microbial Cell Death 1. Electrical Breakdown: • The cell membranes are susceptible to electric fields because of the dipolar nature of phospholipid monolayers • The pulsed electric fields results both Thermal and Electrical impacts on the cell membranes • PEF makes microbial membranes unstable and creates electromechanical compression that leads to the formation of pores
- 12. 2. Electroporation • Cells exposed to high-voltage electric pulses temporarily destabilize the lipid bi-layer and the proteins of cell membranes • Electroporation results in a considerable increase in membrane rupture and permeability which is termed as “electro-permeabilization” • Electro-permeabilization can be Reversible or Irreversible • Degree of membrane organizational changes Death of microbial cells • The strength of the electric field Membrane permeability • Membrane permeability Inactivation of microbial cells • Electrolysis Results highly reactive free radicals which act as antimicrobial agents
- 14. Sale & Hamilton Theory • When an external electric field applied • A transmembrane potential (TMP) is developed across the cell membrane • The TMP of the cell membrane in the direction of an applied electric-field strength (E) is given as follows; 𝑼 𝒕 = 𝟏. 𝟓𝒓𝑬 • When TMP reached around 1 V, cells loss their membrane integrity (Breakdown TMP) • Exercise: If the radius of the cell is 3 µm how much is the strength of the electric field we must apply to breakdown the membrane integrity of the cell? U(t)= Transmembrane potential (TMP) in the direction of applied electric field strength (V) r = Radius of the cell (µm) E = Applied electric field strength (kV/mm)
- 15. • Calculation of PEF treatment time (Sale and Hamilton, 1967) 𝑻 = 𝒏 ∗ 𝒕 • Calculation of Electrical Intensity 𝑬 = 𝑽 𝒅 T: Treatment time n: Pulse number t: Pulse duration E: Electrical Intensity V: Voltage across two points, separated by a non-conductive material (Potential difference) d: Distance between two points
- 16. The Inactivation Rate Modeling • In 1981, Hulsheger et al., developed a mathematical model to find the survival rate as a function of electric field strength and treatment time 𝒔 = 𝒕 𝒕𝒄 − 𝑬−𝑬𝒄 /𝒌 S= Survival ratio; (ratio of number of mo present in the food after treatment and initial number of mo present before the treatment t= Treatment time (µs) tc= Critical treatment time, a threshold value above which inactivation occurs (µs) E= Electric field strength (kV/cm) Ec= Critical electric field strength, which is a threshold value above which inactivation occurs (KV/cm) K= Specific constant for a microorganism
- 17. − log 𝑠 = 𝐸 − 𝐸𝑐 𝑘 ∙ 𝑙𝑜𝑔 𝑡 𝑡𝑐 • Inactivation ratio is directly proportional to • The strength of applied electric field • logarithm of applied treatment time • Electric field strength has more significant effect on microbial inactivation than the treatment time • Exercise: • Find the PEF treatment time for E.coli k=12 strain at Electric field strength of 20 Kv/cm to achieve 5 log reduction. Ec= 4.9kv/cm, tc=12 µs, k= 6.3
- 18. Effect of PEF on Microbial Inactivation PEF Conditions Microorganism Effect on Microbes Reference PEF treatment at 38.4 kV/cm for 160 μs for green tea beverage E. coli 5.6 log reductions Zhao, et al., 2008 PEF treatment at 38.4 kV/cm for 200 μs for green tea beverage S. Aureus 4.9 log reductions PEF treatment for must and wine microbiota by continuous flow of PEF-processing at 15 to 25 kV/cm and 175 to 148 kJ/kg, parameters applicable at industrial scale at 1 ton/h. CFU/mL 3-log reductions Carlota et al., 2023 PEF treatment at 40KV/cm, 10 pulses and lower flow rate with an outlet temperature of 39.2°C for protein functional beverages (Whey protein concentrate + Soy protein isolate) E.coli 4.2 log reductions Alzahrani et al 2022 PEF treatment at 25KV/cm, 25 pulses and lower flow rate with an outlet temperature of 38.4°C for protein functional beverages (Whey protein concentrate + Soy protein isolate) E.Coli 4.2 log reductions
- 19. Effect of PEF on Chemical/ Nutritional Properties PEF Conditions Chemical/ Nutritional Property Effects Reference PEF treatment at 38.4 kV/cm for 200 μs for green tea beverage Colour No considerable effect Zhao, et al., 2008 GTP Total Free Amino Acids PEF treatment at 28KV/cm, 200 µs of treatment time and pulse width 40 µs for Almond milk Lipoxygenase 50% reduction Muhammad et al., 2020 Peroxidase 45% reduction PEF treatment at 24.6KV/cm, 0.02ms treatment time for apple extract Polyphenol oxidase (Cause enzymatic browning) Reduced from 38% to 3.15% Jiner et al., 2001
- 20. System Components • Power Supply • High-Power Capacitors • Switches • High Voltage Pulse Generator • Treatment Chamber
- 21. Power Supply • High-voltage electric pulses are supplied to the system via a high-voltage pulse generator at the required shape, intensity, and duration • The high-voltage power supply can either be, • A source of DC • A Capacitor charging power supply with high-frequency AC inputs • The capacitor is generally charged by the high-voltage power supply and store the energy • The total power of the system depends on how many times a capacitor can be charged and discharged in a given period • A smaller capacitor requires less time and power to be charged than a larger one
- 22. High-Power Capacitors • The high-power sources has storage capacitors and on/off switches • The energy stored in capacitors is used to generate either electric or magnetic fields • In PEF it generates a pulsed electric field that leads to cell membrane breakdown
- 23. Switches • Switches play a considerable role in the efficiency of the PEF system • Switches connect elements between the storage device and the load • Switches used in PEF • Ignitrons • Spark gaps • Trigatrons • Thyratrons • Semiconductors • Solid-state semi-conductor switches Future of high-power switching • Two types of switches: ON switches and ON/OFF switches • On Switches: • Provide complete discharging of the capacitor, can turn off only when the discharging completed • Can handle higher voltages with comparatively lower cost • Disadvantage: short life and low repetition rate • Examples: Gas Spark Gap, Trigatron, The Ignitron, Thyratron
- 24. • On/Off Switches: • Able to control the pulse generation process with partial or full discharge of the capacitors • Advantage: Longer life spans and better performance • Examples: • The insulated gate bipolar transistor (IGBT) • The symmetrical gate commutated thyristor (SGCT) • The gate turn-off (GTO) thyristor
- 25. High-Voltage Pulse Generator • Provides electrical pulses using a pulse-forming network (PFN) • PFN: An electrical circuit consists of • One or more DC power supplies • A charging resistor • A capacitor bank • One or more switches • Pulse-shaping inductors and resistors • The DC power supply charges the capacitor bank to the required voltage • Pulse generator converts • AC power from the utility line High-voltage AC power Rectified to high-voltage DC power
- 26. Treatment Chamber • The treatment chamber is used to keep the product during the pulsing • Consists of two electrodes held in position by an insulating material which also forms a cabin to keep the sample • The uniformity of the process depends on the design of the treatment chamber > • Treatment chamber designs • Parallel Plate • Coaxial • Co-linear The strength of the applied electric field The electric field strength of the food Food Breakdown
- 27. Parallel Coaxial Co-linear Used in batch modes Used in continuous modes. In this type medium is pumped at a known flow rate and a known pulse frequency is applied. This type is used in continuous operations for liquid foods. This is the most used type of treatment chamber. Consists of ring-shaped electrodes and insulators in an alternating order. The parallel configuration of electrodes produces a field improvement problem at the edges The co-axial chamber is composed of two hollow cylinders with the direction of the electric field directed radially This configuration is characterized by relatively high electrical resistance, that leads to high electric field strength levels necessary for many cell disruption tasks while limiting electrical current flow, energy input, and the associated temperature increase Provide homogenous treatment but display poor uniformity The dispersion of the electric fields is non- homogeneous from the internal towards the external cylinder Co-linear electrodes overestimate the actual exposure of the electric fields inside the treatment area Different Designs of Treatment Chamber Cross-sectional views of arrangements of treatment chambers; (a) parallel plate, (b) Co-axial, (c) Collinear
- 28. Applications of Pulsed Electric Field Technology • PEF is used to pasteurize foods such as juices, milk, dairy products, soups, liquid egg products, etc. at a temperature below 30-40°C • Limitations: • Product must be free from air bubbles (Dielectric breakdown occurs) • Food must have lower electrical conductivity • Particle size < the gap of the treatment chamber ensure appropriate treatment • Generally, not suitable for solid foods that are not pumpable • PEF Technology can be used to enhance the extraction of several bioactive components and sugars from plant cells • Alternative applications of PEF technology • Drying enhancement through diffusion process • Enzyme activity modification (The PEF affects enzyme activity by changing mainly the secondary (α-helix, β-sheets, etc.), tertiary (spatial conformation), and quaternary (number and arrangement of protein subunits) structures of the enzyme.) • Improves extraction rate of juices
- 29. PEF in Processing of Fruits • Used in the citrus industry: for inactivating microorganisms and preventing the development of off-flavours • Can extend the shelf-life of orange juice from a few days to a few weeks • Enhance the diffusion coefficient of soluble substances in apple juice • PEF can disintegrate biological tissues and enhance the extraction of intracellular compounds from fruits • Ex: Extraction of pectin from fruits (Short pulses avoid excessive heat and undesirable electrolytic reactions that enhance the extraction rate of pectin from fruit pomace)
- 30. Milk Preservation By PEF • Milk is spoiled by several spoilage-causing and pathogenic bacteria (E.coli, Listeria, Pseudomonas spp., etc.) • PEF pasteurizes the milk without adversely affecting the quality attributes of the milk • According to the recent studies, • PEF is effective for microbial reduction in simulated milk ultra-filtrate and skim milk (Sobrino-López, A., & Martín-Belloso, O. 2010). • The presence of fat and protein molecules limits the adeptness of PEF in whole milk because these molecules protect the microbial cells during the treatment (Sharma, P., et al, 2014) • PEF can be successfully used in collaboration with other preservation techniques. Use of PEF with heating up to 55-60°C resulted in a considerable decline in microbial load (Guerrero-Beltrán, J. Á., et al, 2010) • At 50 °C and 22-28KV/cm, Gram (+) and Gram (-) bacterial load in whole milk can be reduced by 5-6 log cycles (Sharma, P., et al, 2014)
- 31. PEF in the Meat Industry • Meat quality is considered the most critical factor for consumer attraction. • PEF has the potential for different applications in some solid foods • Structural modifications • Changing physical quality parameters • Extraction of bioactive compounds • Extending shelf-life • In meat, PEF can, • Enhance the cell permeability of meat which Improves enzyme release and glycolysis that are essential for proteolysis which leads to meat tenderization • Reduce the microbial load and extend the shelf-life • Maintain the natural aromatic profile of meat during storage period (Faridnia, F., et al, 2015) • Significant improvement in tenderness of meat by PEF treatment at 5-10KV/cm in different pulse frequencies (20, 50, and 90 Hz) on beef muscles (Reduced the shear force and improved the textural properties) (Suwandy, V., et al, 2015)
- 32. Factors Affecting The Outcome of Pulsed Electric Fields Treatments • The efficacy of PEF against pathogenic and spoilage microorganisms in food materials basically depends on three factors • Technological Factors • Biological Factors • Media Factors
- 33. Technological Factors Factor Effects on the efficacy of the PEF outcome 1. Electric field Intensity • Microbial inactivation increases with an increase in the electric field intensity above the transmembrane potential (Quin et al., 1998) • Electric field intensities of smaller than 4-8 KV/cm generally do not affect microbial inactivation (Peleg, 1995) • In general, to inactivate the microbial cells in foods the required electric field intensity is 12-45 KV/cm 2. Treatment time and temperature • Effective time duration that microorganisms are subjected to the electric pulses • It depends on the Intensity of the pulses and the width of the pulses applied • Treatment time and the electric field strength are the main factors that determine the efficacy of PEF treatment in microbial inactivation (Wouters et al., 2001) • When temperature increases lethality from PEF increases up to a certain extent • Inactivation of E.coli by PEF considerably improved when temperature increased from 7 to 20°C. Further increase in temperature from 20 to 33 °C did not further increase the inactivation of microbial cells (Zhang et al., 1995) 3. Frequency of pulses • If the same number of pulses are applied, Killing of microorganisms does not depend on the number of pulses applied per second (Alvarez et al., 2003) 4. The type of wave form • Generation of exponential waves are much easier than square wave forms. A pulse pulse-forming network (PFN) with an array of capacitors and inductors is required to generate square wave forms • Square waveforms are more detrimental and energy-efficient than exponentially decaying pulses. Compared to exponential pulses the square pulses have longer peak voltage duration (Amiali et al., 2006)
- 34. Biological Factors • Intrinsic parameters of the microorganisms: size, shape, species, growth state • Gram (+) vegetative cells are less susceptible to PEF than Gram (-) bacteria • Yeasts show are more susceptible than bacteria • Induction of electric fields into cell membranes is higher when larger cells are exposed to PEF treatment (Zhang et al., 1994) • Bacillus cereus spores were more resistant to mild PEF treatment at an electric field strength of 20KV/cm and 10 pulses in a study conducted on apple juice (Cserhalmi et al., 2002) • Bacillus cereus spores were not affected by PEF treatment of 60KV/cm for 75 pulses at room temperature (Pagan et al., 1998) • Mold Condi spores were sensitive to PEF in fruit juices and Neosartorya fischeriasco spores were resistant to PEF treatments (Raso et al., 1998)
- 35. Media Factors • Properties of the liquid food affect on the effect of PEF on microbial inactivation • Microorganisms are more susceptible to PEF than the enzymes and proteins of the food matrix • The physical and chemical characteristics of the food products affect on the effectiveness of PEF treatment (Wouters et al., 2001) • The media’s chemical and physical properties affect on the recovery of injured microbial cells and the continuation of their growth after being exposed to the PEF. • Fats and proteins in the media protect microorganisms and act against the effectiveness of PEF (Martin et al., 1997) • Media’s intrinsic factors • Conductivity • Resistivity • Dielectric Properties • Ionic Strength • pH • Composition • The above factors affect alone or in combination on the efficiency of PEF
- 36. Consumer Acceptance of PEF-treated Products • Consumer acceptance is limited due to several reasons • Consumers are not aware of the nature of the technology • Labeled as “minimally processed foods” creates a negative impression as not properly processed • It is important to make consumers aware about the technology and its advantages in food safety
- 37. Conclusion • PEF is a cost effective, efficient non-thermal food preservation technique • Can use to replace conventional thermal preservation techniques • Can extend the shelf-life of the foods (specifically liquid and semi-solid) without harming the quality (nutritional and sensory) • Involved in microbial inactivation, retardation of chemical and enzymatic reactions, retention of functional components • More research is needed in the following areas • Changes in functional properties of foods with PEF • PEF for viscous and particulate foods • Optimization of process conditions for different types of foods • Innovative developments in high-voltage pulse technology
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