PULSED ELECTRIC FIELD PROCESSING OF FOODS

Transcription

1 PULSED ELECTRIC FIELD PROCESSING OF FOODS Michael Ngadi, PhD., PEng William Dawson Scholar and Associate Professor Department of Bioresource Engineering McGill University, Montreal

2 OUTLINE Background PEF Overview Microbial inactivation case studies Biofilm control Quality aspects Safety monitoring Concluding remarks

3 BACKGROUND FOOD PROCESSING OBJECTIVES maintain safety and quality of food products for as long as possible impact unique attributes to products

4 THERMAL TREATMENT preferred method used by the food industry to inactivate pathogenic microorganisms and enzymes PROBLEMS high temperatures and overcooking result in quality deterioration affecting sensory and nutritive attributives energy requirements

5 NON THERMAL PROCESSES being explored to address the adverse effects of thermal processing. high pressure processing ultrasound ultraviolet light pulses magnetic fields pulsed electric fields (PEF)

6 PEF SYSTEMS: PRINCIPLES PULSED ELECTRIC FIELD PROCESSING involves the application of pulses of high electric voltage (in the range of 20 to 80 kv/cm) across food contained between 2 electrodes. ELECTRIC FIELD results in inactivation of microorganisms suspended in the food medium without significantly increasing the temperature of the food product.

7 electropermeabilization Knorr & Angersbach (1998) = electroporation = Weaver & Chismadzhev (1996) electroplasmolysis McLellan, Kime & Lind (1991) u m - transmembrane potential increase in cell walls permeabilization E

8 Inactivation par CEP Electroporation Cytoplasm a) b) Medium c) d) Cell membrane objectif a) E=0 : Absence of EF b) E< E R : Presence of EF c) E>E R : Reversible breakdown d) E>>E R : Irreversible breakdown

9 ELECTROMECHANICAL MODEL OF CELL RUPTURE For spherical cells: V 15. mre cos( ) 1 e i o t t R V R 15. re cos( ) R where: V i = induced potential V R = rupture electric field r = cell radius (m) E o = applied electric field (V/m) E R = rupture electric field (V/m) = angle (rad) between the vector of the cell radius and the vector of the electric field at a point on the membrane t = duration of application of Eo (s) t r = relaxation time between electric field application and the moment when the membrane acquires a stable electric potential m = term related to surface conductance and cell resistivity

10 10 FOOD ENGINEERING liquid food pasteurization mass transfer process enhancement: dehydration, pressing, diffusion, drying MEDICINE, BIOLOGY Genes and drugs transfer into the cells Electropermeabilization Cell suspension in liquid medium Biological tissues

11 PEF SYSTEMS: COMPONENTS Charging Resistor Rc Discharge Switch HV Power Supply Energy storage capacitor, Co Treatment Chamber R Food E = V/d where V = voltage across the electrodes and d = gap between electrodes. V = V C - V R - V L V Q C Ri L di dt V C = voltage across a capacitor V R = voltage across a resistance V L = voltage across an inductance The resistance (in ohms) of a treatment R d A d V 2 A = cross sectional area of active electrode (m 2 ) = conductivity of medium (S/m)

12 SYSTEM CHARACTERISTICS Energy per pulse: For square wave pulse Capacitance of the capacitor Energy stored in a capacitor Energy density Temperature increase E C o Q Q s 1 2 t 0 R V p C o V o VI n v ( t). I( t) dt E P t V I t T peak ne mc p 2 max A d max 2 V n Rv 2 V R n max a E 2

13 PEF WAVEFORMS Long tail of low voltage Maximal use of high energy

14 PULSE CHARACTERISTICS

15 PEF SYSTEMS: TREATMENT CELL COAXIAL CHAMBER BASIC CONTINUOUS STATIC CHAMBER CHAMBER

16 HIGH VALTAGE SWITCHES Spark Gaps Solid State

17 MICROBIAL INACTIVATION BY PEF Electric field strength Treatment time Number of pulses Pulse wave shape Type of microorganism Microbial growth stage Ionic strength of medium Other medium characteristics Processing temperature

18 BEYOND LAB STUDIES

19 MATERIALS AND METHODS Thermocouple High Voltage DC Pulse Generator GND Treatment chamber HV probe HC probe GND Oscilloscope Water bath Peristaltic Pump Treated milk samples Untreated milk samples Schematic of experimental setup

20 PEF Generator PFN 30 kv, 100, 9 MW, 18J/p

21 TREATMENT CHAMBER Top view Top view 1.5 cm 5.0 cm 0.5 cm Gap 3.0 cm Side view Water Inlet Food Outlet Water Inlet Water Outlet Food Inlet Water Outlet

22 Ground Polyoxymethylen Derlin HV probe Inlet Outlet HV Line Materials and methods

23 TYPICAL WAVEFORMS

24 Log (N/No) Time (µs) T=18 C T=35 C T=45 C T=50 C Inactivation of E. coli O157:H7 in whole milk using electric field strength of 30 kv/cm

25 Log (N/No) Time (µs) T=18 C T=35 C T=45 C T=50 C Inactivation of E. coli O157:H7 in whole milk using electric field strength of 20 kv/cm

26 Kinetic Rate (s -1 ) 1.20E E E E E E E Temperature ( C) E=30 kv/cm E=20 kv/cm Kinetic rate constants for inactivation of E. coli O157:H7 in whole milk.

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28 Log reduction s log( N TE ) log( N T ) log N N TE T 0 E=0 kv/cm E=6 kv/cm E=15 kv/cm -2-4 Additional inactivation o Initial temperature ( C)

29 Additional log reduction Additional inactivation 0 E=3 kv/cm -1 E=6 kv/cm -2-3 E=15 kv/cm -4 n= o Initial temperature ( C)

30 Log reduction E=0 kv/cm E=15 kv/cm } T due to PEF heating additional thermal synergy o Initial temperature ( C)

31 Additional log reduction 0-1 E=15 kv/cm synergistic effect T=4 T= thermal inactivation Initial temperature ( C) o

32 Increase in the rate of inactivation with temperature is attributed to reduced trans-membrane breakdown potentials at higher temperatures. Phospholipids molecules in cell membrane undergo temperature-related transitions, changing from a firm gel-like structure to a less ordered liquid crystal phase at higher temperature, thus reducing the mechanical resistance of the cell membrane.

33 LITERATURE Source Milk type Process conditions Dunn et al Zhang et al Grahl et al Dutreux et al Evrendilek et al Pasteuriz ed milk Batch, 43 C, 43 kv/cm, 23 pulses SMUF Batch, 33 C, 70 kv/cm, 80 pulses UHT milk (1.5% fat) Fat free milk Batch, < 45 C, 20 kv/cm, 20 pulses Continuous, 17 C, 41kV/cm, 63 pulses Skim milk Continuous, 24 kv/cm, 50 pulses Log reduction

34 BIOFILM CONTROL Biofilm: biologically active matrix of cells and extracellular substances in association with a solid surface. Food-borne pathogens and spoilage microorganisms accumulate as biofilm on stainless steel, aluminium, glass, Buna-N, teflon steel and nylon materials typically found in food-processing.

35 MECHANISM OF BIOFILM FORMATION Conditioning of a surface the accumulation of macromolecules at the solid-liquid interface on food-contact surface. Adhesion of cells attachment of microorganisms to the conditioned surface. Formation of microcolony layer of cells covering the surface. Biofilm formation continuous attachment of the bacterial cells with associated EPS production.

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37 QUALITY ASPECTS OF PEF

38 Proteolysis in Cheddar-type Cheese made from Pulsed Electric Field Treated Milk Major biochemical changes during cheese ripening include proteolysis, glycolysis and lipolysis Proteolysis is considered the most important issue in terms of flavor and texture development

39 RESULTS AND DISCUSSION hydrophilic peptide hydrophobic peptide Typical HPLC profile of the water soluble peptides in Cheddar cheese curd slurry

40 Ratio THE RATIO OF HYDROPHOBIC TO HYDROPHILIC PEPTIDES IN CHEESE SLURRIES RM PEF80 PEF120 PM Day(s) RM: raw milk PM: Heat pasteurized milk PEF80: PEF treated milk with 80 pulses PEF120: PEF treated milk wit h120 pulses

41 PEF treated milk had the similar hydrophobic to hydrophilic proportion as raw milk. PEF treated milk could give similar flavor to Cheddar cheese as raw milk.

42 INFLUENCE OF PEF ON DRYING KINETICS PEF significantly increased drying rate of carrot pieces Electric field strength, capacitance and number of pulses significantly affected the drying rate

43 COMPRESSION TEST Slicing into cylindrical samples: diameter of 57 mm, thickness 15 mm Control PEF treatment Compression test

44 Stress (MPa) COMPRESSION TEST Number of pulses Strain Typical stress-strain curves from the compression test on apple samples treated by different number n of electric field pulses (E=1000 V/cm, t i =300 s, f=1 Hz); curve with n=0 corresponds to untreated sample.

45 Alfalfa Juice Relative quantities of juice extracted from alfalfa mash: (a) juice from PEF treated alfalfa mash and (b) juice from untreated alfalfa mash. (a) alfalfa mash before pressing, (b) untreated mash residue after pressing, and (c) treated mash residue after pressing. PEF treatment: 1.25 kv/cm; 200 pulses at 1 Hz in two successions. Each PEF treatment was followed by pressure application at 2 and 4 MPa, respectively.

46 Average yield of pressed alfalfa mash at two pressures. Moisture content of fresh alfalfa was 68% wb. Juice extracted (% by mass) 2 MPa 4 MPa Total Untreated alfalfa PEF Treated alfalfa PEF treatment: 1.25 kv/cm; 200 pulses at 1 Hz in two successions. Each PEF treatment was followed by pressure application at 2 and 4 MPa, respectively.

47 Entire tissue Simple pressing Combination: pressure-pef-pressure 47 + Combined treatment allows for homogenized tissue structure, intensified plasmolysis of the cells and increased juice yield

48 FOOD SAFETY MONITORING: HYPERSPECTRAL IMAGING Park et al. (2007): Biosys Eng. 96:

49 CONCLUDING REMARKS There is excellent potentials for application of PEF to ensure safety of processed high quality foods. Major efforts are being made to define inactivation kinetics, processing conditions that allow a more reliable comparison among different systems, impact of food additives and the use of hurdle technology, and effects of the process on specific food components, as well as modeling the electric and flow fields and their interaction, developing monitoring and control systems that assure adequate treatment, and developing new products.

50 Thank You