Novel food processing: extrusion, high-pressure processing and pulsed electric fields

In short

Novel (or minimal) processing uses mechanical, pressure or electrical energy instead of prolonged heat to cook, texturise or preserve food. Extrusion is a high-temperature short-time cooking and forming operation that makes snacks, pasta, breakfast cereals, pet food, texturised vegetable protein and India's fortified rice kernels; high-pressure processing (HPP) and pulsed electric fields (PEF) inactivate vegetative microbes with little loss of vitamins, colour and flavour. The topic appears in GATE (Food Technology, XE/Food Engineering section), in university papers under "Novel/Emerging food processing technologies", and in interviews at ITC, Haldiram, Bikaji, Balaji, Nestlé, Hindustan Unilever, Hector Beverages, Britannia and equipment firms such as Clextral, Bühler, Hiperbaric and Basiron/Indian extruder makers. FSSAI regulates both the products and, through the Non-Specified Food and Food Ingredients Regulations, 2017, any genuinely novel process or ingredient. Examiners want the mechanism, the operating windows, the governing equations (SME, expansion index, pressure D-value, PEF specific energy) and the limits — especially that HPP and PEF do not kill bacterial spores at ambient temperature.

Explained

What makes a process "novel" or "non-thermal"

• Definition: a process that achieves cooking, texturisation or microbial inactivation using a form of energy other than conduction/convection heat, or heat for far shorter times than conventional retorting.
• Driver: conventional sterilisation destroys thiamine, ascorbic acid, anthocyanins and volatiles; novel processes aim at the same safety with higher nutritional and sensory retention.
• Classification: (a) mechanical/thermomechanical - extrusion; (b) pressure-based - HPP, pressure-assisted thermal sterilisation; (c) electrical - PEF, ohmic heating, cold plasma; (d) radiative/acoustic - UV, pulsed light, irradiation, ultrasound; (e) membrane/chemical hurdles.
• Hurdle concept: novel processes are usually combined with low pH, water activity, refrigeration or preservatives; alone they are rarely a full sterilisation.
• Regulatory position in India: a food made by a process "not historically used" or an ingredient without a standard needs prior FSSAI approval under the Non-Specified Food and Food Ingredients Regulations, 2017; the finished food must still meet the FSS (Food Products Standards and Food Additives) Regulations, 2011 and labelling/claims rules. Irradiation has its own FSSAI schedule and licensed plants; HPP and PEF are treated as processing steps validated through HACCP, not as separate licences.

Extrusion: principle, equipment and process

• Definition: forcing a plasticised food mass through a shaped die by one or two rotating screws in a barrel; the screw does mechanical work and the barrel is jacketed, so the dough is simultaneously mixed, sheared, cooked, pressurised and formed.
• Types by temperature: cold/forming extrusion (pasta, noodles, some confectionery; 40-60 °C, screw cooled) and cooking extrusion (snacks, cereals, TVP; 120-180 °C, 10-60 s residence time, HTST).
• Types by screw: single-screw - cheaper, conveying by drag flow, needs friction, poorer mixing, poor tolerance of high fat and high moisture (slippage); twin-screw - co-rotating intermeshing (self-wiping, positive displacement, most common in food), counter-rotating (high shear, poor mixing), non-intermeshing.
• Barrel sections: (1) feed zone - deep flights, conveying; (2) compression/transition zone - decreasing pitch or depth, compaction, melting, starch gelatinisation; (3) metering/cooking zone - shallow flights, maximum shear and pressure (up to 10-20 MPa), temperature peak; then the die and a rotating face cutter.
• Key geometry: screw L/D ratio (typically 10-25 food extruders), compression ratio (feed channel volume ÷ metering channel volume, about 2:1 to 4:1), helix angle, flight clearance, die hole diameter and number.
• Physico-chemical changes: starch gelatinisation and dextrinisation, protein denaturation and realignment into fibres, lipid-amylose complexing, Maillard browning, enzyme and antinutrient (trypsin inhibitor, lipase) inactivation, microbial reduction, loss of heat-labile vitamins and some lysine.
• Expansion mechanism: at the die the melt is above 100 °C and under pressure; on exit, pressure falls to atmospheric, superheated water flashes to steam, bubbles nucleate and grow, and the starch matrix sets as it cools and dries - giving a puffed, low-density extrudate.
• Control variables: feed moisture (usually 12-18% for puffed snacks, 25-35% for TVP, 40-70% for high-moisture meat analogues with a long cooling die), screw speed, feed rate, barrel temperature profile, die size, screw configuration.
• Effect of variables (standard exam answer): raising moisture or lowering screw speed lowers SME and expansion and raises density; raising barrel/die temperature raises expansion up to an optimum, beyond which cell walls rupture; small die holes raise shear and expansion.
• Advantages: continuous, high throughput, very high energy efficiency (no separate dryer-cooker), low water use, versatile shapes, no effluent, inactivates antinutrients, enables fortification.
• Limitations: high capital cost, loss of heat-sensitive vitamins and volatile flavours (so flavour is usually sprayed on afterwards), high wear on screws and barrel liners, needs skilled operation; extrudates are hygroscopic and need moisture-barrier packaging.

High-pressure processing (HPP)

• Definition: packaged or pumpable food is held in a vessel filled with water and pressurised to 100-600 MPa (commonly 400-600 MPa) for seconds to a few minutes at ambient or chilled temperature.
• Governing principles: isostatic principle - pressure is transmitted uniformly and instantaneously in all directions, independent of size and shape, so there is no "come-up" gradient like heat conduction; Le Chatelier's principle - any reaction accompanied by a volume decrease is favoured by pressure; microscopic ordering - at constant temperature, pressure increases ordering of molecules.
• What pressure does: disrupts cell membranes, denatures and dissociates proteins and enzymes, collapses ribosomes, acidifies the cell interior - killing vegetative bacteria, yeasts, moulds, parasites and most viruses.
• What it does not do: covalent bonds and small molecules are barely affected - so vitamins, pigments, amino acids and flavour compounds survive (the basis of "cold pasteurisation"); bacterial spores (Bacillus, Clostridium) survive 600 MPa at room temperature.
• Adiabatic compression heating: compression raises the product temperature by roughly 3 °C per 100 MPa for water (more for fats and oils, 6-9 °C per 100 MPa); the heat is lost again on decompression, so the process is near-isothermal overall.
• Pressure-assisted thermal sterilisation (PATS/PATP): preheat to 70-90 °C, then pressurise to 600-700 MPa; adiabatic heating takes the product to 110-121 °C for a very short time and rapid decompression cools it - this does inactivate spores with far less cook damage than retorting.
• Equipment and cycle: thick-walled forged or wire-wound vessel, intensifier pump, yoke, pressure-transmitting fluid (potable water, sometimes with a trace of soluble oil); steps = load packs, close, fill, come-up (2-5 min to 600 MPa), hold (1-5 min), decompression (a few seconds), unload. Cycle time 5-10 min, so capacity depends on vessel volume and cycle count.
• Packaging requirement: product must be pre-packed in flexible or semi-rigid packaging with at least 10-15% headspace-free compressibility - pouches, PET bottles, thermoformed trays; glass and rigid metal cans are unsuitable.
• Suitability: high-moisture foods; low-acid solids and powders are not suitable (no water to transmit pressure uniformly at cellular level, and spores survive). Air pockets cause pack deformation.
• Advantages: in-pack treatment so no post-process recontamination; retains vitamin C, colour, fresh flavour; extends chilled shelf life 3-10×; tenderises meat; aids shucking of oysters and shelling of crustacea; allows clean-label, preservative-free claims.
• Limitations: batch operation, very high capital cost (a commercial vessel runs into several crores of rupees), no spore kill, enzymes such as polyphenol oxidase and pectin methylesterase are pressure-resistant, limited to chilled distribution for low-acid foods, cost per kg much higher than thermal pasteurisation.

Pulsed electric fields (PEF)

• Definition: a fluid or soft food between two electrodes is exposed to repeated very short (1-100 µs) pulses of high-voltage field, typically 20-80 kV/cm for microbial inactivation and 0.5-5 kV/cm for tissue permeabilisation, with total specific energy of roughly 50-200 kJ/kg.
• Mechanism - electroporation: the field induces a transmembrane potential; when it exceeds a critical value (about 1 V across the membrane), hydrophilic pores form. Below a threshold the pores reseal (reversible electroporation, used to improve extraction and drying); above it the membrane ruptures irreversibly and the cell dies.
• Equipment: high-voltage pulse generator (capacitor bank charged by a DC supply, switched by a thyratron/IGBT), treatment chamber - parallel plate, coaxial or co-field (the industrial choice, small cross-section so uniform field and low electrode area), pump, and a heat exchanger because some ohmic heating is unavoidable.
• Pulse shapes: exponential decay (simple RC discharge, wastes energy in the tail) and square wave (uniform field, more energy-efficient, preferred); bipolar pulses reduce electrode fouling and electrolysis and are more lethal than monopolar.
• Critical factors: field strength E, pulse width, number of pulses/total treatment time, specific energy, pulse shape and polarity, product electrical conductivity (0.1-0.5 S/m typical for juices; high conductivity raises current and energy loss), temperature (mild warming to 35-50 °C greatly increases lethality), pH and ionic strength, and the microbe - large cells are easier to kill, so yeasts > Gram-negative > Gram-positive > spores (not inactivated).
• Applications split: (a) preservation of pumpable, low-conductivity, particle-free liquids - fruit juices, liquid egg, milk, smoothies; (b) cell disintegration pre-treatment at low field strength - potato before cutting (smoother cut, less breakage, less oil uptake), higher juice yield from apple/grape/sugar beet, faster drying and better extraction of oils, pigments and proteins.
• Advantages: continuous operation, very short treatment, excellent retention of vitamins and fresh flavour, low energy compared with thermal pasteurisation, no pressure vessel, easy in-line integration.
• Limitations: liquids only (no gas bubbles, no large particles, no high-viscosity products), no spore inactivation, enzymes only partly inactivated, electrode corrosion/metal migration, dielectric breakdown risk, non-uniform field gives survivors, and in India it is still mainly at pilot/R&D scale (CFTRI Mysuru, IIFPT Thanjavur, NIFTEM, ICAR institutes) rather than commercial.

Comparing the three, and the kinetics used

• Extrusion is a cooking and forming operation whose "novelty" is thermomechanical energy input; HPP and PEF are preservation operations that avoid heat.
• Microbial inactivation under pressure or PEF is often non-log-linear, so Weibull or Hülsheger models fit better than first-order; exams still accept a first-order treatment with a pressure D-value.
• Pressure D-value (D_P): time at constant pressure for one log reduction; z_P: pressure change for a ten-fold change in D_P (analogous to z in thermal processing, expressed in MPa).
• Tail populations (pressure- or PEF-resistant sub-populations) are the practical limit on achieving a 5-log pathogen reduction - the benchmark international juice-HACCP requirement, and the figure validation studies target.

Key points

• Extrusion - continuous HTST cooking and forming by screw(s) through a die; mixing, cooking, shearing, forming and expansion in one unit, 10-60 s.
• Twin-screw co-rotating - positive conveying, self-wiping, wide moisture and fat tolerance; single-screw relies on drag flow and friction, cheaper but less flexible.
• Barrel zones - feed, compression/transition, metering/cooking, then die and cutter; compression ratio 2:1 to 4:1, L/D 10-25.
• SME - specific mechanical energy, kJ/kg, is the single best predictor of extrudate expansion, density and starch conversion.
• Expansion - caused by flash vaporisation of superheated water at the die exit; measured as sectional expansion index (SEI) and bulk density.
• Moisture - higher feed moisture lowers SME, lowers expansion, raises density; it is the most powerful control variable.
• HPP - 400-600 MPa, 1-5 min, in-pack, isostatic and size-independent; follows Le Chatelier's principle.
• Adiabatic heating in HPP - about 3 °C per 100 MPa in water, reversed on decompression.
• Spore limitation - neither HPP at ambient temperature nor PEF inactivates bacterial spores; low-acid HPP products need refrigeration, or PATS.
• HPP packaging - flexible/semi-rigid only, minimal air; glass and rigid cans are excluded.
• PEF - 20-80 kV/cm, µs pulses, electroporation of the cell membrane once the transmembrane potential exceeds roughly 1 V.
• Pulse shape - square-wave bipolar pulses are the most energy-efficient and least fouling; exponential decay wastes energy in the tail.
• Conductivity - high product conductivity raises current, energy consumption and ohmic heating in PEF.
• Low-field PEF - 0.5-5 kV/cm as a pre-treatment to raise juice yield, cut quality and drying rate (reversible electroporation).
• D_P and z_P - pressure analogues of the thermal D and z values; inactivation is often non-log-linear (Weibull).
• Enzymes - PPO, PME and peroxidase resist both pressure and PEF, so a short thermal step or low pH is often still needed.
• Indian regulation - novel processes/ingredients need FSSAI approval under the Non-Specified Food and Food Ingredients Regulations, 2017; products must meet the FSS (Food Products Standards) Regulations, 2011.
• Indian flagship application - extruded fortified rice kernels for the public distribution system, blended 1:100 with milled rice, made on twin-screw extruders to the BIS standard for fortified rice kernels.

Formulas and examples

Example 1 - SME of a snack extruder. Twin-screw extruder, screw speed 300 rpm, net torque 250 N·m, feed rate 120 kg/h.

• N = 300/60 = 5 rev/s; ṁ = 120/3600 = 0.03333 kg/s.
• Power = 2πNT = 2π × 5 × 250 = 7,854 W = 7.854 kW.
• SME = 7.854/0.03333 = 235.6 kJ/kg - in the normal 150-400 kJ/kg band for a well-expanded corn snack. Doubling the feed rate at the same torque would halve SME and give a dense, under-cooked extrudate.

Example 2 - moisture conditioning and expansion. Corn grits at 14% moisture are fed at 100 kg/h; in-barrel moisture must be 18%. Die holes are 3.0 mm; the extrudate measures 9.6 mm diameter and the collected product has bulk density 110 kg/m³.

• Dry solids = 100 × 0.86 = 86 kg/h. At 18% moisture the total mass M satisfies 0.82M = 86 → M = 104.88 kg/h.
• Water to inject = 104.88 − 100 = 4.88 kg/h (≈ 4.9 L/h).
• SEI = (9.6/3.0)² = 10.24.
• Expansion ratio against solid starch (≈1,400 kg/m³): 1,400/110 ≈ 12.7-fold overall volume expansion.

Example 3 - HPP: heating and lethality. Juice at 20 °C is pressurised to 600 MPa and held 3 min. For the target spoilage yeast, D_P = 0.5 min at 500 MPa and 0.05 min at 650 MPa.

• Adiabatic heating ≈ 3 °C/100 MPa × 6 = +18 °C → product reaches about 38 °C at pressure, returning to ~20 °C on release.
• z_P: D falls ten-fold over 650 − 500 = 150 MPa, so z_P = 150 MPa.
• At 600 MPa: log D_600 = log 0.5 − (600 − 500)/150 = −0.301 − 0.667 = −0.968 → D_600 = 0.108 min.
• Log reduction in 3 min = 3/0.108 = 27.8 logs - far beyond the 5-log juice benchmark, which is why hold times of 1-3 min suffice for vegetative cells. Spores, with D_P effectively infinite at 38 °C, are untouched - hence chilled distribution.

Example 4 - PEF: field, energy and temperature rise. Co-field chamber, electrode gap 0.6 cm, pulse voltage 30 kV, capacitance 0.1 µF, frequency 100 Hz, juice flow 200 L/h, c_p = 3.9 kJ/kg·K.

• E = 30/0.6 = 50 kV/cm.
• W_p = ½ × 0.1×10⁻⁶ × (30,000)² = ½ × 10⁻⁷ × 9×10⁸ = 45 J per pulse.
• Pulses per hour = 100 × 3,600 = 360,000; per litre = 360,000/200 = 1,800 pulses/L.
• Specific energy w = 1,800 × 45 = 81,000 J/L ≈ 81 kJ/kg (typical commercial range).
• ΔT = 81/3.9 = 20.8 K - so the juice leaving is about 21 °C hotter and must be chilled immediately; this mild heating itself contributes to lethality.

Example 5 - Hülsheger prediction. For *E. coli* in a juice, E_c = 8 kV/cm, k = 6.3 kV/cm, t_c = 10 µs. Treatment: E = 30 kV/cm, total time 100 µs (50 pulses × 2 µs).

• Exponent = (E − E_c)/k = (30 − 8)/6.3 = 3.49.
• S = (100/10)^−3.49 = 10^−3.49 = 3.2 × 10⁻⁴ → 3.5 log reduction. To reach 5 logs, raise E or time: at E = 40 kV/cm the exponent is (40−8)/6.3 = 5.08, so the same 100 µs gives 5.1 logs.

Where it is used

• Extruded snacks and cereals: Haldiram, Bikaji, Balaji, Parle, ITC (Bingo), Britannia, Kellanova and hundreds of MSME units running single- and twin-screw lines for kurkure-type collets, puffs, ring snacks and breakfast flakes; flavour and oil applied in a post-extrusion tumble drum.
• Nutrition programmes: extruded fortified rice kernels blended into PDS/ICDS/mid-day-meal rice, and extruded fortified blended foods (take-home rations) under the Ministry of Women and Child Development - a large, tender-driven Indian market.
• Plant protein: texturised vegetable protein (soya chunks/granules - Nutrela, Sakthi) by low-moisture extrusion, and high-moisture meat analogues (Blue Tribe, GoodDot, Shaka Harry) by twin-screw extrusion with a long cooling die.
• HPP in India: cold-pressed juices, tender coconut water, smoothies, hummus and dips, marinades and ready-to-eat chicken from a small number of commercial vessels (used by brands such as Paper Boat-type beverage lines and premium juice/HoReCa suppliers), plus contract HPP tolling services.
• PEF and research infrastructure: CSIR-CFTRI Mysuru, IIFPT Thanjavur, NIFTEM, ICAR-CIPHET and IIT/NIT food-engineering labs run pilot PEF, HPP and extrusion facilities; incubation and capital support come through PMKSY components and the PLI Scheme for Food Processing.
• Job roles: extrusion process engineer and die/screw-configuration specialist, NPD scientist for snacks and plant protein, HPP plant/validation engineer, QA-HACCP officer validating a 5-log reduction, equipment sales and application engineer, FSSAI regulatory affairs for novel foods.

Interview questions

• Q: Why does an extrudate puff when it leaves the die? A: Inside the barrel water is superheated under pressure; at the die exit the pressure drops to atmospheric, the water flashes to steam and expands the molten starch matrix, which then sets as it cools and dries.
• Q: What is specific mechanical energy and why do you measure it? A: SME is the mechanical work the screws put into each kilogram of product (kJ/kg), computed from net torque, screw speed and feed rate; it correlates with starch conversion, expansion and bulk density, so it is the standard scale-up and control parameter.
• Q: Twin-screw or single-screw for a new snack line - how do you choose? A: Twin-screw if you need flexibility, high fat or high moisture, sticky or sugary recipes, accurate fortification and good mixing; single-screw if the recipe is fixed, low-fat and low-moisture and capital cost matters more.
• Q: Give three ways to reduce the density of an extruded puff. A: Lower the feed moisture, raise screw speed/SME and barrel-die temperature, and use smaller die holes (also increase amylopectin-rich starch content).
• Q: Why is HPP called isostatic, and why does that matter? A: Pressure in the fluid acts equally in all directions and reaches every point effectively instantaneously, so treatment is independent of package size, shape and thermal conductivity - unlike heat, which must conduct inwards.
• Q: Can HPP sterilise a low-acid food at room temperature? A: No - bacterial spores survive 600 MPa at ambient temperature; you need pressure-assisted thermal sterilisation (preheat plus pressure-induced adiabatic heating to 110-121 °C) or chilled distribution with a short shelf life.
• Q: Why can't you HPP a product in a glass jar? A: The pack must compress by roughly 10-15% with the water and recover; rigid glass or metal cannot, so it cracks or deforms - flexible pouches and PET are used, with air excluded.
• Q: What is adiabatic compression heating and is it a problem? A: Compression raises temperature by about 3 °C per 100 MPa in water (more in fats); it is reversed on decompression, so it is usually a bonus for lethality, but it must be accounted for in validating temperature-sensitive products.
• Q: Explain electroporation in one sentence. A: The applied field induces a transmembrane potential of about 1 V, which destabilises the lipid bilayer into hydrophilic pores - reversible at low field strength, irreversible (lethal) above a critical field strength and treatment time.
• Q: Why are square-wave bipolar pulses preferred in PEF? A: A square wave keeps the field at peak value for the whole pulse instead of decaying, so it is more energy-efficient; reversing polarity stresses the membrane more, reduces electrode fouling and limits electrolysis.
• Q: PEF treatment of a juice raises its temperature 20 °C - what does that tell you? A: That the specific energy input is roughly c_p × ΔT (about 80 kJ/kg) dissipated ohmically; you must chill immediately or the "non-thermal" claim and the vitamin retention are both compromised.
• Q: A client wants PEF for a mango pulp with fibre pieces - what do you say? A: PEF needs a homogeneous, bubble-free, moderately conductive liquid; particles distort the field and shield microbes, so either screen the pulp and treat the liquid phase, or choose HPP instead.
• Q: Which Indian approval route applies to a genuinely new process or ingredient? A: Prior approval from FSSAI under the Food Safety and Standards (Approval for Non-Specified Food and Food Ingredients) Regulations, 2017, with safety data; the finished product must still comply with the FSS product standards and labelling/claims regulations.

Common mistakes

• Quoting SME in kW or kJ without dividing by the mass flow rate, or using screw speed in rpm directly in 2πNT (it must be rev/s).
• Calling HPP and PEF "sterilisation" - both leave spores viable; they are pasteurisation-equivalent processes.
• Assuming HPP time depends on package size or that bigger packs need longer - the isostatic principle makes hold time size-independent.
• Forgetting adiabatic compression heating, or applying the water figure (3 °C/100 MPa) to a high-fat food where it is two to three times larger.
• Mixing up field strength and voltage in PEF (E = V/d) and quoting kV instead of kV/cm; also forgetting that pulses received per litre depends on frequency and flow rate, not just frequency.
• Treating pressure or PEF inactivation as strictly log-linear and ignoring tailing - this under-predicts the time needed for the last decimal reductions.
• Confusing expansion indices: SEI is a diameter ratio squared, not a diameter ratio, and bulk density is not the same as true (particle) density.
• Claiming extrusion retains all nutrients - heat-labile vitamins, lysine and volatile flavours are lost, which is why flavouring and most vitamins are added after the die.

Related topics

Engineering properties of food materials · Food processing engineering · Thermal processing of foods: canning, sterilisation and F-value · Size reduction and screening · Food dehydration: spray, freeze and drum drying · Food packaging and shelf-life.