Food processing engineering

In short

Food processing engineering applies the principles of fluid flow, heat and mass transfer, thermodynamics and reaction kinetics to converting raw agricultural produce into safe, stable, convenient foods. It is organised around unit operations — cleaning, size reduction, mixing, separation, heating, evaporation, drying, freezing, extrusion, packaging — each with its own design equations. It matters because India produces huge quantities of milk, fruit, vegetables and grain but processes only a small share, and losses plus value-addition gaps are both large. The topic is a core part of GATE Agricultural Engineering (Processing section), ICAR/ASRB and university exams, and is interviewed by dairy, snack, beverage and food-machinery employers (Amul/GCMMF, Mother Dairy, Nestlé India, ITC Foods, Britannia, Parle, Hindustan Unilever, Haldiram's, Alfa Laval, GEA, Tetra Pak, Bühler) and by FSSAI-regulated QA roles.

Explained

Scope, properties and classification of unit operations

• Unit operation: a physical step (drying, evaporation) common to many processes; unit process: a chemical/biochemical change (fermentation, Maillard browning, hydrogenation).
• Grouped as: mechanical (cleaning, grading, size reduction, mixing, filtration, centrifugation, extrusion), heat transfer (blanching, pasteurisation, sterilisation, evaporation, refrigeration, freezing), mass transfer (drying, distillation, extraction, membrane separation, crystallisation).
• Engineering properties needed for design: bulk and true density, porosity, angle of repose, terminal velocity (pneumatic conveying/aspiration), coefficient of friction, thermal conductivity k, specific heat c, thermal diffusivity α = k/ρc, viscosity (Newtonian for milk/juice; non-Newtonian — pseudoplastic for purees, Bingham plastic for ketchup/butter — described by power law τ = K γ̇ⁿ).
• Water activity a_w = p/p₀ = ERH/100: governs microbial and chemical stability. Bacteria generally stop below ~0.90, yeasts ~0.87, moulds ~0.80; lipid oxidation rises again at very low a_w. Intermediate-moisture foods sit at ~0.65–0.85.
• Sorption isotherm relates equilibrium moisture content to a_w at a given temperature; shows hysteresis between adsorption and desorption; monolayer moisture (BET) is the most stable storage point. Models: BET, GAB, Henderson, Chung–Pfost.
• Moisture content on wet basis M_wb = water/total; dry basis M_db = water/dry solids. Conversion is a standard exam step; drying calculations must use dry basis.

Thermal processing: kinetics and classification

• Microbial death is treated as first-order: D value = time at a constant temperature for a 10-fold (one log) reduction. z value = temperature rise (°C) for a 10-fold reduction in D. F value = equivalent time at a reference temperature.
• F₀ = equivalent minutes at 121.1 °C with z = 10 °C, referenced to *Clostridium botulinum* spores (D₁₂₁ ≈ 0.21 min). The 12-D "botulinum cook" requires F₀ ≈ 3 min minimum for low-acid canned foods (pH > 4.6).
• Q₁₀ and activation energy E_a (Arrhenius) describe quality/nutrient loss; nutrient destruction has a larger z (~25–35 °C) than spore destruction (~10 °C), which is why high temperature–short time (HTST) processing retains quality better for the same lethality.
• Blanching (85–100 °C, water or steam, 1–5 min): inactivates peroxidase/polyphenol oxidase, expels gases, sets colour; adequacy judged by the peroxidase test. Steam blanching loses fewer solubles than water blanching.
• Pasteurisation (mild, <100 °C, kills vegetative pathogens, extends shelf life under refrigeration): milk LTLT 63 °C/30 min (batch/holder), HTST 72 °C/15 s plate pasteuriser, higher-heat-shorter-time 89 °C/1 s. Index organism historically *Mycobacterium tuberculosis*, now *Coxiella burnetii*. Adequacy checked by the phosphatase test (alkaline phosphatase must be negative; peroxidase positive confirms no over-heating in the Storch test).
• Sterilisation / UHT: 135–150 °C for 2–15 s followed by aseptic packaging (tetrahedral/brick cartons) gives ambient shelf life of several months. In-container sterilisation (retorting) at 115–121 °C; rotary and hydrostatic retorts improve heating by induced convection.
• Canning sequence: preparation → blanching → filling → exhausting/hot filling → seaming (double seam) → retorting → cooling (chlorinated water) → labelling. Headspace and vacuum prevent strain and oxidation; thermal centre is the slowest heating point (geometric centre for conduction packs, about one-third up from the base for convection packs).
• Heat penetration test: thermocouple at the thermal centre; plot log(T_retort − T) against time → f_h (time for one log cycle) and j (lag factor); Ball's formula and Stumbo's tables give the process time. Derivation outline: assume straight-line semi-log heating, integrate lethal rate L = 10^{(T−121.1)/z} over time to get F₀ = ∫L dt.
• Non-thermal and emerging: high-pressure processing (300–600 MPa), pulsed electric field, ohmic heating (I²R generated internally, good for particulates), microwave/RF (dielectric, 2,450 MHz domestic / 915 MHz industrial), irradiation (permitted doses notified under the Atomic Energy (Radiation Processing of Food and Allied Products) Rules, 2012 and FSSAI regulations), UV for clear liquids, cold plasma.

Evaporation, drying and dehydration

• Evaporation concentrates a liquid by boiling off water. Types: open pan, short-tube/calandria, long-tube rising-film and falling-film, forced circulation, plate, agitated thin-film (for viscous/heat-sensitive), centrifugal/Centritherm.
• Economy measures: multiple-effect (vapour from one effect heats the next; steam economy ≈ number of effects × ~0.85), thermal vapour recompression (TVR) with a steam ejector, mechanical vapour recompression (MVR) with a fan/compressor. Vacuum lowers boiling point, protecting quality.
• Problems: fouling/burn-on, foaming, boiling-point elevation (Dühring's rule), entrainment losses, flavour loss (recovered by essence/aroma recovery units in juice plants).
• Drying removes water by evaporation into a gas stream, with simultaneous heat and mass transfer. Constant-rate period: surface is saturated, drying rate set by external heat/mass transfer, product stays near wet-bulb temperature. Falling-rate period: internal moisture movement (liquid diffusion, capillary flow, vapour diffusion) controls; product temperature rises toward air temperature.
• Critical moisture content separates the two periods; equilibrium moisture content (EMC) is the limit of drying at that air condition. Thin-layer drying is usually fitted by Page or Lewis models; deep-bed drying is modelled as stacked thin layers.
• Dryer types: tray/cabinet, tunnel (co-current, counter-current), belt/conveyor, bin, fluidised bed and spouted bed, pneumatic/flash, rotary, drum (single, double, twin), spray (pressure nozzle, rotary atomiser; two-stage with fluid-bed agglomeration for instant milk powder), freeze dryer (lyophiliser — sublimation below the triple point, 611 Pa / 0.0098 °C; best quality, highest cost), vacuum, osmotic dehydration (partial water removal in sugar/salt solution), solar (direct cabinet, indirect, mixed-mode, tunnel, PV-assisted), microwave-assisted and puff drying.
• Psychrometrics underpins air drying: dry-bulb and wet-bulb temperature, dew point, humidity ratio, relative humidity, enthalpy, specific volume; heating air moves horizontally on the chart (constant humidity ratio), adiabatic drying along the constant wet-bulb/enthalpy line.

Refrigeration, freezing and cold chain

• Chilling (0–8 °C) slows metabolism and microbial growth without ice formation; freezing (−18 °C and below) immobilises water, dropping a_w sharply.
• Freezing curve: pre-cooling (sensible), plateau of latent heat removal with the bulk of ice forming between −1 and −5 °C (the "zone of maximum ice crystal formation"), then tempering to the storage temperature. Fast freezing → small intracellular crystals → less drip loss on thawing.
• Equipment: air-blast and blast tunnel, plate (contact) freezer, fluidised-bed IQF for peas and green chillies, spiral freezer, immersion/brine, cryogenic (liquid N₂ at −196 °C, CO₂ snow).
• Freezing time by Plank's equation (ignores pre-cooling and variable properties, so it under-predicts); refinements by Cleland and Earle and by Pham.
• Storage disorders: freezer burn (sublimation from the surface), recrystallisation from temperature fluctuation, chilling injury in tropical produce (banana, mango, tomato below ~12 °C), PPP–TTT concept (product, process, packaging; time–temperature tolerance).
• Controlled and modified atmosphere: lowered O₂ (2–5%) and raised CO₂ (2–5%) slow respiration and ethylene action; modified atmosphere packaging (MAP) uses selected films; CA stores are used for apple in Himachal and J&K.

Mechanical operations: size reduction, mixing, separation, extrusion

• Size reduction increases surface area and uniformity. Mechanisms: compression, impact, attrition, shear/cutting. Equipment: roller mill, hammer mill, disc/attrition mill, ball mill, pin mill, colloid mill, cutters/dicers, pulper for fruits, homogeniser (two-stage, 150–200 bar for milk) to reduce fat globules below ~1 µm and prevent creaming.
• Energy laws: Rittinger (surface-area based; fine grinding), Kick (volume/coarse crushing), Bond (intermediate; work index W_i). Fineness modulus and sieve analysis (IS test sieves) characterise ground product.
• Mixing/agitation: paddle, ribbon and Z-blade for solids/dough; propeller, turbine and anchor impellers for liquids; mixing index tracks approach to uniformity; over-mixing can segregate free-flowing particles of different size/density.
• Separations: screening (grading by size), aspiration/pneumatic (terminal velocity), cyclone (collection of flour/powder), filtration (plate-and-frame, rotary vacuum, filter aid; cake filtration equation), centrifugation (cream separator — Stokes' law, disc stack; clarifier; decanter), expression (screw/expeller and hydraulic press for oil, juice), solvent extraction (hexane for oil; supercritical CO₂ for spice oleoresins), distillation (essential oils), crystallisation (sugar, salt), ion exchange and adsorption (decolourising).
• Membrane processes (pressure-driven, by decreasing pore size): microfiltration (0.1–10 µm; bacteria removal, cold sterilisation of milk), ultrafiltration (protein concentration, whey protein concentrate, paneer/chakka standardisation), nanofiltration (partial demineralisation), reverse osmosis (water removal, 20–60 bar). Concentration polarisation and fouling limit flux; countered by cross-flow velocity, turbulence promoters and CIP.
• Extrusion cooking: material is conveyed, compressed, sheared and heated in a barrel, then forced through a die; sudden pressure drop flashes off water and puffs the product. Single-screw versus twin-screw (co-rotating, better mixing and wider moisture range). Controls: screw speed, L/D, compression ratio, barrel temperature profile, feed moisture, specific mechanical energy. Products: breakfast cereals, pasta, fryums, textured soy protein (TSP/soya chunks), pet food, weaning foods, modified starch.
• Frying (deep-fat, vacuum frying), baking (oven heat transfer by radiation, convection and conduction; starch gelatinisation and Maillard browning), coating/encapsulation, agglomeration/instantising.

Plant utilities, heat exchange and CIP

• Heat exchangers: plate (high U, easy to clean, favoured in dairy — regeneration up to 90%+), tubular/triple-tube, scraped-surface (viscous and particulate), spiral, shell-and-tube for utilities; direct steam injection/infusion for UHT (requires culinary-grade steam and later flash cooling).
• LMTD and effectiveness-NTU methods size exchangers; fouling factor must be allowed. Counter-current gives a higher LMTD than co-current for the same duties.
• Steam as the main heat carrier: boiler, dry saturated steam, steam traps, pressure-reducing stations; latent heat at 1 bar(g)/120 °C ≈ 2,200 kJ/kg. Refrigeration by vapour compression with ammonia (R-717) or HFC/HFO refrigerants; ice-bank/chilled water for milk cooling.
• Hygienic design and CIP: sanitary stainless steel AISI 304/316, crevice-free welds, self-draining slopes, no dead legs; CIP cycle = rinse → alkali (1–2% NaOH, 70–80 °C) → rinse → acid (0.5–1% HNO₃) → rinse → sanitise; validated by riboflavin/swab/ATP tests.
• Packaging: tinplate and ETP cans, aluminium, glass, HDPE/LDPE/PP/PET, multilayer laminates and aseptic paperboard; barrier properties (WVTR, OTR), retort pouches, nitrogen flushing, shrink/stretch wrap.
• Water, effluent and by-products: potable water standards, cooling towers, ETP (screening, equalisation, aerobic/anaerobic treatment, high BOD/COD in dairy and fruit effluents), by-product use (whey, rice bran, mango kernel, fruit pomace as feed or pectin source).

Indian regulatory and institutional framework

• Food Safety and Standards Act, 2006 with FSS (Licensing and Registration of Food Businesses) Regulations, 2011, FSS (Food Products Standards and Food Additives) Regulations, 2011, Labelling and Display Regulations, 2020 and the Food Safety and Standards (Food Safety Auditing) framework — administered by FSSAI. Every processing unit needs registration or a state/central licence by turnover and capacity; Schedule 4 sets hygiene and GMP/GHP requirements.
• HACCP and prerequisite programmes; IS 15000 (HACCP) and ISO 22000 certification; BIS standards and AGMARK grading (Agricultural Produce (Grading and Marking) Act, 1937) for ghee, honey, spices and edible oils; Legal Metrology (Packaged Commodities) Rules, 2011 for net quantity declaration.
• Export: APEDA for processed fruit, vegetables and cereals; MPEDA for marine products; EIC/EIA for mandatory export inspection and health certificates.
• Promotion schemes: Ministry of Food Processing Industries — PMKSY (Pradhan Mantri Kisan Sampada Yojana) components including Mega Food Parks, cold chain and Integrated Cold Chain infrastructure, PM Formalisation of Micro Food Processing Enterprises (PMFME), and the Production Linked Incentive Scheme for Food Processing (PLISFPI). Scheme names, outlays and windows change with each plan cycle, so verify current guidelines before quoting.
• Factory-side law: the four Labour Codes in force from 21 November 2025 (Code on Wages; Industrial Relations; Social Security; Occupational Safety, Health and Working Conditions) replaced the Factories Act and the earlier wage/dispute statutes — safety, working-hours, welfare and canteen duties for a food plant now flow from the OSH&WC Code and its rules. GST on food products falls in the 5%/18%/40% structure applicable since 22 September 2025, with most staples nil or 5%; confirm the rate for a specific product, since classification drives it.

Key points

• Unit operation - a physical processing step (drying, evaporation) repeated across many food processes.
• Water activity a_w - ratio p/p₀; stability indicator, not total moisture; bacteria stop below ~0.90, moulds ~0.80.
• Moisture basis - dry basis for all drying calculations; wet basis for trade and specifications.
• D value - time for one log reduction at constant temperature; z value - °C rise that cuts D ten-fold.
• F₀ - lethality in equivalent minutes at 121.1 °C, z = 10 °C; low-acid canned foods need F₀ ≥ ~3 min (12-D botulinum cook).
• HTST advantage - quality factors have a larger z than spores, so hotter and shorter gives the same safety with less damage.
• Pasteurisation - 63 °C/30 min (LTLT) or 72 °C/15 s (HTST); verified by a negative alkaline phosphatase test.
• UHT + aseptic - 135–150 °C for seconds plus sterile filling gives ambient shelf life.
• Blanching - enzyme inactivation (peroxidase test), not a preservation step by itself.
• Drying periods - constant rate (surface-controlled, product at wet-bulb temperature) then falling rate (internally controlled).
• Multiple-effect evaporation - steam economy roughly rises with the number of effects; MVR/TVR cut steam further.
• Spray drying - atomisation, hot-air contact, evaporation, powder separation in a cyclone/bag filter; the route for milk powder.
• Freeze drying - sublimation under vacuum below the triple point; best retention of aroma and structure, highest cost.
• Zone of maximum ice crystal formation - about −1 to −5 °C; crossing it fast limits cell damage and drip loss.
• Plank's equation - freezing time from latent heat, surface and conduction resistance; ignores sensible heat, so it under-estimates.
• Grinding laws - Rittinger (fine), Kick (coarse), Bond (intermediate, work index).
• Membranes - MF → UF → NF → RO by decreasing pore size and rising pressure; fouling and concentration polarisation limit flux.
• Extrusion cooking - shear plus heat plus pressure drop at the die; puffed snacks, pasta, TSP, weaning foods.
• Regulation - FSS Act, 2006 and FSSAI licensing with Schedule 4 hygiene; HACCP/ISO 22000; AGMARK and BIS grades; APEDA for exports.

Formulas and examples

Example 1 — moisture basis and water removal (drying of paddy). Paddy at 22% (wb) is dried to 13% (wb). Feed = 1,000 kg/h.

• Dry solids = 1,000 × 0.78 = 780 kg/h (conserved).
• Final product mass = 780/(1 − 0.13) = 780/0.87 = 896.55 kg/h.
• Water removed = 1,000 − 896.55 = 103.45 kg/h.
• Check with the formula: 1,000 × (22 − 13)/(100 − 13) = 9,000/87 = 103.45 kg/h ✓.
• Dry-basis moistures: M₁ = 0.22/0.78 = 28.2% db; M₂ = 0.13/0.87 = 14.94% db.

Example 2 — D, z and F₀ for a canned low-acid food. *C. botulinum* spores: D₁₂₁.₁ = 0.21 min, z = 10 °C. Required reduction = 12 log.

• F₀ required = D × log cycles = 0.21 × 12 = 2.52 min (industry takes ≥3 min for safety margin).
• D at 110 °C = 0.21 × 10^{(121.1 − 110)/10} = 0.21 × 10^1.11 = 0.21 × 12.88 = 2.70 min.
• Time at 110 °C for the same 12-D kill = 2.70 × 12 = 32.4 min — i.e. ~13× longer than at 121.1 °C, which is why retorting is done hot.
• If a process holds 118 °C for 8 min, lethal rate L = 10^{(118 − 121.1)/10} = 10^−0.31 = 0.49 → F₀ contribution = 8 × 0.49 = 3.92 min > 2.52, so the hold alone is adequate.

Example 3 — HTST milk pasteuriser heat load and regeneration. 5,000 L/h milk (ρ = 1,030 kg/m³, c = 3.9 kJ/kg·K) is taken from 4 °C to 72 °C, held 15 s, cooled back to 4 °C; regeneration = 85%.

• Mass flow = 5,000 × 1.030 = 5,150 kg/h = 1.431 kg/s.
• Total rise = 68 K → total duty = 1.431 × 3.9 × 68 = 379.5 kW.
• Regenerator recovers 85% → 0.85 × 379.5 = 322.6 kW; hot water/steam section supplies 379.5 − 322.6 = 56.9 kW.
• Chiller section must remove the remaining 15% of the cooling load = 56.9 kW ≈ 56.9/3.517 = 16.2 TR.
• Holding tube length for 15 s at 1.5 m/s flow velocity = 15 × 1.5 = 22.5 m.

Example 4 — freezing time by Plank's equation (pea-sized sphere). Green peas, d = 8 mm = 0.008 m, ρ = 1,050 kg/m³, latent heat L_f = 250 kJ/kg, k_frozen = 1.2 W/m·K, freezing point T_f = −1 °C, air at T_a = −35 °C, h = 60 W/m²·K.

• ΔT = −1 − (−35) = 34 K. For a sphere P = 1/6, R = 1/24.
• P a/h = (0.008/6)/60 = 0.001333/60 = 2.222 × 10⁻⁵
• R a²/k = (0.000064/24)/1.2 = 2.667 × 10⁻⁶/1.2 = 2.222 × 10⁻⁶
• Sum = 2.444 × 10⁻⁵
• t_f = (1,050 × 250,000/34) × 2.444 × 10⁻⁵ = 7.7206 × 10⁶ × 2.444 × 10⁻⁵ = 188.7 s ≈ 3.1 min — consistent with fluidised-bed IQF practice.

Example 5 — double-effect evaporator for tomato juice. Feed 4,000 kg/h at 6% total solids, concentrated to 30% TS (paste).

• Solids balance: 4,000 × 0.06 = 240 kg/h → product = 240/0.30 = 800 kg/h.
• Water evaporated = 4,000 − 800 = 3,200 kg/h.
• Single effect at steam economy 0.9 → steam = 3,200/0.9 = 3,556 kg/h.
• Double effect at economy 1.7 → steam = 3,200/1.7 = 1,882 kg/h; saving ≈ 1,674 kg/h of steam (~47%).
• At ₹2.5 per kg of steam and 4,000 operating hours, annual saving ≈ 1,674 × 2.5 × 4,000 ≈ ₹1.67 crore (illustrative; steam cost varies with fuel).

Where it is used

• Dairy plants (Amul/GCMMF, Mother Dairy, Nandini, Hatsun, Nestlé India): HTST/UHT pasteurisers, homogenisers, cream separators, spray-drying for milk powder, UF for paneer and whey protein, CIP systems.
• Fruit and vegetable processing (Dabur, ITC, Varun Beverages, Jain Irrigation's dehydrated onion lines, Mahindra's fruit pulp units): pulping, enzymatic clarification, evaporation to aseptic pulp and concentrate, IQF lines, CA stores for Himachal apple.
• Snack, bakery and cereal plants (Parle, Britannia, Haldiram's, Bikaji, ITC Foods, PepsiCo India): extrusion cooking, frying, baking ovens, continuous mixers, nitrogen-flushed laminate packaging.
• Oil, spice and plantation processing: expellers and solvent extraction for mustard/soy/groundnut, supercritical CO₂ oleoresin units in Kerala and Karnataka, instant coffee spray/freeze-drying plants.
• Job roles: process/production engineer, project engineer in a food machinery OEM (Alfa Laval, GEA, Tetra Pak, Bühler, SPX Flow, Ishida), QA/food-safety officer handling FSSAI licensing and HACCP audits, R&D product development, cold-chain and MFPI project consultancy, FSSAI/BIS/APEDA technical roles.
• Public sector and academia: ICAR institutes (CIPHET Ludhiana, IARI, NDRI Karnal, CFTRI Mysuru, DFRL Mysuru, IIFPT Thanjavur), state agro-industries corporations, and MFPI-funded Mega Food Parks and PMFME-supported micro units.

Interview questions

• Q: What is the difference between pasteurisation and sterilisation? A: Pasteurisation is a mild heat treatment (below 100 °C) that kills vegetative pathogens and needs refrigerated storage; sterilisation uses higher temperatures to destroy spores, giving commercially sterile, ambient-stable product.
• Q: Why is HTST better for quality than LTLT at equal lethality? A: Spore destruction has a small z (~10 °C) while quality and nutrient losses have a large z (~25–35 °C), so raising temperature speeds microbial kill much faster than it speeds quality damage.
• Q: What does F₀ = 3 min mean? A: The process delivers lethality equivalent to 3 minutes at 121.1 °C with z = 10 °C — enough for a 12-log reduction of *C. botulinum* spores in a low-acid food.
• Q: How do you verify that milk has been correctly pasteurised? A: By the alkaline phosphatase test — the enzyme is destroyed just above pasteurisation conditions, so a negative result confirms adequate heating; a positive peroxidase (Storch) test confirms it was not overheated.
• Q: Why is water activity more useful than moisture content for predicting spoilage? A: Microorganisms respond to available (unbound) water; two foods at the same moisture content can have very different a_w depending on solutes and binding, so a_w predicts growth while moisture does not.
• Q: What happens during the constant-rate and falling-rate drying periods? A: In the constant-rate period the saturated surface evaporates at a rate set by external heat and mass transfer, with the product near wet-bulb temperature; in the falling-rate period internal moisture movement limits the rate and product temperature rises.
• Q: Why does fast freezing give better product than slow freezing? A: Rapid passage through −1 to −5 °C forms many small, largely intracellular ice crystals that damage cell walls less, so there is less drip loss and texture change on thawing.
• Q: Why does Plank's equation under-predict freezing time? A: It accounts only for latent heat removal at a single freezing point and ignores sensible pre-cooling, post-freezing cooling and the change of properties with temperature.
• Q: How does a multiple-effect evaporator save steam? A: The vapour from each effect is used as the heating medium for the next, which operates at lower pressure and boiling point, so roughly one kilogram of steam evaporates several kilograms of water.
• Q: Which grinding law applies to fine grinding and why? A: Rittinger's law, because the energy needed is proportional to the new surface area created, which dominates when particles are reduced to fine sizes.
• Q: What makes a food extruder a "cooker"? A: Mechanical shear plus barrel heating gelatinises starch and denatures protein inside the barrel, and the sudden pressure drop at the die flashes water to steam, expanding the product.
• Q: What licences and food-safety systems does a new fruit-pulp plant need in India? A: An FSSAI licence under the FSS Act, 2006 (state or central depending on capacity and turnover) with Schedule 4 hygiene compliance, usually HACCP/ISO 22000 certification, Legal Metrology-compliant labels, APEDA registration for export, and compliance with the OSH&WC Code for plant safety.
• Q: Why is the thermal centre not always the geometric centre of a can? A: In convection-heating liquid packs the circulating product carries heat, pushing the slowest-heating point lower — about one-third of the height from the base — while in conduction packs it is the geometric centre.

Common mistakes

• Mixing wet-basis and dry-basis moisture, or using wet basis in drying-rate and EMC calculations.
• Treating D and z as interchangeable, or adding F values at different temperatures without converting each to equivalent time at the reference temperature.
• Quoting F₀ without the reference temperature and z value, which makes the number meaningless.
• Assuming the whole retort process time equals the holding time — come-up and cooling phases also contribute lethality and must be integrated from the heat-penetration curve.
• Forgetting latent heat in freezing, drying or evaporation energy balances, or using ice's latent heat where water's is needed.
• Confusing blanching with pasteurisation: blanching targets enzymes, not pathogens, and does not preserve the food.
• Reporting a_w as a percentage of moisture, or ignoring that lipid oxidation increases again below the monolayer value.
• Dropping units in Plank's equation (mm instead of m, kJ instead of J) — the commonest source of answers off by 10³.

Related topics

Post-harvest engineering · Grain storage · Dairy and food plant engineering · Heat and mass transfer · Refrigeration and air conditioning · Food quality, HACCP and FSSAI regulation