⚙️ Post-Harvest Processing Technologies

This lesson builds core elective concepts in BSc Agriculture with practical applications and exam-oriented clarity.

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Post-Harvest Processing Technologies

Overview

Post-harvest processing converts raw agricultural produce into storable, marketable, or value-added forms. Processing achieves:

• Reduction of water activity (drying, concentration) → inhibits microbial growth
• Thermal inactivation of microorganisms and enzymes (blanching, pasteurisation, sterilisation)
• Exclusion of oxygen (canning, MAP) → prevents oxidative deterioration
• pH reduction (pickling, fermentation) → bacteriostatic/bactericidal effect
• Reduction of temperature (freezing) → stops biological and chemical reactions

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Drying and Dehydration

Principle: reducing moisture content below the threshold for microbial growth and enzyme activity. Target water activity (aw) below 0.70 (most moulds) or 0.85 (most bacteria). Typically corresponds to moisture content of 5–14% depending on product.

Sun Drying

• Oldest, cheapest method; relies on solar radiation and natural air movement
• Advantages: zero energy cost; simple technology; widely accessible in rural areas
• Disadvantages: weather-dependent; slow (2–7 days); poor hygiene (dust, insects, animals); uneven drying; significant quality loss (bleaching, contamination)
• Suitable for: low-value, high-volume crops — chilli, turmeric, ginger, paddy, copra
• Solar tunnel dryers: improvement; HDPE tunnel traps heat; faster than open sun drying; protects from rain and insects

Hot Air Drying

Mechanism: heated air removes moisture from product surface; moisture gradient drives diffusion from interior to surface.

Types:

Dryer Type	Working Principle	Suitable Products
Cabinet (tray) dryer	Hot air flows over static trays; batch process	Vegetables, fruits, spices; pilot scale
Tunnel dryer	Continuous belt through hot air tunnel; counter-current flow	Potato chips, raisin, onion flakes
Spray dryer	Liquid atomised to droplets; dried in hot air; seconds	Milk powder, tomato powder, coffee powder
Drum dryer	Liquid applied to hot rotating drum; dried in one revolution	Potato flakes, starch slurries
Fluidised bed dryer	High-velocity air fluidises particles; uniform fast drying	Spices, pulses, granular products

Key parameters: inlet air temperature (50–80°C for most produce); outlet temperature; residence time; product moisture content at exit.

Spray Drying details:

• Liquid feed (juice, slurry, solution) atomised through rotary atomiser or two-fluid nozzle
• Droplets meet hot air (150–200°C inlet); surface moisture evaporates within seconds
• Powder collected by cyclone separator and bag filter
• Products: whole milk powder, skim milk powder, tomato powder, coffee powder, flavour encapsulates, probiotic cultures
• Outlet temperature critical: too high = scorching; too low = high residual moisture

Freeze Drying (Lyophilisation)

Principle: ice sublimation — solid ice converted directly to vapour without passing through liquid phase, under vacuum (pressure < 611 Pa, i.e., below triple point of water).

Process stages:

1. Freezing: product frozen to −40 to −50°C (ice crystal formation)
2. Primary drying (sublimation): vacuum applied; shelf heaters provide sublimation energy; most water removed (90–95%)
3. Secondary drying (desorption): temperature raised; bound water removed; product reaches final moisture <3%

Advantages:

• Highest quality retention — nutrients, flavour, colour, shape, texture, and biological activity preserved
• Excellent rehydration — porous structure from ice sublimation absorbs water rapidly
• Shelf life: 2–5 years at ambient; can be stored without refrigeration

Disadvantages: very high capital cost (₹2–5 crore for pilot unit); high energy consumption; slow process (8–24 hours); not cost-effective for bulk commodities

Applications: premium mushrooms, strawberries, raspberries, herbs, instant coffee, pharmaceuticals, blood plasma, probiotic cultures

Osmotic Dehydration

• Partial water removal by immersing produce in hypertonic solution (sugar syrup 40–70°Brix, or salt 15–20%)
• Water diffuses out of product (osmosis); solutes partially enter product
• Pre-treatment before final drying: reduces overall drying energy; improves product flexibility (prevents case hardening)
• Products: osmo-dried mango slices (sweetened), pineapple pieces, cranberries (Craisins)

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Canning (Thermal Processing)

Principle: fill product into hermetically sealed metal cans or glass jars and apply heat (retorting) sufficient to commercially sterilise the product — destroy all pathogenic and spoilage organisms.

Classification by pH

• Low-acid foods (pH >4.6): most vegetables, meat, poultry, fish, mixed products; require retort processing at 121°C (15 psi) to destroy Clostridium botulinum spores
• High-acid foods (pH <4.6): fruits, tomatoes, pickled products; pathogens cannot grow; process at 90–100°C (boiling water bath) sufficient

Canning Process Steps

1. Raw material preparation: washing, sorting, grading, peeling, slicing
2. Blanching: brief heat treatment (85–100°C, 2–5 min) to inactivate enzymes (peroxidase, polyphenol oxidase); prevent browning and off-flavours during canning; also softens tissue for better fill
3. Filling: product filled into can; headspace left (6–10 mm) for vacuum
4. Seaming: mechanical double-seam (tin cans) or lug-cap closure (glass); vacuum seaming removes air → prevents oxidation
5. Retorting: cans in pressurised steam retort (batch) or continuous hydrostatic steriliser; temperature-time profile monitored
6. Cooling: rapid cooling to 40°C prevents overcooking and thermophilic spoilage; residual heat dries exterior
7. Labelling and warehousing

Thermal Process Calculations

• F₀ value: equivalent lethality of process at reference temperature 121°C; target F₀ ≥ 3 minutes for low-acid canned foods (margin for Cl. botulinum destruction, 12D concept)
• D value: decimal reduction time (time at given temperature to destroy 90% of organisms); Cl. botulinum D₁₂₁ = 0.21 min
• Z value: temperature increase needed to reduce D value by 10-fold; Cl. botulinum z = 10°C

Can Types

• Tin plate (electrolytic tin-coated steel): most common; lacquered inside (epoxy or oleoresin) to prevent metal contact with acidic foods
• Tin-free steel (TFS / ECCS): chromium-coated; no tin; requires lacquer
• Aluminium: lighter; corrosion-resistant; beverage cans, tuna, pet food
• Glass jars: inert; allows visible product; reusable; heavier; breakable

India: MOFPI supports canning units; major canned products: mango pulp, lychee, pineapple slices, mixed vegetables, pickle.

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Freezing

Principles

Freezing removes heat and drops product temperature below −18°C; biological and chemical reactions essentially cease.

• Ice crystal formation: slow freezing → large crystals → cell wall rupture → mushy texture on thaw → quality loss
• Rapid freezing → small ice crystals → less cell damage → superior texture after thawing

IQF (Individually Quick Frozen) Technology

• Products individually separated and frozen rapidly at −35 to −40°C in blast freezers or fluidised bed freezers
• Fluidised bed IQF: cold air fluidises small particles (peas, corn kernels, diced vegetables) at high velocity; frozen in 2–5 minutes; free-flowing frozen product
• Blanching before freezing (vegetables): inactivates enzymes causing off-colour and off-flavour during frozen storage; particularly lipoxygenase (rancidity in peas) and peroxidase (discolouration)
• Products: peas, sweet corn, french beans, mixed vegetables, berries, shrimp

Frozen Storage

• Standard temperature: −18°C or below; shelf life 12–24 months for most products
• Temperature fluctuation causes recrystallisation (small crystals merge → large crystals → quality loss); maintain stable temperature in storage

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Pickling and Fermentation

Salt Pickling

• High salt concentration (10–20% NaCl) reduces water activity (aw) → inhibits most pathogens
• Traditional Indian pickles (Achaar): mango, lemon, amla, green chilli; salt + spices + oil; shelf stable up to 1–2 years at ambient
• Oil acts as additional barrier layer against moisture and oxygen

Acid Pickling (Vinegar-based)

• Acetic acid (vinegar) lowers pH below 4.0 → bacteriostatic
• Mixed pickle, gherkins, cauliflower, onion; western-style pickles
• pH monitoring critical for food safety

Lactic Acid Fermentation

• Naturally occurring Lactobacillus, Leuconostoc bacteria ferment sugars → lactic acid → pH drops to 3.5–4.0
• Sauerkraut: shredded cabbage + salt; Lactobacillus mesenteroides initiates, Lb. plantarum completes
• Kimchi (Korean): cabbage + spices + fish sauce; consumed worldwide
• Gundruk (Nepal/NE India): fermented leafy vegetables; tribal food security product
• Kanji (India): fermented carrot/beetroot drink; probiotic beverage

Fermented Beverages

• Cider: apple juice fermentation by Saccharomyces cerevisiae
• Vinegar production: alcoholic fermentation → acetic fermentation (Acetobacter); submerged fermentation (Frings acetator) produces vinegar in 24–48 hours

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Juice Extraction and Processing

Extraction Methods

• Fruit crushing + belt press: grapes, apples, pears; 50–75% juice yield
• Screw press (expeller): citrus juice extraction; reamers or rasping; 40–50% yield
• Enzymatic extraction: pectinase + cellulase treatment before pressing; increases juice yield and clarification; mango, guava pulps

Clarification and Filtration

• Centrifugation: removes coarse particles; produces clearer juice
• Fining agents: bentonite (negative charge; removes proteins), gelatin (removes tannin-phenolic haze), isinglass, activated charcoal
• Filtration: diatomaceous earth (DE) filter; membrane filtration (microfiltration 0.1–0.45 µm; ultrafiltration removes bacteria)

Heat Treatment

• LTLT (Low Temperature Long Time): 63°C for 30 minutes; batch; used in smaller dairies
• HTST (High Temperature Short Time): 72°C for 15 seconds; continuous plate heat exchanger; standard for milk and juices
• UHT (Ultra High Temperature): 138–140°C for 4 seconds; aseptic packaging; shelf life 6 months ambient; Tetra Pak

Juice Concentrate

• Single Strength Juice (SSJ): natural not-from-concentrate (NFC) juice
• Concentrate: evaporated in multi-effect vacuum evaporator; 65–70°Brix; reconstituted with water at destination
• India's mango pulp export: major foreign exchange earner; Alphonso and Totapuri varieties; APEDA certified

FSSAI Standards for Fruit Beverages

Product	Minimum Fruit Content	Minimum TSS	Preservative Allowed
Fruit juice	100% juice	Natural	None
Nectar	25–40% juice	≥15°Brix	SO₂ up to 350 ppm
Squash	25% juice	≥40°Brix	SO₂/benzoate
RTS (Ready-to-Serve)	10% juice	≥10°Brix	SO₂/benzoate
Syrup/Sharbat	25% juice	≥65°Brix	SO₂/benzoate

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Drying methods (sun, hot air, spray, freeze), canning, freezing, pickling, fermentation, and juice processing — principles, processes, and quality parameters.
Section context	Revise this lesson with the rest of Processing Technologies for stronger conceptual continuity.