🔬 Seed Processing and Quality Testing

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

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Seed Processing and Quality Testing

Objectives of Seed Processing

Seed processing transforms harvested grain/seed into marketable quality seed that meets certification standards. The objectives are:

1. Clean seed of inert matter, weed seeds, diseased seeds, undersized/broken seeds
2. Dry seed to safe storage moisture content
3. Size-grade seed for uniformity and better plantability
4. Treat seed with protectants against seed-borne pathogens and storage pests
5. Package and label for distribution

The sequence of operations is called the seed processing flow or seed processing line.

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Unit Operations in Seed Processing

1. Pre-Cleaning (Scalping)

• Removes coarse material: straw, clods, large weed seeds, stones larger than seed
• Equipment: scalper (coarse screens), air screen cleaner (first pass)
• Done immediately after harvest before drying (reduces bulk, speeds drying)

2. Drying

• Most critical operation; must reduce moisture to safe storage level before further processing and storage
• Safe moisture targets: cereals ≤12%, pulses ≤8%, oilseeds ≤7%, groundnut ≤8%

Drying Methods:

Method	Description	Suitable for
Sundrying	Spread seed on tarpaulin; natural solar drying	Small quantities; traditional
Batch dryer	Fixed-bed; hot air forced through grain; timed batches	Medium lots
Continuous flow dryer	Seed flows continuously through hot air stream	Large processing plants
Recirculating dryer	Seed recirculates; gentle; lower temp	Vegetable seeds; sensitive seeds

Temperature limits: cereals 43°C max, vegetables/legumes 38°C max, onion 35°C max. Exceeding temperature limits kills embryo and reduces germination.

3. Basic Cleaning — Air-Screen Cleaner

• The most versatile and widely used seed processing machine
• Separates by size (width and length) using screens, and by density/shape using air blast
• Components: scalping screen (top), sizing screen (middle), aspiration channel
• Removes: small seeds, large seeds, light chaff, heavy material

4. Length Grading — Indent Cylinder / Disc Separator

• Separates by length of seed
• Indents (small pockets) in rotating cylinder or disc pick up short seeds; long seeds remain in main flow
• Used to separate: broken grain, short weed seeds (e.g., wild oat from wheat), short seeds from long seeds
• Critical for crops with length-based weed seed contamination

5. Width and Thickness Grading

• Width grading: slotted screens — seed passes through if narrower than slot
• Thickness grading: round-hole screens — seed passes through if thinner than hole diameter
• Used to remove thin/shrivelled seeds (often low vigour)

6. Gravity Separation — Specific Gravity Separator

• Separates by density (specific gravity)
• Seed flows on a vibrating, inclined, air-fluidized deck
• Heavy seeds → uphill end (high quality)
• Light seeds (hollow, immature, diseased) → downhill end (low quality, discarded)
• Destoner: specific type; removes stones and clods of same size as seed
• Especially important for pulse seed production (removes soil clods)

7. Seed Coating and Pelleting

• Coating: seeds covered with a thin layer of material containing fungicides, insecticides, or colorants; seeds retain original shape
• Pelleting: each individual seed is coated with an inert material to make it round and uniform in size (important for precision planting of vegetables, sugarbeet, carrot)
• Colorants used to: (a) indicate treatment, (b) allow easy visual detection of treated seed (worker safety)

8. Seed Treatment

• Application of protective chemicals to the seed surface before packaging
• Fungicides: Thiram (contact, broad-spectrum), Carbendazim (systemic, against ascomycetes), Captan
• Insecticides: Imidacloprid (systemic; protects against sucking pests in early crop), Chlorpyrifos (contact)
• Bio-inoculants: Rhizobium (pulses), Azotobacter (cereals), Trichoderma (biocontrol)
• Methods: drum treater (slurry), continuous treater, hand treatment (small scale)

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Seed Processing Plant Layout

A typical seed processing plant has the following flow:

Intake → Pre-cleaning → Drying → Basic cleaning →
Length grading → Gravity separation → Treating →
Coating/Pelleting → Packaging → Labelling → Dispatch

Key infrastructure requirements:

• Receiving area with weigh bridges
• Dryer (batch or continuous flow)
• Processing line (sequential machines)
• Finished seed storage (controlled humidity)
• QC laboratory on-site

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Seed Lot — Definition and Size

A seed lot is a definite quantity of seed with uniform origin, variety, and quality.

Crop	Maximum Lot Size
Rice	20,000 kg (20 t)
Wheat	20,000 kg (20 t)
Maize	20,000 kg
Soybean	20,000 kg
Cotton	10,000 kg
Vegetables	10 kg (small-seeded) to 100 kg (large-seeded)

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Seed Quality Testing — ISTA Procedures

ISTA (International Seed Testing Association) publishes the International Rules for Seed Testing — the globally accepted standards for testing seed quality.

A. Purity Analysis

Purity analysis determines the composition of a seed lot by weight percentage.

Four fractions:

1. Pure seed: seeds of the species being tested; includes immature, small, shrivelled if identifiable
2. Other crop seeds: seeds of other crop species
3. Weed seeds: seeds of weed species
4. Inert matter: stems, soil, stones, husks, broken seeds <50% size, insect debris

Procedure: Weigh out working sample (ISTA prescribed size by crop); separate into 4 fractions by hand under illuminated magnifier; weigh each fraction; express as % of total weight.

Standards: Physical purity minimum is notified under Seed Act (typically 97–99% pure seed for major crops).

B. Germination Test

Determines the percentage of seeds capable of producing normal seedlings under optimal conditions.

Procedure:

• Work with 400 seeds (4 replicates × 100 seeds) per lot
• Substrates: between paper (BP), top of paper (TP), sand (S) — substrate specified by ISTA rules for each crop
• Temperature: specified by ISTA (e.g., rice 25°C, wheat 20°C, maize 25°C)
• Count at first count (early germination) and final count (ISTA schedule)

Three categories:

• Normal seedlings: meet ISTA criteria for normal development
• Abnormal seedlings: defective, malformed, diseased — not counted
• Ungerminated seeds: hard seeds, fresh seeds, dead seeds

Result: % germination = (normal seedlings / 400) × 100

C. Vigour Tests

Germination tests only indicate maximum germination potential under optimal lab conditions. Vigour tests assess the ability of seed to perform under field stress.

Vigour Test	Principle	Conditions
Accelerated Ageing (AA)	Expose seed to high heat + humidity → weaken low-vigour seed → germinate	41°C, 100% RH, 72h; then normal germination test
Cold Test	Simulate cold, wet soil conditions	10°C, 7 days in cold; then germination at 25°C
Electrical Conductivity (EC) Test	Low-vigour seed leaks more electrolytes into water; measure EC of soak water	Soak 50 seeds in 250 mL water, 20°C, 24h; measure EC (μS/cm/g)
Tetrazolium (TZ) Test	2,3,5-triphenyl tetrazolium chloride reduced to red formazan by living cells	Soak seed; cut; stain in 0.5–1% TZ solution, 30°C, 3h; evaluate staining pattern

Tetrazolium test is valuable as a rapid viability test (results in 6–8 hours vs. 7–14 days for germination test). Red/pink staining = viable tissue; white/cream = dead.

D. Moisture Content Determination

Oven method (standard):

• Grind seed; weigh; dry in oven at 103°C for 17 hours (cereals, pulses)
• Or 70°C for 48 hours (low-temperature method for oilseeds)
• Calculate: MC% = (fresh weight − dry weight) / fresh weight × 100

Moisture meter (field/quick method):

• Electrical resistance or capacitance of seed changes with moisture
• Quick but needs calibration for each crop; less accurate than oven method

E. Seed Health Testing

Tests for seed-borne pathogens (fungi, bacteria, viruses, nematodes).

Method	Principle	Use
Blotter method	Seeds incubated on moist blotter paper; fungi grow and sporulate; identified under microscope	Broad-spectrum fungal detection; most common
Agar plate method	Seeds plated on nutrient agar (PDA, WA); incubate; isolate and identify fungi	Quantitative; specific pathogens
ELISA	Enzyme-linked immunosorbent assay; detects specific virus antigens	Virus detection (e.g., TMV in tomato, SMV in soybean)
PCR	Amplifies specific pathogen DNA/RNA	Highly specific; for regulated pathogens
Washing test	Wash seed; centrifuge; examine pellet	Externally-borne spores (e.g., bunt in wheat)

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Minimum Seed Standards for Major Crops

Crop	Min. Germination (%)	Min. Physical Purity (%)	Max. Moisture (%)
Rice	80	98	13
Wheat	85	98	12
Maize	90	98	12
Sorghum	75	97	12
Groundnut	70	96	9
Soybean	70	98	12
Cotton	65	97	12
Sunflower	75	98	9
Chickpea	75	98	9
Mustard	75	97	8

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Overview

Seed processing converts harvested crop into quality seed through a series of unit operations — pre-cleaning, drying, cleaning, length grading, gravity separation, treatment, and packaging. Seed quality testing following ISTA procedures covers purity analysis, germination, vigour, moisture, and health testing. Together, these ensure that certified seed reaching farmers meets the minimum standards prescribed under the Seed Act, 1966.

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Summary Cheat Sheet

Topic	Key takeaway
Main focus	Unit operations in seed processing, ISTA quality testing procedures, germination, vigour, purity, moisture, and seed health testing methods.
Section context	Revise this lesson with the rest of Post-Harvest Seed Technology for stronger conceptual continuity.