Chapter 2: Sort food processing raw materials

Sorting of food processing raw materials is a critical initial step that directly influences the quality, safety, and marketability of the final product. In Kenya, industries such as county referral hospitals, agricultural cooperatives, and hospitality enterprises rely heavily on effective sorting and grading to meet health standards and consumer expectations. Proper sorting removes contaminants and inferior materials, ensuring that only suitable raw materials proceed to processing. This chapter explores the various methods and factors involved in sorting and grading raw materials, providing students with foundational knowledge applicable across diverse professional sectors.

2.1 Sorting and grading of raw materials

Sorting and grading are distinct but complementary processes used to classify raw materials by quality attributes. Sorting separates materials into groups according to a physical characteristic such as weight, size, shape or colour. Grading assigns a quality level to each group against a set standard. Together they give a processing line uniform material, make equipment settings dependable and let the plant meet the specification of its buyers. A milk processing plant sorts and grades its own raw materials in a different way from a hotel kitchen preparing vegetables, but both rely on the same idea: material that is alike is processed alike.

2.1.1 Sorting methods

Sorting methods are grouped in two ways. By the property used, material is sorted by weight, size or shape. By the way the work is done, it is sorted manually, mechanically, optically or by gravity. The property chosen depends on what the next processing step is sensitive to: a peeler is sensitive to size, a portion cutter to weight and a slicer to shape. The way chosen depends on the raw material, the volume handled and the cost the plant can carry. Colour sorting has its own sub-topic and is not repeated here.

2.1.1.1 Weight sorting

Weight sorting separates raw materials into classes by mass, measured in grams or kilograms. It is used where items of similar size differ in weight, or where a later step depends on mass: portion cutting, pack size, cooking or drying time and yield. Two fruits of the same size can differ in weight because their density and water content differ, so they can fall into different classes.

Weight can be measured in two ways:

• By hand: An operator weighs each item or crate on a bench scale and places it in the crate for its class. This suits small volumes and fragile material, but it is slow and depends on the operator reading the scale correctly.
• On a weighing conveyor: Each item passes over a belt fitted with a load cell, which reads its mass in a fraction of a second. A gate, pusher or air jet then moves the item into the bin for its class. This is called weigh-and-eject sorting and it suits large volumes of items that are handled one at a time, such as dressed poultry or fish.

For example, a poultry processing plant may sort dressed chickens into four weight classes so that each class goes to the pack size it fits. The classes below are for illustration only: a plant sets its own from the specifications of its buyers.

The counts add up to 90 + 270 + 210 + 30 = 600 birds, and the shares to 15 + 45 + 35 + 5 = 100%. Records of this kind show the plant how its supply is spread, so it can plan which pack sizes to produce.

Good weight sorting depends on a few habits:

• Zero and tare first: The scale is checked at zero before every batch, and the weight of any crate or tray is subtracted.
• Calibrate: The scale or load cell is checked with certified test weights at the interval the plant sets, and the result is recorded.
• Drain wet material: Surface water adds weight, so washed produce is drained before it is weighed.
• Keep the scale level and still: A tilted bench or a vibrating floor gives readings that wander.
• Guard the machine: Ejector gates and belts are guarded, and floors around wet weighing stations are kept free of slippery spills.

2.1.1.2 Size sorting

Size sorting separates raw materials by one dimension: length, width, thickness or diameter. It is based on a simple idea. An item passes through an opening only if its dimension is smaller than the opening, so a series of openings of increasing size divides a mixed lot into size classes. Weight and size are related but are not the same: a long thin item and a short round item can weigh the same.

Size is sorted by these means:

• Sieves and screens: A flat or rotating screen with holes of a set size lets small items fall through while larger ones ride over. Round holes grade by width or diameter, while slotted holes grade by thickness.
• Rotary drum screens: A slowly turning drum has sections with holes that grow larger along its length, so each size class drops out at a different point.
• Diverging belts or rollers: Two belts or rollers start close together and move apart along the line, so small items drop through early and larger ones later.
• Gauges and templates: Operators check items against a ring, card or calliper set to the class limits. This is slow but needs no power and suits small batches.
• Camera systems: A camera measures the outline of each item and a gate or air jet moves it to its class.

For example, a potato crisps plant may sort tubers into three diameter classes before peeling, such as under 40 mm, 40 to 60 mm and over 60 mm (classes for illustration only). Small tubers lose a larger share of their flesh in a peeler, and slices cut from very large tubers differ in diameter from the rest, so each class is run with its own peeler time and slicer setting.

Size sorting is also used on dry raw materials. A stack of sieves with decreasing apertures splits a lot of grain into fractions, and this is how a laboratory finds the particle size spread of a flour sample. The separation of dirt and foreign matter from grain by screens is a cleaning step and is covered in Raw material cleaning methods.

Common faults are screens overloaded so that small items ride over with the large ones, damp or sticky material that blocks the holes, and worn holes that have grown larger than the class limit. Screens are therefore fed at an even rate, cleaned between lots and checked for wear. Fruit that rolls over hard screens can be bruised, so soft produce is sorted on padded or slow-moving surfaces.

2.1.1.3 Shape sorting

Shape sorting separates raw materials by their geometry: round or flat, long or short, straight or curved, whole or broken. Shape matters where a machine needs a regular form, such as a slicer, a corer or a filling line, and where appearance in the finished product is part of its quality. A shape that departs from the usual is not always a defect: a forked carrot is perfectly safe, but it does not slice evenly.

Shape is sorted by these means:

• Rolling and sliding: On an inclined belt, round items roll down and away while flat or irregular items are carried up by the belt. A spiral separator works on the same principle: round seeds roll fast to the outside of a spiral chute and flat or broken ones slide to the inside.
• Indented cylinders: A turning cylinder has small pockets in its wall. Short or broken pieces fit into the pockets and are lifted out, while long pieces stay in the cylinder. This separates broken grains from whole ones by length.
• Cameras and software: A camera records the outline of each item and the software calculates its length-to-width ratio and roundness. Items outside the set limits are removed by air jets.
• Visual inspection and templates: Trained operators pick out misshapen items by eye or compare them with a template card. This is flexible but depends on the operator staying alert.
• Disc separators: A stack of slowly turning discs has small pockets on its faces. Short or broken grains drop into the pockets and are lifted out, while long grains stay in the mixture and move on. It follows the same idea as the indented cylinder.

For example, a groundnut processing plant may separate whole, round kernels from split and flat ones before roasting, because split kernels roast unevenly and scorch. A vegetable processing plant may send straight, evenly tapered carrots to the slicing line and divert forked or twisted ones to a puree line where shape does not matter. Diverting material to a line where its shape is no problem keeps it in use and reduces waste.

Shape is usually judged together with size, because a small item and a broken item can both pass through the same opening. Plants therefore use shape sorting after size sorting, not instead of it.

Manual sorting

Manual sorting involves workers inspecting and separating raw materials by visible or tactile cues. It is widely used in small operations or where delicate handling is required, such as picking shrivelled, mouldy or split passion fruit from a belt at a fruit processing unit. It allows flexibility in identifying defects that a machine might miss, such as a subtle bruise or the first sign of disease. Sorters work on a table or a slow belt with good lighting, wear clean gloves and hair covering, and take short breaks because attention falls as the work becomes repetitive.

Mechanical sorting

Mechanical sorting uses machines to separate materials by size, weight or other physical properties. For example, a flour mill may use vibrating screens and sieves to separate grain from debris. Mechanical sorting improves speed and consistency but requires capital investment and maintenance. The machines themselves are described in Sorting and grading equipment.

Optical sorting

Optical sorting uses cameras and sensors to detect differences in colour, shape or texture, and it is used in large plants where rapid and precise sorting is essential. The camera and air jet work as described under Colour sorting technology. For instance, a tea processing factory may use optical sorters to remove discoloured leaf and stalk so that the finished tea is uniform.

Gravity sorting

Gravity sorting separates materials by differences in density. For example, a maize mill may use a gravity table to remove stones and lighter impurities from grain. The table is a slightly tilted, vibrating deck through which air is blown: heavy particles settle on the deck and work uphill, while light ones float and drift downhill. This method improves purity and reduces damage during milling.

2.1.2 Colour sorting

Colour sorting is a quality control step that separates raw materials by visible colour, which often shows ripeness, contamination or damage. For example, a tomato sauce plant needs tomatoes that are fully red, because green or yellow shoulders give a pale product with a different taste. A tomato with a brown patch may be rotten, and a grain kernel with dark spots may carry mould.

Importance of colour sorting

Colour differences often signal variations in quality or safety. Discoloured vegetables may indicate spoilage or fungal infection, which could harm the consumer if the material were processed. Colour sorting helps maintain product standards and reduces customer complaints.

Types of colour sorting

Colour sorting can be manual or automated. Manual colour sorting relies on trained workers, such as those at a cashew processing unit picking discoloured kernels from a belt. Automated colour sorting uses sensors and cameras to identify and separate material rapidly, and is increasingly adopted in large processing plants.

Colour sorting technology

Modern colour sorters use high-resolution cameras combined with software to detect colour differences. Each item passes the camera in a thin stream, falling or on a belt, and the software compares its colour with the accepted range. An item outside the range is removed by a short blast of air. These machines can handle large volumes quickly and remove defective items without damaging good raw material.

Challenges in colour sorting

Colour sorting accuracy can be affected by lighting, surface moisture, dust on the camera and variation within the material itself. For example, uneven lighting in a food processing plant can make good produce read as discoloured, and wet leaves reflect light differently from dry ones. Cameras are therefore cleaned and calibrated against reference samples at regular intervals, and operators check the rejected stream to make sure good material is not being thrown out.

2.1.3 Manual grading

Manual grading is the assessment of raw materials by people, who assign quality categories based on appearance, size and other senses. It remains widely used in food processing units because it adapts easily to different materials and costs little to set up.

Criteria used in manual grading

Graders judge size uniformity, colour, texture and the presence of defects. For example, in a fish processing unit, graders place whole fish into quality classes by the clearness of the eyes, the colour of the gills, the firmness of the flesh and the smell. Fish that fall below the lowest accepted class are rejected before they reach the filleting line.

Skills required for manual grading

Effective manual grading needs trained personnel with sharp observation and a working knowledge of the quality standards. In a food processing plant, graders are trained with reference samples and photographs for each grade, and are checked from time to time against the quality laboratory's results to confirm that their judgement has not drifted.

Limitations of manual grading

Manual grading can be subjective and inconsistent, especially with large volumes or tired workers. This variation affects product uniformity and may lead to disputes with buyers over what a grade means. Speed is also limited, and a grader can examine only the surface of an item.

Enhancing manual grading efficiency

Standard grading charts and regular refresher training improve consistency. For instance, a hotel kitchen may use colour-coded grading cards so that staff apply the same limits when sorting fresh produce. Good lighting, rotating graders between tasks and testing two graders on the same sample to compare their results also help.

2.1.4 Machine grading

Machine grading uses automated equipment to classify raw materials by measurable attributes such as size, weight or firmness. It raises throughput and removes much of the personal judgement of manual grading, especially in commercial food processing.

Types of machine grading equipment

Common machines include size graders, weight graders and firmness testers. A mango packhouse may use size grading machines to separate fruit into classes for different packaging specifications, and a firmness tester to check ripeness before the fruit goes to the pulping line. How each machine is built and selected is covered in Sorting and grading equipment.

Advantages of machine grading

Automated grading reduces human error, increases processing speed and records data for quality control. A weight grader, for example, can print the number of items and the total mass in each class at the end of a shift, so that supply and yield can be reviewed.

Challenges in machine grading

Initial investment and maintenance costs can be barriers for small processors. Machines also need calibration to suit different varieties of raw material, and a worn screen or drifting load cell grades wrongly without any obvious sign.

Integration with sorting processes

Machine grading is often combined with sorting to control quality more closely. For instance, a fresh-cut salad unit may run size grading and colour sorting in sequence to select the grade for its premium buyers.

2.1.5 Product damage

Understanding and managing damage to raw materials during sorting and grading is essential to keep quality and reduce losses. Damage can occur through mechanical handling, environmental factors or improper storage.

Types of product damage

Damage includes bruising, cracking, microbial contamination and dehydration. For example, a fruit processing plant loses material when fruit is bruised during sorting, because bruised tissue spoils faster and shortens the shelf life of the pulp.

Causes of damage during sorting

Rough handling, wrong machine settings and overcrowding contribute to damage. In a pickling unit, overloading sorting bins crushes the cucumbers at the bottom and increases waste. Long drops onto hard surfaces, fast belts and worn, rough screens add to the damage.

Impact of product damage

Damaged raw materials may spoil faster, pose health risks and lose market value. This reduces profitability for a processor such as a tea factory that depends on quality leaf, because broken and bruised leaf gives a different cup quality.

Prevention strategies

Gentle handling, regular equipment maintenance and proper training reduce damage. For instance, a juice plant may fit soft belts and lower the drop height between conveyors to reduce bruising of fruit during sorting. Bins are filled to the marked level only, and damaged items are separated at once so that they do not contaminate sound ones.

2.1.6 Reasons for sorting and grading

Sorting and grading serve several purposes that improve product quality, safety and market acceptance. Food processors in every sector use them to remain competitive.

Ensuring product uniformity

Uniform raw materials give consistent processing results and a consistent final product. A flour mill that sorts wheat before milling gets even grinding and a flour that behaves the same way in the dough from one batch to the next.

Removing contaminants and defects

Sorting removes foreign matter and inferior items that could compromise safety. For example, a sesame processing unit removes stones, twigs and damaged seed to prevent equipment damage and contamination of the product.

Facilitating pricing and marketing

Graded products can be priced by quality class, which lets a processor sell to different markets. A coffee processing factory grades beans by size and defects, and each grade is sold at its own price.

Enhancing processing efficiency

Uniform raw materials reduce machine downtime and improve throughput. A vegetable processing plant gains from sorted produce that feeds smoothly into automated peeling machines.

Complying with regulatory standards

Food safety regulations require processors to meet quality and hygiene criteria. Sorting and grading help food processors meet the requirements of the Kenya Bureau of Standards (KEBS) and of public health officers, who inspect processing premises.

2.1.7 Grading factors

Grading assesses the factors that decide whether a raw material suits processing and consumption. Processors judge these factors against the written specification for each grade.

Size and weight

Consistent size and weight allow uniform processing and packaging. For instance, a canning line for French beans grades pods by length and thickness, so that cans fill evenly and the pods cook at the same rate.

Colour and appearance

Colour indicates freshness and ripeness, and a uniform appearance supports product appeal. A chilli sauce plant, for example, grades chillies by colour so that only fully red fruit goes into a red sauce.

Texture and firmness

Texture affects behaviour in processing and consumer preference. A jam-making unit separates firm, just-ripe guavas from soft, over-ripe ones, because ripeness affects the setting of the jam.

Moisture content

Moisture influences shelf life and processing performance. A sorghum mill measures the moisture of incoming grain with a moisture meter, because grain that is too damp spoils in store and mills poorly.

Presence of defects or contaminants

Detecting bruises, insect damage or foreign matter is essential for safety and quality. A fruit processing plant rejects fruit with visible defects at the grading table before it reaches the pulping machine.

Chapter Summary

This chapter examined the processes of sorting and grading raw materials in food processing, highlighting the essential methods used to classify inputs by weight, size, and shape. It explored the role of colour sorting as a critical quality determinant, alongside the distinction between manual and machine grading techniques. The chapter emphasized the impact of product damage on sorting outcomes and detailed the various reasons why sorting and grading are integral to maintaining product standards. Key grading factors were identified, providing a framework for consistent quality assessment. An overview of the equipment employed in sorting and grading was presented, illustrating the technological support that enhances efficiency and accuracy. Finally, the importance of thorough documentation was discussed as a means to ensure traceability and accountability throughout the sorting and grading stages.