Agro-processing machinery plays a crucial role in enhancing the value of horticultural produce in Kenya by transforming raw fruits, vegetables, and flowers into finished or semi-finished products. Mastery of these machines enables horticulture professionals to improve product shelf life, quality, and marketability, which directly impacts income and food security. This chapter focuses on the different types of agro-processing machinery relevant to horticulture and their operation, providing essential knowledge for efficient farm machinery use in the horticultural sector.

6.1 Agro-processing machinery

Agro-processing machinery encompasses a range of equipment designed to process agricultural products into consumable or market-ready forms. In Kenya, horticultural produce such as mangoes, passion fruits, tomatoes, and avocados require specialised machinery for peeling, pulping, drying, and packaging. Understanding the types of machinery available and their specific functions helps horticulture professionals select and operate equipment that maximises efficiency and product quality.

6.1.1 Types of Agro-processing machinery

The diversity of horticultural products necessitates various machinery types tailored to specific processing needs. This subtopic explores the main categories of agro-processing machinery used in horticulture, highlighting their functions and relevance in smallholder and commercial farms. The five machine groups covered first are mills, grinders, shellers, dryers and elevator conveyors. Other machines used along the processing line follow after them.

6.1.1.1 Mills

A mill is a machine that reduces dry, hard produce into flour, meal or coarse particles by impact, crushing or shearing. On a horticultural farm, mills are used on dried cassava chips, dried sweet potato slices, dried banana slices and dry maize, and on dried leaves such as moringa. Example: a farm group that dries sweet potato chips on trays can mill them into flour, which sells for more than the fresh roots and keeps longer.

Main types of mills:

• Hammer mill: Swinging hammers on a fast-rotating shaft strike the material until the particles are small enough to pass through a screen. It handles many dry materials and the screen size sets the fineness.
• Plate (disc) mill: Material is crushed and sheared between a fixed plate and a rotating plate. The gap between the plates is adjusted to make coarse or fine meal.
• Roller mill: Material passes between pairs of rollers that crush it progressively. It gives uniform particles and is used where flour must be graded.
• Stone mill: Material is ground between two stones, one fixed and one turning. It is slow and produces a coarse, cool product, so it suits small-scale work.
• Pin mill: Pins on rotating and fixed discs shatter dry material into fine powder. It suits brittle dried products such as dried herbs.

Main parts of a mill:

• Hopper: Holds the material and feeds it into the machine.
• Feed control: A gate or slide that regulates how quickly material enters, preventing overload of the chamber.
• Milling chamber: The space where hammers, plates or rollers act on the produce.
• Screen or sieve: Retains oversize particles until they are reduced, which controls product fineness.
• Drive unit: An electric motor, engine or tractor power take-off that turns the shaft through belts or pulleys.
• Discharge outlet: Delivers the milled product into a bag or collecting bin.
• Frame and guards: Support the machine and cover moving belts and pulleys to protect the operator.

6.1.1.2 Grinders

A grinder reduces produce to small particles, paste or powder by abrasion between rough surfaces, burrs or blades. Grinders differ from mills mainly in scale and in the material handled. They often process smaller quantities of spices, herbs and moist produce such as ginger, garlic and chilli. Example: a farm that grows chilli and ginger can use a grinder to produce chilli powder and ginger paste for sale in packs.

Main types of grinders:

• Hand-operated grinder: Turned by a crank, it suits very small quantities and places without electricity. It is slow and tiring for large batches.
• Motorised dry grinder: An electric motor drives blades or burrs that powder dried spices and herbs. It gives fast, fine output.
• Wet grinder: Uses stones or blades to make pastes from moist produce such as ginger, garlic, tomatoes and chilli. It is built so that wet material does not clog the chamber.
• Burr grinder: Two serrated surfaces crush the material to a set particle size. The gap between burrs is adjusted to choose coarse or fine grind.
• Pulveriser: A high-speed machine that turns dried produce into very fine powder, such as dried moringa leaf or dried chilli.

Points that guide the choice of a grinder:

• Moisture of the produce: Dry material needs a dry grinder, while moist material needs a wet grinder or it will clog and cake.
• Required fineness: Powder, coarse meal and paste each need a different grinding surface and gap setting.
• Capacity: Output per hour must match the quantity the farm processes, otherwise the machine is either overloaded or underused.
• Power source: Electric, engine or manual drive must suit what is available at the processing site.
• Material of construction: Parts in contact with food should be of food-grade stainless steel or another easily cleaned material so the product stays clean and uncontaminated.
• Ease of cleaning: Strong flavours such as chilli and garlic remain in the machine, so it should be easy to dismantle and wash between products.

6.1.1.3 Shellers

A sheller removes seeds, grains or kernels from pods, cobs or husks without crushing the seed. Horticultural farms use shellers on maize cobs, groundnut pods, beans, peas and some nuts. Example: a farm producing groundnuts for roasting uses a groundnut sheller to separate kernels from pods much faster than hand shelling.

Main types of shellers:

• Hand-operated sheller: A small crank or lever machine for household or small-group use. It is cheap and portable but has low output.
• Motorised cylinder-and-concave sheller: A rotating cylinder with spikes or bars rubs the produce against a curved fixed surface so that the kernels are released. It suits maize and beans.
• Beater-type sheller: Rotating beaters strike the pods to break them open. It is gentler on seeds and suits groundnuts and peas.
• Groundnut sheller: An oscillating or rotating beater rolls the pods against a perforated concave sieve. Kernels and broken shells fall through the holes and a fan blows the shells away.
• Nut cracker: Used for hard-shelled nuts such as macadamia, it cracks the shell between adjustable surfaces without crushing the kernel.

Main parts of a sheller:

• Feed hopper: Receives the cobs or pods and directs them to the shelling unit.
• Shelling unit: The cylinder, beaters or rubbing bars that detach the seeds from the husk or pod.
• Concave or sieve: The fixed curved surface or perforated screen against which produce is rubbed, with openings sized to allow kernels to fall through.
• Cleaning fan or blower: Blows away light chaff and husks so the grain outlet delivers cleaner product.
• Outlets: Separate exits for shelled grain and for waste such as cobs or shells.
• Drive and guards: A motor or engine drives the unit through belts, and guards cover the belts and the feed opening.

The moisture content of the produce affects shelling. Produce that is too wet does not detach cleanly, while produce that is too dry makes seeds break more easily.

6.1.1.4 Dryers

Dryers remove moisture from fruits, vegetables, herbs and grains to extend shelf life and reduce storage challenges. By lowering the water content, they slow the growth of moulds and bacteria that spoil produce. Common types include solar dryers, cabinet dryers, and tunnel dryers, each suitable for different scales of operation. Example: a solar dryer at a horticultural cooperative dries mango slices into fruit snacks, using renewable energy to reduce costs.

Main parts of a dryer:

• Drying chamber: The insulated enclosure holding the produce so that heat and airflow are kept inside.
• Trays or racks: Hold the produce in thin layers so air passes around every piece.
• Heat source: A solar collector, electric element, gas burner or biomass furnace that warms the drying air.
• Fan or blower: Moves warm air through the produce and carries moist air out.
• Air vents and outlets: Allow moist air to escape and fresh air to enter, preventing humidity building up inside.
• Controls and thermometer: Used to monitor and set drying temperature, since too much heat cooks or scorches the produce.

Principles that apply to all dryers:

• Heat: Warm air provides the energy to evaporate water from the produce.
• Airflow: Moving air carries water vapour away so that drying continues.
• Produce preparation: Washed, peeled and evenly sliced produce dries uniformly, while thick or uneven pieces dry slowly and spoil.
• Drying end point: Produce is dry when it reaches a safe moisture level for storage, which differs between products.

6.1.1.5 Elevator conveyors

An elevator conveyor lifts or moves bulk produce from one level or place to another without hand carrying. It saves labour, reduces damage and speeds up work along the processing line. Example: at a farm store, a bucket elevator lifts shelled maize or dried produce from the sheller outlet up to a storage bin or to the hopper of a mill.

Main types of elevator conveyors:

• Bucket elevator: Buckets fixed to a belt or chain scoop material at the bottom and tip it out at the top. It lifts grain and dried chips vertically to bins and hoppers.
• Screw (auger) conveyor: A rotating spiral screw in a tube or trough pushes material along. It can move material horizontally or at an incline, for example from a mill outlet to a bagging point.
• Belt conveyor: A continuous belt on rollers carries produce along a flat or inclined path. It is used for sorting, washing and packing lines where produce must not be damaged.
• Chain or drag conveyor: Flights attached to a chain drag material along a trough. It suits dry, free-flowing products.
• Pneumatic conveyor: A stream of air carries light, dry material through pipes. It suits fine or light products but needs a strong blower.

Main parts of an elevator conveyor:

• Boot or feed point: The lower section where material enters and is picked up by buckets, screw or belt.
• Carrier element: The buckets, belt, chain or screw flight that carries the produce.
• Head section and discharge chute: The upper end where material is released into a bin, hopper or machine.
• Casing or trough: Encloses the moving parts and the produce, reducing spillage and dust and keeping hands away from moving parts.
• Drive unit: An electric motor or engine with gearbox, belts or chains turning the head pulley or screw.
• Tensioning device: Keeps the belt or chain at the right tension so that it does not slip or sag.
• Guards and emergency stop: Cover drives and pulleys and allow the machine to be stopped quickly.

Washing machines for horticultural produce

Cleaning is a critical first step in agro-processing, removing dirt, pesticide residues, and microbes from harvested produce. Washing machines use water sprays, brushes, or ultrasonic waves to clean fruits and vegetables efficiently. For example, at a fruit processing facility, washing machines prepare mangoes for pulping by ensuring contaminants do not affect product quality or safety.

Peeling and cutting machines

Peeling machines automate the removal of skins from fruits like mangoes, pineapples and citrus, saving labour and reducing waste. Cutting machines then slice or dice produce into uniform pieces, facilitating further processing such as drying or juicing. In a horticulture cooperative, peeling and cutting machines increase processing speed and maintain uniformity, essential for quality control.

Pulping and juicing machines

Pulpers extract juice and pulp from fruits, separating seeds and fibres, which is vital for producing juices, purees, and concentrates. Juicing machines operate similarly but focus on liquid extraction. In a juice processing plant, pulping machines enable the production of high-quality passion fruit juice with minimal manual effort.

Packaging machines

Packaging machinery automates the filling, sealing, and labelling of processed products, ensuring hygiene and market readiness. These machines vary from manual bag sealers to fully automatic flow pack machines. At a horticultural export packhouse, packaging machines help dried fruit products meet export standards for presentation and shelf life.

Chapter Summary

This chapter provides a comprehensive overview of agro-processing machinery, beginning with the classification of different types such as mills, grinders, shellers, dryers, and elevator conveyors. It explains the specific uses and operational procedures for each type of machinery, emphasizing their role in enhancing farm productivity. The chapter also addresses the critical aspects of maintenance, including servicing routines, repair techniques, and proper storage methods to ensure machinery longevity. Detailed guidance is offered on the operation of farm-based agro-processing equipment, highlighting practical steps for mills, grinders, shellers, dryers, and conveyors. Finally, the chapter covers the general resources, materials, and supplies necessary for maintaining these machines, ensuring they remain functional and efficient for ongoing farm use.