Processes of Milling and Oilseed Crushing

Every grain of rice on your plate and every drop of mustard oil in your kitchen goes through a series of carefully controlled processing steps before it reaches you. Milling and oilseed crushing are among the most fundamental operations in food processing – they transform raw agricultural produce into consumable, shelf-stable products. Whether it’s converting paddy into polished white rice, wheat into fine flour, whole pulses into split dal, or mustard seeds into cooking oil, these processes determine the quality, nutritional value, and market worth of the final product. Let’s break down each of these processes step by step.

What is grain milling? 🔗

Grain milling refers to the mechanical process of removing the outer layers (husk, bran, or hull) from cereal grains and reducing the inner portion – the endosperm – into a usable form such as polished grain or flour. Each grain type has a distinct milling process, but the general principle remains the same: clean the grain, remove the inedible or less desirable outer layers, and refine the inner starchy portion. According to ScienceDirect, the milling process typically includes removal of the outer hull, followed by toasting, soaking, or grinding to achieve the desired product.

Rice milling: from paddy to polished grain 🔗

Rice is the most widely consumed staple grain globally, and milling is a critical post-harvest step that converts raw paddy into edible white rice. As per the IRRI Rice Knowledge Bank, the basic objective of a rice milling system is to remove the husk and bran layers while producing a kernel that is adequately milled and free of impurities.

Composition of a rice grain 🔗

A typical rice grain is made up of approximately 20% husk, 11% bran layers, and 69% starchy endosperm (the edible white portion). In a well-managed milling process, the ideal output fractions are roughly 20% husk, 8-12% bran, and 68-72% milled white rice. The by-products – husk, bran, germ, and broken grains – all find use in animal feed, rice bran oil production, and rice flour manufacturing.

Steps in modern rice milling 🔗

Modern rice milling is a multi-stage process involving a sequence of machines, each performing a specific function:

1. Pre-cleaning: Raw paddy arrives with impurities like stones, dust, straw, and weed seeds. Pre-cleaning machines – vibrating screens, de-stoners, and air aspirators – remove these foreign materials. This step is essential because impurities can damage downstream machinery and reduce milling efficiency.

2. Dehusking (hulling): The cleaned paddy passes through rubber roll huskers, where two rubber rollers rotating at different speeds create a shearing action that strips off the outer husk. About 80-90% of the husks are removed at this stage, producing brown rice.

3. Paddy separation: Not all grains get dehusked in a single pass. A paddy separator identifies and recirculates unhusked grains back to the husker, while brown rice moves forward. This separation step improves overall milling quality and reduces wear on subsequent machines.

4. Whitening (bran removal): Brown rice still has its bran layer intact. Whitening machines – using either abrasive stones or friction-type polishers – remove the bran to reveal the white endosperm. Typically, 8-10% of the paddy weight is removed as bran during this step.

5. Polishing: A brush or water-mist polisher gives the milled rice a smooth, shiny appearance, improving both visual appeal and shelf life.

6. Grading and sorting: The polished rice is separated into head rice (whole kernels) and broken grains using length graders and sifters. Head rice commands a much higher market price than brokens. Colour sorters may also be used to remove discoloured or defective grains.

7. Packaging: The final graded rice is weighed and packed for distribution.

A key factor affecting milling quality is the moisture content of the paddy at the time of milling. The ideal moisture level is around 14%. If it is too low, excessive grain breakage occurs; if too high, the rice becomes difficult to mill cleanly.

Wheat milling: turning grain into flour 🔗

Wheat milling follows a different approach than rice milling. The goal here is not to produce a whole polished grain but to grind the endosperm into fine flour while separating it from the bran and germ. As noted by UK Flour Millers, the process relies on a series of break and reduction rollers working in combination with sieves.

Steps in wheat flour milling 🔗

1. Cleaning: Wheat grains are cleaned using vibrating screens, magnetic separators, and air aspirators to remove stones, dirt, weed seeds, and metal particles.

2. Conditioning (tempering): Water is added to the cleaned wheat to raise its moisture to a specific level (usually around 15-16%). This conditioning step toughens the bran, making it easier to separate in large flakes rather than shattering into tiny fragments that contaminate the flour. It also softens the endosperm for efficient grinding.

3. Gristing (blending): Millers may blend different wheat varieties before milling to achieve specific flour characteristics – for example, a higher protein content for bread flour or a softer blend for cake flour.

4. Break rolling: The conditioned wheat passes through corrugated cast-steel break rollers set slightly apart. The top roller spins faster than the lower one, creating a shearing action that cracks open the grain and separates the endosperm from the bran without pulverising it. A typical mill may have up to four sets of break rollers.

5. Sifting and purifying: After each break pass, the material is sifted through a series of sieves (called plansifters). Coarser bran particles are separated, and the endosperm fragments – called semolina or middlings – are sent to purifiers for further cleaning.

6. Reduction rolling: The cleaned endosperm particles pass through smooth reduction rollers (a mill may have up to 12 sets) that progressively grind them into fine flour. This step determines the final flour quality, including its protein content, baking performance, and texture.

7. Blending and enrichment: Different flour streams from the mill are blended to meet specific product requirements. In many countries, flour is also enriched with vitamins and minerals (like iron and B-vitamins) to compensate for nutrients lost during milling.

8. Packaging: The finished flour is packed in bags of various sizes for retail or bulk distribution.

White flour is produced using only the endosperm, while wholemeal (whole wheat) flour includes all components – endosperm, bran, and germ – blended back together.

Corn (maize) milling 🔗

Corn processing is done through two primary methods: dry milling and wet milling. In dry milling, the corn kernel is cleaned, tempered, and then passed through degerminating machines that separate the germ from the endosperm. The endosperm is ground into grits, meal, or flour depending on particle size. The germ, which is high in fat (about 10%), is processed separately to extract corn oil.

Wet milling is a more complex industrial process used mainly for starch, sweetener, and ethanol production. The corn is steeped in a dilute sulphurous acid solution for 24-48 hours, after which the softened kernels are ground and the starch, protein, fibre, and germ are separated through centrifuging and washing steps.

Pulse milling: making dal from whole pulses 🔗

Pulse milling – or dal milling – involves removing the outer husk of pulse grains (like chickpea, pigeon pea, lentil, mung bean, and urad) and splitting them into two cotyledon halves. In India, over 75% of pulses are consumed as dehusked splits, making dal milling one of the country’s most important agro-processing activities. According to Enterclimate, there are about 7,000 dal processing mills across India, each handling an average of 10 tonnes per day.

Why pulse milling is challenging 🔗

Unlike rice or wheat, the husk in most pulses is tightly bound to the kernel. This makes dehusking difficult and is the primary reason pulse milling involves a unique pre-treatment step to loosen the husk before mechanical removal. As IndiaAgroNet notes, traditional milling methods only achieve 65-75% recovery of split dal, compared to a potential yield of 82-85% with improved techniques.

Steps in pulse milling 🔗

1. Cleaning and grading: Pulses are cleaned using rotary screens and aspirators to remove dust, chaff, stones, and other impurities. They are also graded by size for uniform processing.

2. Pitting (scarification): An emery roller machine lightly scratches the surface of the cleaned pulses. This creates micro-cracks in the husk, allowing oil or moisture to penetrate and loosen the bond between the husk and the kernel.

3. Oil treatment and conditioning: A small quantity of edible oil (usually linseed oil or castor oil) is applied to the pitted grains. After oiling, the pulses are sun-dried or mechanically dried. This alternate wetting-and-drying cycle is the key to loosening the husk effectively. The cycle may need to be repeated two or three times depending on the pulse variety.

4. Dehusking: The conditioned pulses pass through emery-coated roller dehuskers (also called “gota machines”). In a single pass, about 50% of the grains are dehusked. The dehusked grains (called gota) are separated by sieving, while the remaining unhusked grains are recycled through the process again.

5. Splitting: Dehusked whole pulses are split into two halves using under-runner disk machines or impact splitters. The split dal is then separated from unsplit gota and any remaining whole grains.

6. Polishing: The split dal is polished using a small quantity of oil and/or water to improve its appearance and give it a shiny, attractive finish that consumers prefer.

7. Grading and packaging: The finished dal is graded by size and quality, then packaged for sale.

There are several recognised methods for pulse milling in India, including the wet milling method, dry milling method, and improved institutional methods like the CFTRI method and the Pantnagar method. Among these, dry milling is the most widely used in commercial mills.

Oilseed crushing: extracting oil from seeds 🔗

Oilseed crushing is the process of extracting vegetable oil from oil-bearing seeds such as mustard, groundnut, soybean, sunflower, sesame, coconut (copra), and rapeseed. According to French Oil Mill Machinery Company, the main extraction methods include mechanical pressing (using screw presses or expellers), solvent extraction using hexane, and newer techniques like supercritical CO₂ extraction.

Traditional method: the ghani 🔗

The ghani (also known as “chekku” or “kol” in different regions) is one of the oldest oil extraction devices, used extensively across the Indian subcontinent for centuries. It consists of a heavy wooden or stone mortar fixed to the ground and a pestle attached to a long lever, traditionally turned by a bullock. As the pestle rotates inside the mortar at about 5-7 rpm, it grinds the oilseeds through friction and pressure. Water is added to the ground mass, and the released oil drains out through a hole at the bottom of the mortar, while the oil cake is scooped out from the top.

Ghanis are most commonly used to process mustard, sesame, groundnut, and copra. A single bullock-driven ghani can process about 5 kg of oilseed per hour. While oil yield from a ghani is relatively low, the oil produced is valued for its natural flavour and the absence of chemical residues.

Modern method: screw press expellers 🔗

The screw press expeller is the modern workhorse of mechanical oil extraction. It works by continuously feeding oilseeds into a barrel-shaped pressing chamber where a rotating helical screw compresses the material under high pressure. The oil is squeezed out through small slotted openings in the barrel cage, while the solid residue (press cake or oil cake) is pushed out from the other end.

Before pressing, the oilseeds undergo several preparatory steps: cleaning to remove impurities, cracking or flaking to break open the seed structure, and conditioning (adjusting temperature and moisture) to optimise oil release. As Wikipedia notes on expeller pressing, friction during the process can raise temperatures to 60-99°C, though cold-pressing techniques keep temperatures below 49°C to preserve oil quality and nutrients.

Screw press expellers can process a wide range of oilseeds – from groundnut and mustard to soybean and sunflower – and offer several advantages over traditional ghanis: much higher processing capacity (from less than 1 tonne to over 50 tonnes per day), significantly better oil recovery, continuous operation, and consistent product quality. However, even with a screw press, some residual oil (typically 4-8%) remains in the press cake.

Solvent extraction 🔗

For large-scale industrial operations processing over 500 tonnes per day, solvent extraction is the preferred method. In this process, the oilseeds are first flaked and conditioned, and then the prepared material is treated with a petroleum-based solvent – most commonly hexane – which dissolves and separates the oil. The solvent-oil mixture (miscella) is then heated to evaporate the solvent, leaving behind crude oil. The defatted solid residue (meal) is also desolventised separately.

Solvent extraction is highly efficient, reducing residual oil in the meal to less than 1-2%. It is commonly used in combination with mechanical pre-pressing – where expellers first remove 65-75% of the oil, and solvent extraction recovers the rest. However, the method raises concerns about solvent residues in the final oil, high temperatures during processing, and environmental safety of hexane use.

Comparing oil extraction methods 🔗

Here is a quick comparison of the three main methods:

Ghani: Low capacity (2-5 kg/hr), low oil yield, no chemicals used, retains natural flavour, suitable for household or village-scale operations.

Screw press expeller: Medium to high capacity, good oil recovery (residual oil 4-8% in cake), no chemicals, suitable for small to large commercial operations, versatile across different oilseeds.

Solvent extraction: Very high capacity, maximum oil recovery (residual oil below 2%), uses hexane, best for industrial-scale processing, requires higher capital investment and safety precautions.

Why milling and crushing efficiency matters 🔗

The efficiency of grain milling and oilseed crushing directly impacts food availability, farmer income, and consumer cost. In rice milling, for instance, even a small improvement in head rice recovery – say from 60% to 65% – translates to significantly higher revenue for millers, since head rice is worth nearly twice as much as brokens. In pulse milling, transitioning from traditional methods (65-75% dal recovery) to improved techniques (82-85% recovery) can substantially reduce post-harvest losses. Similarly, in oilseed processing, choosing the right extraction method for the scale of operation ensures maximum oil yield with minimum waste.

Investments in modern machinery, proper maintenance of equipment, correct moisture management, and skilled operation are all critical factors that determine whether a milling or crushing operation achieves its full yield potential.

Key takeaways 🔗

Grain milling and oilseed crushing are multi-step, technology-driven processes tailored to the specific characteristics of each crop. Rice milling focuses on removing husk and bran while preserving whole grains. Wheat milling uses a break-and-reduce system to separate endosperm into flour. Pulse milling requires unique pre-treatment to loosen tightly bound husks before dehusking and splitting. Oilseed crushing ranges from the centuries-old ghani to modern screw press expellers and industrial solvent extraction, each method suited to a different scale and quality requirement.

What do you think? Given the increasing demand for cold-pressed and chemical-free oils, do you think traditional methods like the ghani have a viable commercial future – or will modern expellers and solvent extraction continue to dominate? How might improvements in pulse milling technology help reduce India’s reliance on pulse imports?

References

1. https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/grain-processing
2. http://www.knowledgebank.irri.org/step-by-step-production/postharvest/milling
3. https://www.ukflourmillers.org/flour-milling/the-milling-process
4. https://www.sriboga.com/news/flour-milling-process
5. https://enterclimate.com/setup-pulse-milling-industry
6. https://indiaagronet.com/indiaagronet/post_harvest/pulses.htm
7. https://frenchoil.com/articles-events/oilseed-extraction-using-mechanical-extraction-and-beyond/
8. https://ijeab.com/upload_document/issue_files/1468041564-4%20IJEAB-JUN-2016-5-Technologies%20for%20Oil%20Extraction_%20A%20Review.pdf
9. https://en.wikipedia.org/wiki/Expeller_pressing