Evaluating the mill performance

07 May 202115 min reading

The assessment of how well the mill is performing involves measuring each production day’s flour yield for each milling unit. While variations between daily production yields are expected to occur, it is nevertheless important for the mill manager to understand what factors have the greatest impact on daily yield. These factors might include grain characteristics such as moisture content or kernel weight, or environmental characteristics such as temperature and/or humidity in the mill.

Kehinde Peter Owonifari Mill Technologist Crown Flour Mills Nigeria

The wheat flour milling process involves breaking open the grain, scraping the endosperm from bran & germ, and then gradually reducing the chunk of the endosperm into flour by a series of grindings, with intermediate separation of products by sifters & purifiers.

When we talk about milling performance, it simply means the running efficiency of the milling and cleaning machines at optimum capacity. In other words, it means consistency in the operation of the milling equipment and production activities without downtime or breakdown of equipment and maximum utilization of the milling machines' capacity.

Improving milling performance could have different meanings to many people like improving the machine performance, maximization of mill yield/extraction, reduction of production waste, improvement in the preparation of the raw material and, improvement in the finished product quality. For the purpose of this write-up, three (3) key performance indicators (KPIs) will be considered in evaluating mill performance; • Quality of the raw material. • Milling yield • Capacity Utilization.

To achieve the above KPIs, improvement must start from the preparation of the raw materials (Wheat) which includes; the cleaning of the grains, conditioning and tempering of the wheat. Good preparation will have a positive effect on the mill performance.


Quality starts from the raw material preparations. It should be noted that this is dependent on standard measured by the millers based on customers’ requirements which include; the moisture content, protein, ash, wet gluten, gluten index, starch damage.

• Wheat quality: We start from the consideration that the raw material meets the quality parameters needed based on the type of finished product quality to be produced and as defined by the industry standards. At the reception of wheat, an organoleptic test is carried out by visual checking, smelling and feeling, to confirm if it is the exact wheat required, free from any form of infestations and good for milling. Also, all the quality parameters need to be check, in other to be able to adjust and set the parameters to the required standard to achieve the required final product. • Efficient Cleaning system The wheat delivered to the mill is not pure but contains all kinds of foreign materials, sand, dust in the crease of the wheat, weed-seeds of all kind, as well as sometimes shrinkled wheat. These impurities may vary between 1-3% .or 3-10% according to the country of origin. A poor cleaning system can drastically affect quality. If inorganic materials like Iron, dust, stones and other impurities escape to the mill, it will not only impair the quality of the product but cause harm for human health when consumed, damages of machines' components, and could cause dust explosion in the plant.

Therefore, the raw grains must be properly prepared by passing through different cleaning machines to remove the impurities before conditioning and tempering of the wheat. The cleaning machines make use of the different principle of separation such as; Separation Principle Machines • Size Drum sieve, Separator • Terminal air velocity Aspiration channels • Magnetic reaction Magnet separator • Density De-stoner, Combinator • Length Indented separator • Shape Spiral separator • Reflection of visible light spectrum Optical sorter

This is needed to separate particles and impurities that differ from wheat and must be carefully removed. There is a true saying in milling: “A well-cleaned wheat is half grinded!. The cleaning machine must work continuously at high efficiency, without a repetition of that operation.


For example, the insufficient performance of one of the cleaning machine like the aspiration machine will lead to:

• A significantly higher presence of mineral dust particles in the mass of the wheat grain and on the surface of wheat kernels. This dust presents a high risk of moving to flour mass, generating color problems, ash, etc. • Unseparated foreign bodies in the cleaning section, because of higher friability than endosperm’s, may generate particles that give the same problems of color, i.e. ash, and also affects extraction by inflating the bran percentage.


The purpose of a milling activity is the transformation of a mass of milling grain into finished products. The process itself is a collection of operations that involves several steps which includes; preparing for milling raw material by proper tempering, and efficient grinding. These are factors which can influence the mill extraction. • Dampening and tempering of wheat Tempering is the process of adding water to wheat before milling to toughen the bran and mellow the endosperm of the kernel and thus improve the efficiency of flour extraction. After the separation of all the impurities and the adhering surface dirt, the cleaned wheat is then dampened using an automatic dampener(Turborlizer) and thoroughly mixed in a special type of dampening worm or in a rapid dampener.

Depending on the hardness, temperature of the wheat, initial moisture content, and condition of the outer bran layers, the wheat is allowed to rest in so-called conditioning bins for varying durations of time, in order to allow the required time for the water to penetrate inside the grain and get evenly distributed over the entire grain. This is called the Tempering time or conditioning time.

Especially the epidermis and seed coat, distribution of the water onto the kernel surface, quantity of protein. The tempering time indicated in Table:

However, as tempering moisture within the kernel increased, flour extraction rate decreased. Therefore, the tempering procedure should balance the level of flour extraction and the acceptable level of bran in flour. The preparation of wheat by repeatedly dampening and conditioning is aimed at bringing the wheat to an ideal condition suitable for milling. By ensuring the ideal distribution of water within the kernel, optimum milling results can be achieved.

Most tempering studies have focused on tempering mechanism and milling yield but not flour quality. Few reports link tempering effects to flour qualities such as ash content, farinograph, and extensograph quality. Wheat with constant humidity will create conditions for stable milling, a loading at relatively constant flows of products, cleaner intermediate products, finished products with lower and constant ash content, products of a granulation corresponding to the final goal and not ultimately minimal specific power consumption.

• Grinding system This is the section where the real milling of raw wheat to flour takes place. It consists of the Break system, Grading or Sifting, Purification, Scratch system, and Reduction system; in which the number of passages is decided on the mill flow sheet based on the type of raw material e.g. (Wheat), to be milled and the finished product to be produced.

Fig 1: Parts of a wheat kernel // Source: Canadian Wheat Board.

A wheat kernel is made up of three main parts: bran, germ, and endosperm. One of the main goals of the wheat miller is to separate the wheat kernel as efficiently and completely as possible into these three parts as shown in (fig 1).

This separation determines the percent of the kernel that is converted to flour. A key consideration is not to remove too much of the bran layers with the endosperm. Removing bran with endosperm causes flour ash content to increase which decreases the baking quality of the flour.

Equipment within the grinding section has different technological features. Relating to the equipment, we classify the factors influencing the extraction in two major groups, namely: a. Factors related to the technological features of the equipment b. Factors related to the plant concept, the mill flow diagram

Each machine within the grinding section has certain technological features. They are not chosen at random but are a whole. An incorrect corrugation at B1 will immediately affect the plant performance. This is valid also for sifting, semolina purifiers, etc.

Break System

The aim of the break system is to break up the wheat kernel, separate the bran from the endosperm as efficiently as possible. To achieve this aim the following keynote needs to be put into consideration: • Proper wheat dampning and tempering • Gentle grinding of the break roll • Good condition of the roll flutes and proper roll differential • Use of bran finisher The break system and stock distribution is one of the key factors that need to be well set for an effective mill performance. Millers must adjust the entire plant according to his intuition and ability, to obtain the best results in terms required by the expected quality of the finished product.


This is the separation and stratification of the intermediate stock( mixture of semolina, bran particles and germ) into clean semolina/middlings particles, endosperm with bran attached, bran particles and germ, using the principle of terminal air velocity. The terminal air velocity principle is influenced by factors such as; Density, shape and size of the stock particles.

The purifier helps to improve the quality of the finished product, especially in a situation where production of good yield with low ash flour is required. For an optimal purifier performance the following point must be achieved: • Uniform feeding of stock across the surface area of the sieves. • Proper air settings. • Good condition of the sieve mesh. • Efficient brush cleaning system.

Reduction system

The aim of the reduction system is to reduce the semolina of different sizes and ash content step by step into flour. During grinding the temperature in the grinding gaps are to reach a certain limit in order not to damage the gluten characteristics which could affect the baking characteristics of the flour.

To achieve an efficient reduction of semolina and middlings to flour the following important points need to occur: • Constant feeding of stock and distribution of stock over the entire length of the rolls. • Correct roll surface, differential speed and taper of the roll. • Appropriate flake detacher underneath the reduction rolls. To improve extraction and mill performance the above points need to be seriously put into consideration.


This is defined as the percentage of the equipment's capacity used during the milling process. This is based on the production in a steady state, and is strategically pursued to maximize output and ensure the finished product is of the required standard. Milling equipment needs to be in its perfect state to achieve maximum utilization.

The factors seriously influencing the final outcome of the finished product are ruled by three parameters: the processing capacity, the extraction and the type of finished product. Any changes in one of the 3 factors will influence the other two parameters as highlighted below: • Increasing the capacity is at the expense of extraction. It leads to overloading the milling machines with could affect the distribution of stocks. • Operation of the mill under-capacity conditions increases the extraction and alters the flour color, increasing their ash content and over sifting occurs. • Increasing capacity alters the granulation of flours, meaning it lowers the average diameter of flour fractions.


• Maintenance Policies (Preventive and Predictive Maintenance)

Maintenance is human activities conducted to maintain or take care of an object/hardware so that it can continue to be in use. A poor maintenance policy gradually affects the efficiency and performance of machines and equipment, which can end up in low capacity utilization. For example; a poor pneumatic filter performance can lead to low pressure and reduce the capacity of the pneumatic lifts which could cause blockage in the pneumatic lift.

Also, fault in feed roll or mechanical part of a roll could affect the grinding performance of the roll in which it affect the capacity utilization and milling performance.

Recommendation: A preventive and predictive maintenance is essential to be carried out on the milling machines in other to achieve maximum utilization and not wait for equipment failure before repair which also increases production downtime.

• Temperature and Humidity Temperature and humidity effects in mills have been studied over a long time ago (80years) and the conclusions are that both temperature and particularly humidity play a role in how the mill performs. Air stabilization has been shown to increase stock uniformity, increase extraction, and improve flour quality.

Most commercial mills do not have air stabilization systems to control temperature and humidity. Controls used were opening and closing windows and air returned into the plant from the pneumatic systems as temperatures dropped.

Recommendation: Building ventilation systems are strongly recommended. It helps to avoid condensation in the mill, balanced temperatures between the floors and increase plant availability.


Downtime occurs when production is shut down for any reason. Downtime is an important variable to track because anytime production is stopped, yield losses occur associated with the milling unit going down and starting back up. Equipment is not running at optimum settings anytime downtime occurs. Thus, a reduction in yield is expected on any day that encounters downtime.

Downtime which occurs due to machine breakdown could be as a result of not carrying out proper maintenance on the machine for a long time. The following maintenance is recommended to enhance the efficient performance of the milling machine in other to maximally utilize the milling or machine capacity.

RECOMMENDATIONS: Preventive Maintenance

This involves physical examinations over the equipment to prevent damage and prolong the age of service equipment within a certain period or duration of the operation of the machine During this period, the maintenance functions are implemented and time-based care activities (Time-based Maintenance – TBM) customarily done. Preventive Maintenance is carried out on the basis of the estimated probability that equipment will experience a decrease in performance or damage at the specified interval. Preventative maintenance performed includes lubrication, cleaning equipment, spare part replacement, adjustment and inspection.

Predictive Maintenance

Predictive maintenance is often also referred to as maintenance based on condition (Condition Based Maintenance – CBM). In this strategy, the action is taken in response to the treatment condition of certain equipment or when the equipment experiencing a declining of performance. Diagnostic techniques are used to measure the condition of physical equipment such as engine temperature, noise, vibration, lubrication and corrosion. When one or more of these indicators reach a predetermined threshold, maintenance initiatives underway to develop equipment to the desired conditions. This means that the equipment is removed from the production line only if there is direct evidence that there has been noticeable performance deterioration.

Predictive maintenance is based on the same principle of preventative maintenance even though it uses different criteria to determine specific maintenance needs. Another plus is that the need to perform maintenance occurs only when the need is real, and not after the passage of a certain period

Corrective Maintenance

This is a concept carried out to avoid equipment failure, equipment reliability improvement being expanded so that equipment failure can be removed (increased reliability), and equipment can be easily maintained (maintenance of equipment upgrade). The main difference between corrective and preventive maintenance is that the problem should exist before any corrective action is taken. The purpose of corrective treatment is to improve the reliability of the equipment, the ability of the maintenance, design flaws (material, shape), equipment undergoing structural reform, reduce damage and failure.

In conclusion, The assessment of how well the mill is performing involves measuring each production day’s flour yield for each milling unit. While variations between daily production yields are expected to occur, it is nevertheless important for the mill manager to understand what factors have the greatest impact on daily yield. These factors might include grain characteristics such as moisture content or kernel weight, or environmental characteristics such as temperature and/or humidity in the mill.

There are many other factors, and technological advancement in milling which can be implemented and use in improving milling performance. Most importantly, managing the process effectively is crucial in any milling industry.

The miller or mill manager must be well equipped, trained and have a good knowledge of how to handle the cleaning and milling processes from the intake of the raw material, cleaning of the wheat, tempering and milling of the wheat to the finished product. The miller must be able to balance and set the mill, using the necessary tools and measures, check the break releases to ensure the break rolls release and ensure consistency in the mill performance.

About the Author: Owonifari Kehinde Peter is a mill technologist. He obtained a degree in Electrical Engineering. Presently pursuing his MBA degree program at the University of Suffolk, UK. He started his milling career in one of the largest flour milling companies in Nigeria, 11 years ago, where he started as trainee miller to senior miller level. He is presently working with Crown Flour Mills Ltd (Olam Grains Pasta) in Nigeria.

He is responsible for leading his team in the production of pasta flour, oversees the milling activities and ensures the flour produced is consistent in quality, hygienic for human consumption and meets customer satisfaction. He also has a passion in training millers in order to develop strong milling skills, organized and manage plant maintenance projects to keep the facility safe, efficient and clean.

Peter is one of the pioneers set of the African Milling School, Kenya-Nairobi in 2015, where he graduated with a diploma in milling and came out as the 1st runner-up best graduating miller. He is also a graduate of the Swiss School of Milling, St Gallen, Switzerland, where he also bagged a diploma as a mill technologist. He also owns a Certificate of Professional in 48hrs Occupational Health and Safety Manager (OSHA), Advance Certificate in NABIM (Uk- City &Guilds). Peter is a trained miller and manager and has handled numerous jobs which include; commissioning of bread flour mill, pasta/ semolina mill, soft biscuit and sorghum mill.

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