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THE MILLING PROCESS

Farmers grow wheat plants and the grain from those plants is used to make flour. Flour is used to make a huge range of food products, such as bread, breakfast cereals, biscuits and cakes. The 'grain chain' is the sequence of events that take place from the planting of the seed through to the food product being purchased. Find out more about the grain chain here.

The milling process is a key step in the grain chain, where the wheat grains are made into flour. The structure of the wheat grain is shown below.

Diagram of wheat seed

A wheat grain is a seed, and each grain of wheat is made up of three distinct parts. There is the coarse outer bran layer then, inside the bran layer, the grain is made up of two main parts. The smaller part is called the germ (which would grow into a new plant) and the larger part the endosperm. The endosperm is the starchy store of food, which the germ would feed on while growing.

During milling, different parts of the wheat grain are used to make different types of flour. White flour is made from the endosperm only. Wholemeal flour uses all parts of the grain: the endosperm, the wheatgerm and the bran layer. Brown flour contains about 85% of the original grain, but some bran and germ have been removed. The miller buys, blends and mills the wheat to produce the right kinds of flour.

The milling process

Milling process diagram
Delivery and storage
  • Each load of wheat is tested against a contract specification for variety, moisture content, specific weight, impurities, enzyme activity associated with sprouting, protein content and quality.
  • It passes through a preliminary cleaning process to remove coarse impurities, such as nails and stones, and may be dried before being stored in silos according to quality.
Assessing wheat quality scientifically
  • The 'Hagberg Falling Number' measures the time, in seconds, a plunger takes to descend through a heated mixture of ground grain and water. The test indicates the alpha-amylase activity in the grain. This enzyme converts starch to smaller sugar units that do not thicken at elevated temperature.
  • Too little enzyme and the mixture will remain viscous. The plunger will take a long time to descend and a high Hagberg Falling Number will be recorded. Too much enzyme and the reverse will be true. High enzyme activity impairs bread quality, producing a very weak and sticky crumb mixture.
Cleaning and conditioning
  • When wheat is drawn from the silos prior to milling, it is thoroughly cleaned. Powerful magnets extract any remaining ferrous metal objects.
  • Machines, which separate by shape, remove barley, oats and small seeds. Gravity separation removes stones and, throughout the cleaning process, air currents lift off dust and chaff.
  • The wheat is then conditioned to a suitable moisture content by tempering it with water and leaving it in conditioning bins for up to 24 hours. This conditioning softens the outer pericarp (bran) layer of the wheat and enhances the release of the inner white endosperm during milling.
Blending
  • Cleaned and conditioned wheat is then blended in a process known as gristing. This combines different wheats to produce a mix capable of yielding the required quality of flour at the lowest possible cost. The protein content of finished white flour tends to be about 1% lower than the wheat due to the removal of the protein-rich germ and outer skins.
  • Although wheat gluten may be added to increase the protein content of milled flours, this is expensive and so is kept to the minimum.
Separating
  • The grist is passed through a series of fluted 'break' rolls rotating at different speeds. These rolls do not crush the wheat but shear it open, separating the white, inner portion from the outer skins.
  • The various fragments of wheat grain are separated by a complex arrangement of sieves. White endosperm particles, known as semolina, are channelled into a series of smooth 'reduction' rolls for final milling into white flour.
Streaming
  • Bran and wheatgerm are streamed into this flour to make brown or wholemeal flour. Baking powder (raising agent) will be added to make self-raising flour at this stage. The nutrients calcium, iron and the B vitamins (niacin and thiamin), which are legally required in all white and brown flours, are also added. (Wholemeal flour already contains these nutrients, although it is lower in calcium.)
  • The whitest flours are produced from the early reduction rolls, with the flour getting less white on later rolls as the proportion of bran particles increases. Brown flour is a mixture of white flour and a portion of the other streams. To produce wholemeal flour, all the streams must be blended back together.
  • In a typical mill, there may be up to four break rolls and 12 reduction rolls, leading to 16 flour streams, a bran steam, a germ stream and a bran/flour/germ wheat feed stream.
Sifting
  • Finally, the flour is sifted before being automatically packed into bags ready for delivery to shops or supermarkets.
  • Bran and wheat feed left over from producing white and brown flours is sometimes used in breakfast cereals or animal feed.

Milling food and safety legislation

  • Bread and Flour Regulations 1998 include rulings on bread fortification, bread names and required contents.
  • Food Labelling Regulations 1996 apply to all food that is ready for delivery to the ultimate consumer or caterers.
  • Weights and Measures Act 1985 imposes requirements on selling by weight, number or in prescribed quantities.
  • Food Safety Act 1990 prohibits the production and sale of food that is not suitable for consumption or is wrongly labelled.

Food safety management: HACCP

Hazard Analysis Critical Control Points (HACCP) is perhaps the best-known food safety management system. It analyses food manufacturing processes and was originally developed in the USA in the 1960s. A requirement of the Food Safety (General Food Hygiene) Regulations 1995 is that all food manufacturing processes utilise a system of identifying critical control points (CCPs).

The essential stages of identifying CCPs are:

  • Select HACCP team including personnel with production, engineering and technical responsibilities.
  • Define scope/terms of reference: understanding the product, how it is to be used and the criteria to be examined.
  • Construct a flow diagram to show the production process, including any relevant time/temperature relationships.
  • Establish CCPs: These materials or parts of the process will require greater emphasis and control.
  • Establish critical limit and tolerances for each CCP. These are target levels that separate acceptability from unacceptability.
  • Establish a monitoring system to measure and observe each CCP in respect of its tolerance/critical limits.
  • Establish a correction action plan to be used to bring the CCP within critical limits if necessary.
  • Establish documentation, essential for the efficient and effective implementation of HACCP.
  • Verify the system to confirm the HACCP system is in compliance with the HACCP plan.
  • Review the HACCP plan when changes occur.

Quality management

Management through the application of quality systems has become increasingly widespread since its inception in 1987. The management system most commonly used in the flour milling industry is based on a series of international standards, ISO 9000, of which ISO 9002 is the most appropriate. Setting up a management system and achieving registration by one of the accrediting bodies can be lengthy and costly, but many customers now request ISO 9000 registration before buying a miller's flour. An effective management system can also show that organisations are working with 'due diligence' under the Food Safety Act. The quality management system can help an organisation to develop and expand its business because it can demonstrate consistent quality of products and services.

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