Chapter 2 Quality assurance of selected commodities
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2.1 Milled products
The process flow chart for selected milled products is shown in Fig. 11 and a summary of the main quality assurance procedures for cereal and oilseed processing is shown in Table 4 (see Table 4: Quality assurance procedures for cereal and oilseed processing). The quality assurance procedures in the table are discussed in more detail in the following Chapter.
In this chapter three types of milled products are considered:
Although the technology differs for each product, there are similarities in the quality assurance procedures that are needed, particularly when considering raw material and process control.
The main quality factors for milled products are the need for correct raw material storage to maintain the characteristic milling qualities and flavour, the absence of moulds or other contaminants and the correct moisture content for processing. During processing, the main quality control measures are concerned with correct operation of mills, dehullers or oil expellers to maximise yields and give the desired quality characteristics in the finished products. These aspects are described in more detail below, together with information on packaging and storage control for individual products.
In general cereal grains and flours are considered to have a low risk of causing food poisoning because when properly dried they have a low moisture content which inhibits the growth of food poisoning micro-organisms. However, cereals may be infected with moulds if allowed to become moist and one group in particular gives cause for concern. These are moulds of the Aspergillus family (especially Aspergillus flavus and A. parasiticus) that produce poisons named 'mycotoxins' which cause serious liver damage and in severe cases, death. Although this problem might occur in cereals it is more prevalent in oilseeds and it is therefore discussed in more detail in Chapter 2.1.2.
Cereal grains have four main components: the germ, the starchy interior, an outer bran layer and a protective fibrous husk. The purpose of milling is to separate these various components to produce a range of products with the particular qualities that are required during further processing. For example, when milling wheat to produce flour for bakeries, the customer (the baker) may require a finely milled white flour that contains mostly the starchy material with little or none of the bran, germ or husk. Similarly, in maize milling to make flours for foods such as tortilla, kenkey or nsima, the aim is to separate the starchy material from other components. In rice milling the aim is to remove only the outer husk from the grain and for 'polished' rice, to also remove the bran layer.
All cereals may pass through a long chain of unit operations from harvest to final consumption and sometimes this chain may be very complex and involve a large number of people. An example of such a post-harvest chain is shown in Fig. 11 (see Figure 11: Process Flow diagram for cereal and oilseed milling). There are three main parts to the chain; harvesting and storage, primary processing, after which the grain is still not edible and secondary processing to convert the grain to an edible form.
The main areas of quality assurance and control are:
In this chapter quality assurance procedures are described for the first and second parts of the chain, including milling and storage of flour. It should however be remembered that the milled flours are then processed further to make the finished product that is consumed. There are a very large range of finished products including baked goods, pasta, snackfoods, fermented beverages, pastes and porridges etc. An example of the control needed for this final processing is given in Chapter 2.2 on bakery products.
Raw material harvesting
There is an optimum time for harvesting which depends upon the type of crop, its maturity and local climatic conditions. Details of optimum times for different crops are given in the reference (FAO, 1970) in Appendix 1. The main quality considerations are to harvest the crop after it has been partially dried by the sun but before there is significant insect infestation.
A common practice that reduces grain quality is to harvest before the grain is sufficiently mature and dry. Farmers may wish to harvest early to obtain the income, repay debts or to avoid theft of the crop. Grains that contain a higher level of moisture deteriorate more quickly because the enzymes in the grain are still active and the moisture supports mould growth and insect damage during storage.
However some grains, particularly maize and paddy that are left in the field after maturity may become repeatedly moist from night-time dew or rain and then repeatedly dried by the sun. This causes the grain to crack and become more likely to be infected with moulds and infested with insects.
Similarly, cracking can occur in very dry, hot regions where moist grains are dried too quickly. This results in bleaching, scorching, discolouration and reduced milling quality. 'Case hardening' can also be a problem. Here the surface of the grain dries and forms a barrier which prevents moisture from leaving the interior of the grain. If such grain is placed in a store, the moisture slowly moves to the surface and results in mould growth and insect infestation.
Where possible the processor should have some control over the way that cereal crops are treated before they arrive at the mill. This can be done by assisting farmers through extension agents who advise on the correct time of harvest if these problems are found to occur. The agent can also advise on the correct use of pesticides to reduce the likelihood of pesticide residues contaminating the harvested crop.
The other two operations that are usually carried out in the field are threshing and winnowing. These may be done by hand or through the use of small machines. The main quality control factors are to use methods or machines that are appropriate for a particular crop so that the grain is not cracked or broken and to ensure that the separated grain is placed into sacks or boxes to avoid contamination by soil, chaff, stalks etc.
Drying and storage
Correct drying and storage are critical stages in the post harvest system for achieving good quality products. Grains are usually transported to a separate area for further drying and storage. Depending on the climate and the level of investment, drying may be carried out in dryers or in the sun on a large flat surface such as a concrete or hardened earth slab, on mats or plastic sheets or on roofs. It is important that the grain is dried quickly to prevent mould growth, germination, discolouration and insect infestation, but not too quickly as this can result in cracking or case hardening. When grain is dried in the sun on open surfaces there is also significant risk of contamination from birds, dust and animals, as well as wetting from rainfall.
There are considerable quality advantages in using one of the many designs of forced air dryer (for example see Figure 12: A bin dryer for drying cereals) in which the temperature, humidity and airflow are controlled. The grain is dried quickly and uniformly and is less likely to become contaminated by dust, insects or animals. The grain is also stored in different types of silo dryers and therefore requires less handling. However, the higher capital investment and operating costs may make this technology inappropriate for individual small scale farmers; although it may be appropriate for groups of farmers or co-operatives.
There have been many attempts to develop solar dryers which have a lower capital cost and do not require the fuel or power needed by forced air dryers. However, because of the large volume of grain that is harvested at one time, the success of solar dryers has been limited to date. This is due to the higher capital cost compared to sun drying and the extra labour required to regularly load and unload the dryers. In addition the rate and uniformity of drying may not be higher than sun drying. The main benefit of solar dryers is protection of the crop from contamination and periodic rainfall.
Methods for improved grain storage are widely documented (see Appendix l) and are not discussed in detail in this book. The main factors that relate to quality assurance are as follows:
Table 5: Guideline moisture content for safe grain storage
|Type of grain||Maximum moisture content (%) for one year's storage at 27°C and relative humidity of 70 %|
Adapted from: FAO, 1970.
Grain millers or rice polishers frequently buy grain from agents or intermediaries, who in turn purchase from the farmers. The disadvantages of this system are similar to those described in the chapters on fruit products and herbs and spices (Chapters 2.3 and 2.4) and where possible millers should have close contact with farmers to advise them on the drying and storage conditions that will maintain the required quality in the grain. As with these other products there are a number of ways in which this can be achieved, such as agreements with farmers.
It may be possible for millers to reach agreements with a number of farmers in an area to supply grain of a specified quality. Typically the specification would include details of the variety to be grown, the required moisture content, acceptable levels of infestation or contamination and degree of maturity. The price is guaranteed provided that the quality is met and there may be a part of the agreement that specifies the maximum amount that will be bought.
It may also be possible for the processor to offer both training for the farmers in how to dry and store the crop to maintain the required quality and also to employ a field worker to advise the farmer at harvest time.
Quality assurance in the mill
Hygiene and safety rules for buildings and equipment
Details of the correct construction of buildings and layout of equipment are described in Chapter 1.2. In mills the major hygiene problem arises from flour or bran dust which can collect on ledges and floors unless a proper cleaning programme is implemented. If dust is allowed to collect, it attracts insects which may breed to large numbers and contaminate good quality flour. If dust accumulates above equipment it may become rancid or infested and then fall as large lumps into the product, causing gross contamination. Accumulations of flour or bran dust also attract rats and birds which contaminate grain and stored flour with hairs, feathers and excrete. A cleaning schedule should therefore be implemented each day by operators who have the responsibility and time to ensure that it is done effectively.
Flour mills and dehullers are potentially dangerous pieces of equipment and safety procedures should be devised and strictly enforced. For example:
Raw material inspection
When grain is delivered to the flour or rice mill it should be inspected by taking a sample from the batch. Most often the grain is delivered in sacks to small scale millers and a sample should be taken from the sacks using a 'Thief Sampler' (Fig 34). The sample should be carefully examined for the following quality characteristics:
Except for moisture determination (Chapter 3: moisture content measurement, grains), these checks may be carried out without specialised quality control equipment. Operators involved in raw material inspection should be fully trained and some form of incentive scheme to reward careful inspection may also be appropriate. The results of the examination and the amount in each batch should be recorded in an Incoming Materials Test Book, noting the number of the batch and the name of the supplier (Fig. 25).
Depending on the outcome of the raw material inspection and the level of contamination that is found, it may be necessary to clean the grain before it is milled. This is necessary for two reasons: to prevent contamination of the finished product and to protect the mill or dehuller from excessive wear or damage by sand or stones. The cleaning method to be used depends to some extent on the contaminants that are present, but it is usually sufficient to sieve the grain to remove dust, sand, insects, small stones, weed seeds, etc. through a fine mesh and retain chaff, leaves and larger stones on a coarse mesh. The weights of contaminants in each batch should be recorded and the results reported back to the supplier. This evidence may be used to re-negotiate the price paid in cases of serious contamination.
Conditioning of grain before milling is important to ensure the proper separation of the component parts of the grain and give a good yield of flour (or in the case of rice, a good separation of the hulls). A quality control check on moisture content should be made (Chapter 3: moisture content measurement, grains). Grain that is too moist should be redried. If it is too dry a small amount of water is mixed in and the grain is stored for 1224 hours and tested again for moisture content. Parboiling of rice and subsequent drying has beneficial effects on both the efficiency of dehulling and the quality of rice after storage. It helps to prevent rice grains cracking and also increases their resistance to insect attack. The main quality control points are the time of parboiling and the rate of drying.
The grain supplied to rural millers is in most cases, milled to a single flour which is then sold. However, if millers are supplying bakers or other secondary processors, there may be a demand for flours that have specific properties (see Chapter 2.2) or for composite flours. It is then necessary to blend different grains before milling. The quality assurance aspects of blending are mostly concerned with control and identification of stocks, so that the miller knows where different grains are stored and with accurate weighing and adequate mixing of the raw materials to achieve a uniform blend for each batch.
The most appropriate types of mill for different cereal products depend on a number of factors, including the type of cereal being milled, the degree of milling (or fineness) of flour required, power consumption, required throughput, the capital that is available for investment, the availability of spare parts and maintenance/repair facilities. Similar considerations apply when selecting a dehuller. There is a very large range of milling equipment available but there is insufficient space in a book of this type to describe the different options in detail. However mills that are commonly used by small scale millers for all types of cereals include plate mills, stone mills and hammer mills. Roller mills for wheat are usually too expensive for small scale processors and are confined to large scale centralised mills. Rubber roller dehullers are commonly used for rice hulling.
Once a mill or dehuller has been set up correctly, following the manufacturer's guidelines, it should be regularly checked as part of a routine quality assurance programme. This is important because the grinding surfaces wear over a period of time and require adjustment or replacement. For example the grinding plates of a plate mill can last for 1-3 months before replacement, but if there is significant contamination of the grain by sand, the replacement time can be reduced to one or two weeks. If the plates are incorrectly adjusted the quality of the flour, as indicated by its fineness, becomes variable. Similarly, the efficiency of dehullers is reduced if the gap between the rollers is not regularly checked and adjusted. The miller should therefore check the adjustment of the mill or dehuller for every batch of grain.
Other moving parts require a daily check to ensure that they have not become loose or out of adjustment. A metal bolt falling into a mill can cause serious damage to the grinding surfaces and metal filings from worn parts cause contamination of the product. The same considerations apply to the routine checks on dehullers.
A further quality control check is to measure the range of particle sizes in the flour which provides information on the degree of milling and the efficiency of the mill. Particle size measurement is described in Chapter 3 (sieving tests). Although the sieves and shaking machine that are used for this check are relatively expensive, the equipment provides valuable information to those millers who produce specialist white flours. The equipment would not normally be recommended for small scale rural millers.
In rural areas it is common for whole-meal flours to be sold by millers and in this case there is no separation of the components of the grain after milling. Typically maize, sorghum, millet and whole-meal wheat flours are produced by dehulling the grain, milling to a flour and then using the flour directly to make the finished products. In other cases there is a demand for a whiter flour in which the majority of the bran and germ is removed to leave predominantly starchy material (for example maize, wheat and rice flours). This separation is achieved by sieving the freshly milled flour through mesh screens, often fitted with brushes to increase the throughput of the screens.
A quality control check at this stage of the process is to examine the sieved flour for bristles that may have fallen from the brushes. The presence of a bristle acts as an early warning that the brushes may need replacement. In addition checks should be made to ensure that the flour does not contain insect eggs, insect parts or other types of contamination. Larger contaminants are separated by a sieve test (Chapter 3: sieving tests) and small contaminants such as insect parts can be isolated using a 'filth test' (Chapter 3: sieving tests, filth test). Flour mites are a particular problem for two reasons: mite excrete causes flour to have a minty smell and make it unusable; and mite hairs can damage the health of operators by causing skin inflammation and lung disorders. If insect parts or mites are found in flour the source should be traced back to either incoming raw materials or infestation in the flour mill and corrective measures should be taken (Chapter 3: insect infestation).
Records should be kept of the amount of raw material milled, batch numbers and the product code numbers that are given to each sack of flour produced. This allows the processor to trace any subsequent faults in a batch of product back to the process and the raw materials used.
Jute or hessian sacks are commonly used to package flour or dehulled grain but multi-layer paper and woven polypropylene sacks are increasingly being used. There are a few visual quality checks that are done on sacks, mainly to ensure that they are properly cleaned, they are not contaminated with other materials and they do not have holes or split seams.
Flours should be carefully filled to ensure that dust is minimised and that the amount in the sack is the same as the net weight described on the label. In most countries it is an offence in law to sell an under-weight product and over-filling means that the company is giving away product at a loss. A large capacity scale (eg 100kg) may be used if the product is filled by hand. Alternatively semi-automatic bagging machines, in which a weighed amount of product is discharged into an open sack, may be cost-effective for larger scale mills. Sacks are then sealed by either tying the neck with rope or by stitching the top of the sack using a stitching machine (Figure 13: An electric sack stitcher).
Storage and distribution
Once the cereal or flour has been sealed into a sack the risks of contamination by insects, dust and micro-organisms are reduced but not eliminated. Incorrect storage conditions such as exposure to sunlight, heat and water, can all result in spoilage. This is particularly important because the expenditure that has already been made during processing makes losses at this stage very damaging financially.
Sacks of flour or dehulled cereals should be stored on pallets to keep them off the floor of the store-room (see Chapter 1.2). The sacks should be carefully stacked (Figure 14: Examples of good and bad stacking of flour sacks) as a poorly stacked block could collapse and injure operators. The store-room should be cool and dark with a good ventilation to maintain a flow of air. In particular windows should be screened against insects and the structure of the roof and walls should prevent rats and birds from gaining entry. The comments above on cleaning schedules should also include storerooms.
It is important to develop a management system to monitor which products are in the store, to control stock rotation and record their destination for delivery. This is particularly important when a range of products is made as slower-selling items may be hidden at the back of a store and spoil. Simple records (Fig. 31) should be kept by storekeepers to show which products and materials are transferred into and out of the store-rooms.
Where a miller is responsible for delivery of products to other processors, the delivery vehicle should be included in the cleaning schedule described above to ensure that products arrive at the processors in a clean and uncontaminated condition. In most countries the risks of dust, rainfall and contamination by birds and insects require the use of a covered delivery vehicle. Storage of flour by secondary processors is described in Chapter 2.2.
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