Flour extraction and mill gain/loss

by Teresa Acklin
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Industry consultant David Sugden explains the procedures needed to assure optimum mill performance.

      The process of monitoring, measuring and recording flour extraction is necessary on a regular basis — daily, weekly or quarterly, at the least — to measure efficiency and to take corrective action if needed. With extraction information, flour millers can ensure optimum financial performance commensurate with the product quality necessary to satisfy all customers. Extraction is also one indication, among several, of product consistency.

   Extraction is defined as the quantity of flour obtained from wheat. Another recommended calculation is mill gain/loss, defined as the difference between raw or dirty wheat weighed into the plant and the sum of all products weighed out of the plant.

   Three methods of recording extraction are shown in Table 1 on page 36; while many variations exist, these are the principal ones. The calculations are worth looking at carefully.

   First, let's clarify the ideal method of measurement and put it in perspective. Accurate weighers are the secret at each and every point. Truck and rail car weighbridges at grain intake points and bulk or bagged product outloading points are a significant benefit.

   Indeed, this weighing equipment, whose measurements typically are components of trade contracts, will normally be licensed and stamped by government inspectors. Intake batch scales and bagging weighers usually are similarly approved.

   Production weighers are not normally inspected by government officials unless there is a trade-related need. Production weighers come in a number of types, including mechanical or electronic batch weighers, gravimetric, constant weight, loss in weight or load cells mounted under garners or bins.

   Ideally, all points before and after any internal process or bulk bins should have weighers. This recommendation applies to bins for wheat, dockage, millfeed, each flour type, bran, germ and any other raw material or product.

   Weighers should be in place at these points to ensure stock/inventory control and to measure production efficiency or extraction. Moreover, they facilitate quality control, total quality management, certification to ISO 9000 standards and process control. This in turn leads to right-the-first-time products, a practice that avoids rework and keeps the accountants happy.

   Table 1 contains a sample of basic data and three main extraction calculation methods using that data. There are several other possible calculation methods, but these are of principal use.

   The first method shown (1), a straight calculation measuring flour produced against wheat at intake, results in extraction of 77%. This information is clearly of use, but note the time delay from wheat intake through storage and milling.

   This method is helpful as a check over a month or more, and it also can be used in conjunction with other methods in case of inventory investigations based on some imbalance. But it cannot be used for daily or hourly calculations.

   The second calculation method (2), known as first break (I Break) extraction, shows an extraction rate of 77.78%; notice that by this time, screenings or dockage have been deducted.

   The I Break method is of particular use for the day to day, hour by hour setting of the mill because it gives an instant as well as continuous record. What it cannot do is provide overall information for the measurement of the entire operation from wheat intake to products delivered.

   The third calculation method (3), known as total products extraction, is widely used by accountants and to a lesser extent by production personnel for recording and monitoring the ratios of all products; it shows an extraction of 75.86%, and notice that there is no wheat figure here.

   The basic data used for calculating extraction also are used to compute mill gain/loss, as shown in Table 2 on page 38. This calculation is also important because it totals up all products and compares the sum to wheat on intake. This example shows a 1.5-tonne, or 1.5%, gain in weight over wheat taken in.

   Obviously, different equation inputs will give different answers. In principle, the reason lies in varying wheat intake and product moistures and evaporative or “invisible” losses, such as dust emission, inaccurate weighing or pilferage.

   For example, a wheat blend with a 12% moisture content that is milled into flour at 14% to 15% moisture generally will give an overall mill gain of 0.5% to 1%. On the other hand, a wheat blend at 15% moisture may well give a mill loss of 1.5% to 2%.

   A number of other factors also come into play here, such as greater or lesser rollermill grinding pressures for a particular flour specification, atmospheric humidity, the number of pneumatic lifts per unit output, the length of positive pressure blowlines or the mill diagram.

   Extraction is similarly affected by these factors. For example, dry wheat will give a higher extraction rate than wet wheat, but the higher extraction is no good if the ash or mineral content of the resultant flour is out of specification.

   How then are all these factors reconciled and adjusted?

   As discussed earlier, accurate weighers regularly checked and calibrated are a must. The mill superintendent also should watch his flour specifications using accurate laboratory data with an eye on the wheats available. Scales indicate the weight of material, and the laboratory shows moisture and other parameters.

   Comparing these theoretical calculations with practice calculations is useful. This can be done by reducing all wheat intake and outbound products to either a dry or “as is” moisture basis. The practice results should differ from the theoretical by no more than 0.5% — more like 0.25%, at best — both for extraction and mill gain/loss.

   When extraction changes unexpectedly, there will be a logical reason. The solution, assuming accuracy of weigher and laboratory data, will be found by investigating several areas.

   Wheat variation can account for extraction changes. Checks should include test weight, all intake and wheat cleaning machinery settings for dockage and screenings removal and the functioning of moisture control equipment.

   Mill settings of rollermills and purifiers also need examination, remembering to look at mill stocks for plansifter blinding and tailing of material that otherwise would end up as flour. Exhaust systems anywhere in the plant, together with their filtration equipment, also should be checked for both dust emission and carryover of excessive material.

   Let's now look at an all too frequently encountered nightmare scenario, that of missing stock or inventory — because this will show up either in poor extraction or an unexpectedly bad mill gain/loss figure or both.

   Unaccounted-for stock loss comes straight through to the bottom line. It can cause friction between production staff and the accountants. The only solution to minimize the degree of this common problem is daily stock or inventory reconciliation.

   This procedure can only be carried out reliably if personnel physically check inventory or stock — by dipping every bin, whether wheat or product, and counting every bag. This practice is, in effect, another form of monitoring.

   The reconciliation should be done daily to prevent the problem from building up over a week or longer. Daily reconciliation provides early notice of trends, allowing action to be taken.

   Comparison of all wheat intake with outbound products by weight is the final arbiter for the financial accounts of the mill; the trouble is that because of time lag, any problem highlighted at this point is already history. The answer for millers is acute watchfulness daily by senior production staff using measuring equipment and the laboratory to optimize results.

Table 1

Flour extraction calculations
Basic data sample:
a. raw/dirty wheat
used in first break100 tonnes
b. flour produced77 tonnes
c. screenings/dockage1 tonne
d. millfeed/offal19 tonnes
e. bran4 tonnes
f. germ0.5 tonne
Calculation methods
FormulaExample with basic data
1. Raw/dirty wheatb/a77/100 = 77%
2. I breakb/(a-c) 77/(100-1) = 77.78%
3. Total productsb/(b+c+d+e+f)77/(77+1+19+4+0.5) = 75.86%

Table 2

Mill gain/loss calculation
Basic data sample:
a. raw/dirty wheat
used in first break100 tonnes
b. flour produced77 tonnes
c. screenings/dockage1 tonne
d. millfeed/offal19 tonnes
e. bran4 tonnes
f. germ0.5 tonne
Calculation method
FormulaExample with basic data
Mill gain/loss(b+c+d+e+f) - a(77+1+19+4+0.5) - 100 = 1.5 tonnes
in percent1.5 tonnes/100 tonnes = 1.5%