“Efficient utilization of resources is a critical aspect of the process for any industry. In the case of wheat flour milling it is of great importance as wheat costs make up over 80% of the cost of flour. Any additional gain in flour yields translates to added revenue. At the same time high level of efficiency must be maintained in ensuring that finished products are being delivered at the optimum acceptable moisture levels. Efficiencies must be maintained in preventing any losses of wheat into screenings and any solids through improper filter system.”
Senior Advisor, Technology
Canadian International Grains Institute (CIGI)
The technical performance of a flour mill is measured by its throughput and by its flour yielding capability. These are quantifiable and therefore always constituted as the most critical part of the production report dating back to the days when manual reports were prepared for hourly performance evaluation of the mill. Importance of flour yield is closely related to the price differential that exist between flour and by-products in a given market. However regardless of the market there is always profitability that is associated with increased flour yield. The only exception to this scenario would be in those markets where flour price is state controlled and by-products values are determined by the open market. In these scenarios at times there can be demand for feed ingredients that could outstrip the supply thus helping to raise the millfeed prices appreciably. Obviously these situations are exception to the rule.
In an ever increasing competitive environment margins are always under pressure. Profitability therefore is closely related to any marginal improvement of yields of flour for competitive advantage. In this article some of the key factors influencing the flour yield and associated efficient utilization of resources will be reviewed.
FACTORS INFLUENCING FLOUR YIELDS
The target is to achieve maximum flour yield of acceptable flour ash and quality without compromising the throughput (rate of production). The factors that influence the flour yield in a broad sense are summarized as follows:
- Raw material
The raw material selection is a very involved topic and requires specific expertise to obtain judicious selection of wheat for blends. Higher flour yield first and foremost can only be achieved with higher wheat quality. While functional properties such as gluten content and quality, soundness and overall physical attributes of wheat are critical for end use utilization, test weight and thousand kernel weights are more often than not regarded by millers as good indicators of flour yield. While the correlation of flour yield and test weight and kernel weight can be a topic of discussion on its own it suffices to say that both will give a fair idea about the flour yield potential and generally speaking works well within a given wheat type for comparative purposes. Both are fairly widely used in the industry for this purpose. Following factors in the raw material have a direct impact on the flour yield and thus on the cost of a tonne of flour:
- Natural moisture
- Foreign material
- Presence of broken & thin immature kernels
- Kernel texture extremes (too hard or too soft)
- Intrinsic yield potential
In order to establish a cost to flour yield varying formats of calculations can be undertaken to obtain an objective evaluation under a given set of circumstances. One such format is shown in Table 1. As can be seen from this exercise how small adjustments in flour yields can affect the cost of wheat that is required to produce a tonne of flour.
Table 1 Impact of flour yield on cost of wheat to produce a tonne of flour
|Parameters||Hard wheat flour||Hard wheat flour||Hard wheat flour|
|Cost per tonne, $||315.30||315.30||315.30|
|Freight Cost per tonne, $||10.25||10.25||10.25|
|Flour yield, %||74.0||75.0||60.0|
|Wheat required to produce 1 tonne of flour, tonne||1.35||1.33||1.67|
|Calculations for cost of wheat required to produce 1 tonne of flour|
|Cost of wheat, $||439.93||434.07||542.58|
|Low grade flour, %||0.0||0.0||14.0|
|Quantity of low grade flour, kg||0.0||0.0||233.33|
|Market value of low grade flour per tonne, $||175.0||175.0||175.0|
|Credit for low grade flour,, $||0.00||0.00||40.83|
|Quantity of millfeed, kg||351.35||333.33||433.33|
|Value of millfeed per tonne, $||100.00||100.00||100.00|
|Credit for millfeed, $||35.14||33.33||43.33|
|Cost of wheat per tonne of flour, $||404.80||400.73||458.42|
In this exercise the second, third and fourth column of data is analyzing the same wheat. The third column is calculating the impact of 1.0% increase in flour yield while the fourth column is working out the impact of highly refined low yielding (60.0%) patent flour on cost of wheat needed to produce a tonne of flour.
It should be noted that for 1.0% increase in flour yield the cost of wheat required is reduced by $4.07. On the other hand in order to produce 1 tonne of the low ash, low yielding (60.0%) patent flour the cost of wheat required goes up by $53.62.
In brief, the calculations shown in the Table 1 can be explained as follows:
The second row and third row have wheat and freight costs per tonne. The fourth row shows the flour yield values while fifth row shows a value that is calculated dividing 100 by the flour yield in the fourth row. The values thus obtained in the fifth row shows how many tonnes of wheat will be required to produce a tonne of flour of that specific yield. In the seventh row the cost of wheat is calculated by multiplying the number in the fifth row with the sum of wheat cost and freight cost per tonne shown in the second and third rows. In the 8th row under column 2nd and 3rd we see zero values shown which means that in these two columns there is no low grade and the flours are straight grade flours with 74.0% and 75.0% of flour yields respectively. Continuing on with the 8th row under column 4th we have a value of 14.0 representing 14.0% of low grade flour production in this case along with 60.0% patent flour. The market value of low grade flour is shown for all the columns as $175.00 per tonne. However, it only is calculated for the 4th column as it gets multiplied with quantity of low grade flour in the 11th row under that column. The value of millfeed gets calculated for all three columns in a similar way. These values are credits so they get deducted from the cost of wheat in row 7 to provide cost of wheat per tonne of flour produced.
There is another format of obtaining cost of flour per tonne that can be performed in a very similar way. That can be modified to take into account the presence of foreign material, natural moisture content and flour extraction corrected to a standard ash content. This format allows to evaluate the impact of these factors on the cost of flour per tonne.
Although for a miller who is running the shift flour yield shown on the computer monitor using commonly available flour yield calculator suffices. This flour yield is based on first break wheat scale and flour scales. It shows him the extraction rate real time basis as well as its trending over a specific period of time to allow him to determine whether the results are acceptable or adjustments are warranted.
Figure 1 CIGI Pilot flour mill Figure 2 Flour yield trending
However for the company management equally important is the fact that wheat that is purchased for milling into flour gets all converted in a most efficient manner so that there are no unnecessary losses of wheat due to improper handling, cleaning and preparation for milling. High efficiency cleaning equipment available from major flour milling equipment manufacturing companies offer cleaning solutions minimizing losses. This helps to improve the bottom line for the company. The goal here is to eliminate the passage of sound wheat into screenings while getting rid of all the foreign material. Sound wheat recovery system may exist in mills where such losses are to be expected. The most efficient way of operating is obviously to prevent such losses to begin with.
Targeted removal of optically differing material than wheat, e.g., discolored material and associated damages such as fusarium, black point or smudge can be precisely removed without much associated loss of sound wheat along with it. The importance of this is not very difficult to understand as largest cost in a bag of flour is due to wheat which makes up about ~80.0% or so.
Apart from cleaning equipment effective and efficient wheat tempering system with accurate automated moisture control hardware is of great importance. Tempering system is a simple step in wheat preparation but often optimum tempering target is compromised resulting in lower performance of the mill. Drier wheat produce flour with higher ash content and inferior color and lower flour moisture. On the other hand wheat with higher moisture result in loss of flour yield and also reduced throughput with medium hard to soft wheat mills being particularly affected by this. The importance of optimum tempering requirement cannot be overemphasized in order to maintain high quality and to avoid economic losses.
Good condition of the corrugations and appropriate buffing on the smooth rolls are just as important in order to maintain higher yields. It is not uncommon to see in flour mills a pair or two rolls with worn out surface. Usually in such cases the slack is picked up by other grinding passages. However if more passages suffer from these defects or the specific roll length is limited then that can have a cumulative negative impact on process yields.
Employment of good filters collecting all the dust particles from the pneumatic air and exhausting partial/all the air out to the atmosphere ensuring clean emission is an important function that is performed by the flour mill much better today. Although this is in the non-production area of the operation but its importance is huge. Given our present stringent standards on emissions non-compliance has serious consequences. Therefore apart from loss of solids meeting social responsibilities being a good corporate citizen has certainly become an important goal to pursue for flour millers in most if not all markets.
Automation has come a long way in helping reduce time and material losses during switch over from one wheat mix to another. This is facilitated by automated roll gap adjustments that is recalled for a specific mix along with all the necessary adjustments are carried out rather quickly and precisely. Additional changes involving valves that are required to be diverted, additive feeders that need to be adjusted, flour being directed to the right bin and any other associated changes are done rapidly. These steps have helped in reducing flour collection in a buffer bin. This reduction of material is significant as in more conventional operations without these options one has to allow sufficient time to stabilize the effects of switching over.
Figure 3 Automated roll gap adjustment for durum and common wheat mode
Mill flow diagrams are designed on the basis of desired level of flour refinement for a given throughput. The measuring index for the refinement that is most commonly used is the ash content. In some regions of the world color may be used as well. Typically 10.5 mm – 11.0 mm/100 kg/24 hours length and
~0.056 m2 /100 kg/ 24 hours sifting area is considered suitable for a ~75.0% flour yield when milling reasonable quality hard wheat producing about 0.50% – 0.54 % ash. In the case of soft wheat the sifting area goes up to ~0.67 m2/100 kg/ 24 hours. In a combination mill millers reduce the throughput by about ~25% or so to enhance the sifting surface for the soft wheat run in order to obtain a good flour yield.
Ash content is the limiting factor for flour yield and this relationship may be different for different flour mills with respect to the same wheat. Evaluating cumulative ash content in a flour mill against cumulative yield of flour streams provides helpful insights. Cumulative ash curves are a function of the milling quality of the wheat and the mill. Same wheat milled in two different mills will provide different results with respect to its cumulative ash as well as the shape of the curve. Likewise different wheat types milled in the same mill will have different cumulative ash curves based on the intrinsic milling quality of the wheat.
Extended flatter curves are indicative of low ash flour of higher flour yield. It is also reflective of the fact that there is a good potential for extracting low ash patent flour. A typical example of this would be a short flow intensive milling system versus an elaborate flow diagram with longer break system allowing gradual breaking down of the wheat material. This enables a more comprehensive grading of the released endosperm particles that may be further purified thoroughly in an elaborate purification system. These steps are further followed by gradual reduction system to obtain an absolute minimum contamination with bran. Such gradual and elaborate approach helps enhance flour refinement along with very good yield but at a higher cost both from the point of view of initial capital expenditure along with the associated operating costs.
There are markets that are extremely quality sensitive that prefer high level of refinement limiting the flour yield at a lower level than what is conventionally expected from rest of the markets that are price sensitive and quality sensitive to a varying degree. The three such flow diagrams are shown in Figures 2, 3 and 4. They may be considered as short, medium and long process. Needless to say that same wheat mix will have improved flour refinement as we move from 2 to 3 to 4.
The value of flour yield has already been extensively covered. Years ago millers on the shift relied on dry and wet slick of flours for comparative purposes throughout the 24 hours of the production run. With the development of dependable on-line quality and production monitoring system the life has become very simple. On-line ash monitoring system coupled with rate of flow of flour through the scale provides miller the critical production data. Tolerances set on various scales that receive flours from various conveyors are expected to have specific range of material going through them. These ranges are set up to a tolerance level that is desired.
Moving a step further an ash control loop can be instituted where the ash coming off the scale will get blended in a proportion until it reaches the desired range level. This ensures optimum yield is realized while maintaining the ash specifications of the resultant flour at all times. These options that are available today is only possible because of the advances in technology in terms of sensing, on-line quality and quantity monitoring capabilities. The on-line capability of speck counting coupled with particle size analysis are very important advancements for the durum mills as such innovations allow processors to push the yield levels to the limit at all times without having the fear of falling out of specification.
Flour yield and quality
Flour yield is very important however the benefit of any enhanced yield obtained is of little value if it lacks in functionality. As the flour yield is increased there are a number of factors that get influenced by this which includes analytical, rheological and end use quality attributes. As flour yield increases many changes occur in flour functionality some of which are listed below:
- Protein content
- Protein quality
- Starch damage
- Amylograph peak viscosity
End use quality that gets impacted are:
- Crumb color
- Water absorption
- Mixing tolerance
- Fermentation tolerance
The above factors impacts dough handling properties and other characteristics of baked goods including the volume.
- Asian noodles
- Color stability
- Cooking properties
For high quality noodles particularly fresh noodles the extraction level is carefully monitored as higher extraction promotes discoloration.
Table 2. Impact of flour yield on quality data
|Hard wheat Patent flour||Hard wheat
Straight grade flour
|Flour yield, %||60.0||76.6|
|Wet gluten, %||37||38|
|Amylograph peak, BU||693||614|
|Starch damage, UCD||24||24.7|
|W (x 10-4), J||402||396|
In order to fulfill the quality requirements suitably for the above typically mills establish appropriate ash contents and flour yield levels for ensuring functionality for a given specific blend of wheat with the basic essential quality characteristics that is required by the end use application.
For example the following Table 3 shows a list of commonly used flour and their specifications in Canada. In this case all the specifications are based on ash and protein contents of the flour only. This is primarily because all of these flours are milled using CWRS wheat of good grades which ensures the basic intrinsic quality is retained and any further modification in quality will come from adjusting the flour yield levels meeting a defined ash content level to attain the targeted quality.
|Table 3 Common Commercial Flours and their Specifications|
|Household||0.37 – 0.39||11.5 – 11.8|
|Large bakers||0.48 – 0.50||12.4 – 12.6|
|Strong bakers||0.53 – 0.55||13.4 – 13.6|
|Whole wheat flour||~1.5||13.5|
Often ash content therefore is considered be all and end all in quality for that reason. Higher ash content in flour is reflective of higher flour yield as we approach the bran layers the increase in ash content is rapid. This is due to the higher ash content of bran (~5.0%-6.0%) compared to in pure endosperm from the center of the kernel of being the lowest in ash (~0.3%). This leads to the conclusion that as ash keeps getting higher the functionality drops due to increased presence of lower grade flour and bran parts in the flour. This relationship would hold true within a given specific wheat type or class. There are wheat types that can have a higher natural endosperm ash content in hard wheat where the flour appears bright in color at comparable extraction rates however relative to ash content they may show higher values which should impact the functionality.
There are also wheat types that have a low endosperm ash content and in such cases once again the ash and flour yield relationship as perceived for a typical hard wheat flour may not hold.
Table 4 Flours from wheat with low and moderate endosperm ash and their Specifications
|Flour yield, %||77.6||76.6|
Efficient utilization of resources is a critical aspect of the process for any industry. In the case of wheat flour milling it is of great importance as wheat costs make up over 80% of the cost of flour. Any additional gain in flour yields translates to added revenue. In a competitive environment any such gain is of importance especially considering the fact that in most markets the price differential between flour and millfeed is anywhere from 3 to 5 times or more. At the same time high level of efficiency must be maintained in ensuring that finished products are being delivered at the optimum acceptable moisture levels. Efficiencies must be maintained in preventing any losses of wheat into screenings and any solids through improper filter system.
We also suggest you to read our previous article titled "EFFICIENT USE OF RESOURCES AND YIELD".