Defining Flour Strength
June 01, 1998
by Teresa Acklin
Industry consultant David Sugden examines the classical definition of flour strength for the practicing miller.
Refer to publication for graphic's on Farinograms and Extensograms
Refer to publication for graphic's on Chopin Alveograms
Imagine for a moment that a baker calls up one day and is patched through to the mill superintendent. The comment is made that the flour is not strong enough. What does the baker mean? Or maybe a cookie or biscuit manufacturer complains of flour too strong; what does he mean?
Of course, mill superintendents, being commercially sensitive, will know that the customer is always right even when he is wrong, he is right.
The issue is complex, and misunderstandings abound. Cereal chemistry as a complete science is insufficiently precise to provide clear and definitive answers in every case, though in most cases it does. Quantified answers are easy, qualified ones are not. How can flour strength be defined? Clearly, flour must be fit for the desired purpose the classical definition of flour quality.
Protein And End Use.
Loosely speaking, flour protein is the critical factor that must be satisfactory, in both quantity and quality, for its end use.
In the case of the baker cited above, he may mean a number of things, but one thing is clear he is not happy. He may be having difficulty with loaf volume or crumb structure or some other trouble. The time-honored immediate solution is to change the flour, take a sample and measure against a control or known sample.
The cookie or biscuit manufacturer may mean that his products, particularly non-fermented ones, will not fit into packaging because of oversize. Protein content may be too high and quality not extensible (free flowing) enough. The instant remedy is as before.
Measurement to give an informative indication is therefore important to the miller. There is only one all embracing test and that is to make the end product, be it bread, rolls, cookies, cakes or biscuits, from the flour. However, this test will indicate that something is wrong (if it is), not what may be at fault.
The approach in every case is to characterize the flour in question, in part to eliminate what is right and on the other hand to highlight faults in comparison. Let us assume that the usual physical and chemical tests on any flour in question are in balance against a control sample. At this stage these tests would probably include moisture, protein content or quantity, alpha amylase and diastatic activity, mineral content, color, percentage of starch damage, additives, fat content and fiber.
The next step is to characterize gluten separated by hand or preferably, by machine (to cut out operator error). The latter is a quick test and will give the first qualitative indication especially when run alongside a known satisfactory sample.
At this point, it is helpful to remember that wheat flour contains two types of protein, namely gluten or insoluble protein usually about 80% of the protein quantity measured and soluble protein. The former is functional or useful, the latter is non-functional (though both are nutritious).
For bread, stronger flours meaning higher protein quantity and quality are required than for biscuits, cookies or wafers. Quality here is defined as gluten that has elasticity; gluten that is elastic recovers to a degree after being stretched.
Conversely, the requirement for non-fermented cookies is for gluten to be extensible, i.e. it does not recover. This allows cookie flour doughs to remain in a relaxed state when sheeted. If they do not, products will be too bold or higher as opposed to flatter, though the individual weight will be similar.
This leads to dough rheology assessment as a particularly useful test not only for dough, but also gluten characteristics. While others exist, the two principal tests are the Brabender Farinograph (and its companion Extensograph) and the Chopin Alveograph. These instruments are shown in figures 1, 2 and 3 and are carefully temperature controlled.
Essentially, the machines operate on flour/water doughs, though variations using sundry additives are also used. Test methodology is supplied by the manufacturers and by AACC/ICC standards. These instruments produce graphs as shown in figures 1, 2 and 3.
The Farinograph measures the water absorption capacity of flour (a function of protein quantity, moisture and damaged starch content) and produces graphs. The graph on the left pictures a weak flour (biscuit/cookie), and the right graph depicts a strong flour (bread).
Measurements are taken from the graph to give numerical values. The instrument works on the principal of measured torque of the motor via bearings and levers to a motorized chart over time.
The Extensograph produces graphs over time of “stretched” doughs made by the Farinograph. The top series of three are biscuit flour dough characteristics and the lower are bread doughs. From left to right, the top and bottom graphs depict the two doughs “stretched” after 45, 90 and 135 minutes rest.
The reader can readily see the graphical/numerical dough characteristics of an extensible, so-called weak (biscuit) flour with little resistance compared to a strong (bread) flour.
The Alveograph, used mainly in Continental Europe, French speaking Africa and some parts of Asia, produces graphs (see figure 3) showing a strong (bread) flour on the top and a weak (biscuit) flour on the bottom. Once more, numerical values are measured comparing the maximum height, length and area within over time. This apparatus works on the principal of water pressure displacing air into a dough, making an increasingly large bubble similar to a balloon until it bursts.
Maintaining Flour Consistency.
All these instruments are used not only to characterize strength or weakness but also to measure functional comparisons between untreated and additive treated flours, blending of selected wheats and various flours and degrees thereof. Because consistency in flours is extremely important, the instruments are able to underline any differences from those intended.
Dough rheology graphs will readily show heat damaged gluten. This is evidenced by a high peak on the X axis and a shortened one on the Y.
This reading means that some, if not most, of the vitality of gluten has been destroyed, heat being the enemy. The causes are normally overdrying with too high a temperature or so called “bin heated” grains. The latter phenomenon can occur with grain stored in a damp condition, breathing ever faster, which creates heat and carbon dioxide in a quickening cycle (at its extreme, spontaneous combustion will take place).
The dough testing instruments also will be able to depict a flour dough to which dried gluten (1% or more, for example) has been added the purpose being to strengthen flour for bread. Further, they can be used to measure the addition of the enzyme protease or the substance sodium metabisulphite (S.M.S.) where allowed by law. Both additives have the effect of making a dough more extensible for cookie/biscuit making. Different treatment levels can be tried and their effects observed.
One problem at this stage is that dough rheology instrumentation will not tell the observer what is wrong only that something may be wrong, or is wrong by degree.
One additional issue is one of gas production in a bread dough. Insufficient gas production is easily correctable by adding fungal enzymes, diastatic malt flour or sugars.
However, a much larger problem occurs if lack of gas retention is identified. Typically, this is a gluten quality issue. Bread dough volume, gas production and retention can be numerically determined using the Chopin Rheofermentometer (not shown). Other equipment also exists.
However, if lack of gas retention is in question because of suspect gluten, a valid solution is to review wheat selection. In some jurisdictions, the dough conditioner supplier also may be able to help by reformulating improvers such as fats, emulsifiers and lecithins for the baker.
Quick screening tests for strength or weakness (suitability) of wheat at intake include the well-known Zeleny and the sodium dodecyl sulphate (S.D.S.) assays. These are excellent tests, though by no means the be-all and end-all. They are indicators rather than precise methods.
Consistency of flour for whatever purpose is vital. So while the meaning of strength (all else being equal) is protein quality as well as quantity, the characterization of protein quality remains difficult. If this is reasonably suspected to be giving trouble to the customer, the ultimate solution will lie in careful wheat selection and/or correction by additives.
David Sugden, independent consultant to the grain industries, may be reached at The Coach House, Killigrews, Margaretting, Ingatestone, Essex CM4 0EZ, U.K. Tel: 44-1245-352048. Fax: 44-1245-251162.