Flour Testing In The Quality Control Laboratory

14 December 201720 min reading

The quality control laboratory is an important part of the flour milling industry.  The objectives of the quality control laboratory at a flour mill are to monitor uniformity throughout the mill run;  ensure uniformity between flour lots or shipments; ensure that the flour meets the customer’s specifications; and verify that the flour has desirable characteristics for a product.

Rebecca Miller Regan

Dr. Rebecca Miller Regan - Kansas State University

It is important to use appropriate standard methods and procedures in the flour quality testing laboratory. Commonly used standard methods used in the food industry that are most applicable to a milling quality control laboratory are published by the American Association of Cereal Chemists International (AACCI) and the International Association for Cereal Science and Technology (ICC).

FLOUR TESTS Moisture The moisture content of the flour is important to ensure that the flour will be stable during storage. Flour containing more than 14.5% moisture is prone to mold and bacterial growth. It is also necessary to know the moisture content of the flour in order to adjust flour test data to a constant moisture basis. Commonly used moisture basis varies by country so the basis used should also be reported.

To convert to a constant moisture basis, 100 minus the desired moisture content is divided by 100 minus the actual moisture content of the flour which is then multiplied by the as-is (actual moisture content) value. If the results are not adjusted, then the results are on an “as-is” basis, meaning the basis is the actual flour moisture content. The example illustrates the importance of the moisture basis. Flour A contains 12.1% protein on a 12.5% moisture basis (the as-is basis) or 11.86% protein on a 14% moisture basis.

Moisture can be measured using several direct or indirect methods. The direct methods use oven heating and assume that the loss in weight of the flour during heating is caused by the evaporation of water. The percentage of moisture in the four is calculated by dividing the weight of moisture lost by the weight of the flour prior to heating and multiplying by 100 to convert to percentage. Oven heating methods take several hours to perform. Indirect methods which use Near Infrared Spectroscopy (NIR) or Radio Frequency (RF) dielectric properties are rapid and simple. NIR methodology measures the absorption of electromagnetic energy at specific wavelengths in the near infrared region and plots it as a spectrum. Peaks in the spectrum at different wavelengths correspond to different flour components such as water, protein, starch, fat, etc. The quantity of each of these components is calculated mathematically using a calibration or reference that is programmed into the instrument. The accuracy of the NIR data is only as accurate as the calibration equation that was programmed into the instrument. It is extremely important to make sure the calibration is good and covers the whole range of values that occur in the product being tested. It is also a good practice to have the instrument serviced every 1 to 2 years to make sure the light source is fully functional as they tend to slowly burn out over time, causing incorrect readings.

Most bench top moisture meters use the RF dielectric method which is also known as the capacitance method. With this method, the sample is scanned with a radio signal at a specified frequency. The radio signal is changed by the dielectric constant as the water in the sample interacts with the radio wave. The change in the dielectric constant is used to calculate how much water is present in the sample. Several factors affect the dielectric constant, most commonly sample density and temperature. Consequently, these two factors are measured and corrected for in the more advanced moisture meters. As with the NIR, the calibration curve programmed into the instrument is critical and the results obtained are only as reliable as the calibration curve.

Protein Protein content is the basis by which wheat and flour are bought and sold and is one of the main factors controlling the price. In the hard wheat classes, price increases as protein content increases while in the soft wheat class, higher prices are paid for lower protein flours being used for specialty products. Protein content is one of the most important features because it affects many flour properties. Flour protein content is categorized as being low, medium or high, depending on the wheat class. Within the soft wheat classes, low protein flours have less than 9% protein while medium protein flours have 9 to 10.5% protein and high protein has greater than 10.5%. Within the hard winter wheat classes, low protein flours have less than 10.5% protein while medium protein flours have 10.5 to 11.5% protein and high protein will have greater than 11.5%. In the spring and durum classes, low protein flours have less than 13.5% protein while medium protein flours will have 13.5 to 14.5% protein and high protein will have greater than 14.5%. In general, dough strength increases as protein content increases. Doughs are referred to as being visco-elastic. This refers to the ability of the dough to be both viscous or extensible but also elastic or strong. The balance of extensibility and elasticity determine the best use for the flour. Low protein flours have a weak dough strength characterized by high extensibility and low elasticity. Low protein flours work best in products that do not need a lot of strength such as cakes, cookies, some types of crackers, and all-purpose flour. Medium protein flours have a medium-high dough strength characterized by having moderate extensibility and moderate elasticity. Medium protein flours work best in products that need an equal balance of extensibility and elasticity such as noodles, tortillas, and white pan bread. High protein flours have strong dough strength and are characterized by having moderate extensibility and high elasticity. High protein flours work best in products that need a highly elastic, strong dough like pasta, hearth breads and whole grain breads.

Protein content is measured by several methods including Kjeldahl, combustion and NIR. The Kjeldahl and combustion methods measure the nitrogen content of the sample which is then multiplied by a factor to calculate the crude protein content. The typical factor used is 5.7 for wheat and wheat flour and 6.25 for feed, bran and shorts. As discussed above, it is important to report protein content on a constant moisture basis. The moisture basis that is used should be reported as well. The Kjeldahl method, developed in 1883, is the universally accepted traditional wet chemistry method. In the method, the sample is digested by heat and sulfuric acid which liberates reduced nitrogen as ammonium sulfate which is then distilled with sodium hydroxide to form ammonia. The level of ammonia is measured by back titration to determine nitrogen concentration which is used to calculate crude protein content. Drawbacks of the method are that it takes longer than an hour to run a single sample, the operator works with harsh chemicals and high temperatures, and hazardous waste must be disposed of properly.

Combustion analysis is an automated and rapid method which is replacing the Kjeldahl method as the standard method of analysis for protein content in food and animal feeds. There are many different protein combustion instruments which all have the same basic operating principle. A sample of known mass is combusted in a high temperature chamber (about 900°C) in the presence of oxygen which causes the release of carbon dioxide, water and nitrogen gases. The carbon dioxide and water are absorbed and the nitrogen is separated out and quantified using special columns. Nitrogen content is then used to calculate crude protein content using conversion factors.

NIR can also be used to directly measure protein content. It is the fastest and easiest method to run. An advantage of NIR is that the moisture and protein contents as well as many other properties of the flour can be measured simultaneously.

Ash Ash is the inorganic or mineral content of the flour. It varies depending on soil and climate. The mineral distribution varies within the kernel but it is most highly concentrated in the bran. Wheat bran contains 30 times more inorganic matter than endosperm so even a small amount of bran in the flour can significantly affect the ash content.

Ash content is often used as a measure of the grade or type of flour because it shows how much bran is present in the flour. Flours milled using a low extraction rate are premium products with low ash contents that can be sold for a higher price than flours milled with a high extraction rate that includes more bran and higher ash. Ash also indicates the completeness of separation of the bran and endosperm during milling or how much bran was removed during sifting. The bran from red wheat affects the color of the flour. The higher the extraction, the darker the color of the flour. Flours milled from red wheat will also have a dull or darker color compared to flours milled from white wheat due to the presence of the red colored bran.

The most accurate way to measure ash is using the muffle furnace method; however, the method takes many hours. In this method, the flour is combusted and burned overnight. The following day, the burned samples are cooled in a desiccator and the residue (ash) is weighed. Ash content is calculated as the weight of the residue divided by the original weight of the sample times 100 to convert to percentage. As discussed above, ash content should be reported on a constant moisture basis.

Many manufacturers and distributors advertise that NIR instruments measure ash. Because the NIR measures the bonds in organic materials and ash is inorganic, NIR does not actually measure ash content. Much of the fiber in wheat is concentrated in the bran layers and flour ash correlates highly with flour fiber content so it is highly likely that the NIR is measuring fiber rather than the mineral ash. The only NIR currently specified in an approved method is the Perten Inframatic 86 series NIR with ash kit (AACCI Approved Method 08-21.01). This instrument allows the user to adjust the bias and slope in the NIR calibration so the measured values are more accurate. The adjustments are calculated by analyzing a reference set of flours using both the NIR and the muffle furnace ash method.

Falling Number The Falling Number (FN) is a hot paste viscometer that is used to measure the alpha amylase activity of wheat and flour. One use of the FN is to determine whether the wheat experienced any pre-harvest sprouting. This is especially important in areas that received excessive rain at harvest time. Once grain reaches maturity, sprouting can occur when the moisture and temperature are right. If these conditions happen at harvest, the kernels can sprout in the field. When sprouting occurs, enzymes in the kernel become active; amylases break down the starch, lipases break down the oil and proteases break down the proteins. The enzymes can become active and influence the functionality of the wheat even before the sprout is visible on the kernel. The impact of sprouting depends on the activity of the enzymes and the degree of breakdown to the components of the kernel. FN values of 150 seconds or less are an indication that the flour has excessively high amylase activity and is unfit for use in most baking applications.

Malt (malted barley) is a flour additive made by the controlled germination and drying of barley. It is often added to flours used for bread making because the alpha amylase in the malt has positive effects on dough handling and machinability, increases oven spring and extends shelf life of the bread. FN is used to verify that the flour meets the customer specifications and was not over or under malted. FN of 200 to 300 seconds are typical values that are specified for bread flours.

Wheat or flour with FN values greater than 400 seconds are sound, meaning that they have no amylase activity and were not sprout damaged or malted. In the FN scale, values higher than 400 seconds (i.e. 500 seconds, 600 seconds, 700 seconds, etc.) are all considered to be sound with no amylase activity and will not perform differently in baking because of the FN value. However, there are great differences in performance when the FN values are less than 400 seconds. It is the best practice for a mill to purchase wheat with a FN near or above 400 seconds and then add malt as needed to produce flours with different FN values.

Flour Color Flour color can be measured by several methods, most of which use older instruments that report color in various instrument specific units of limited value. The Chroma Meter (Konica Minolta) is a recommended modern instrument that utilizes a granular materials attachment for the measurement of flour. It is capable of measuring color using several color scales including L* a* b* which is the most commonly used scale today. The L* value is a measure of the lightness or brightness of the flour. The scale ranges from 0 for pure black to 100 for pure white. The a* scale is a measure of the red to green spectrum using a scale of +60 for pure red to -60 for pure green. The b* scale is a measure of the yellow to blue spectrum where +60 is pure yellow and -60 is pure blue. The L* and b* values are the most important in flour. It is highly desirable for refined flours to have a bright white color (high L*). Bran or other extraneous materials in the flour will decrease the color value. A strong yellow color (high b*) is very desirable in durum semolina while a more neutral (low b*) is more desired in refined flour. Yellowness is caused by natural pigments in the endosperm which lighten during aging or can be chemically lightened with benzoyl peroxide. A typical straight grade flour has an L* value around 92.5, is very slightly green with an a* value of around -2.5 and has a slight yellow tint with a b* of around +6.9. Solvent Retention Capacity Solvent Retention Capacity (SRC) is a test that is becoming popular in analyzing soft wheat flours for use in cookies, some crackers and low moisture snacks. SRC uses different solvents to measure the water holding capacity of different components in the flour. A 50% sucrose solution targets pentosans, 5% lactic acid solution targets glutenin protein, 5% sodium carbonate targets damaged starch and distilled water measures the water holding capacity of the complete flour.

For most low moisture cookies, crackers and snacks, flour with a low water holding capacity is preferred. High absorption is detrimental to cookie spread and increases the baking time (and increases processing costs) to remove the water during baking. A general target value for the water holding capacity of the flour is a distilled water SRC <51%. Sucrose solution measures the impact of pentosans which are highly concentrated in the bran layers. Pentosans and bran are detrimental to the quality of most soft wheat bakery products so a low sucrose SRC <89% is desired. The lactic acid solution measures water holding by the glutenin polymers of the gluten. Although the protein level is low in soft wheat products, the protein needs to absorb water so it can fully hydrate and function properly to set the structure of the final baked good. A reasonable target is lactic acid SRC >87%. Sodium carbonate solution measures water holding by the damaged starch which can have a negative effect on some products, especially cookies. A desirable value is sodium carbonate SRC <64%.

DOUGH TESTS Physical dough testing instruments collect objective data about the physical and rheological properties of flour and dough and are useful for quality testing and process control. Because they only measure some of the properties that determine quality, the usefulness of the data in predicting baking or end-use quality of the flour is limited. The instruments are very useful at characterizing or “fingerprinting” flours to give the baker or end user confidence in the consistency of flour from lot to lot or shipment to shipment.

Farinograph The farinograph is a recording dough mixer which has gained wider acceptance in flour testing laboratories throughout the world than any other experimental dough testing instrument. The farinograph provides information about the mixing properties of flour including water absorption and mixing time and provides information about the consistency or strength of the dough. The most common parameters measured with the farinograph include water absorption, mixing time and dough strength which is derived from dough stability and mixing tolerance index.

In the test, the resistance of the dough against the pins (consistency) during mixing is measured and reported in arbitrary Brabender Units (BU) or Farinograph Units (FU). The vertical (Y) range of the chart is 0 to 1000 BU with each horizontal line indicating 20 BU. On some models, the top section of the chart is blank, presumably for recording information and results from the test. The horizontal (X) axis of the chart is time where each vertical line indicates a 30 second increment. In the test, the level of added water (water absorption) is adjusted until the peak (highest point) of the curve is centered on the 500 BU line. If the center of the curve peak is positioned below the 500 BU line, the dough is too dry and the test needs to be repeated with more water. If the curve peak is positioned above the 500 BU line, the dough is too wet and the test needs to be repeated with less water. The consistency of the dough at 500 BU is an arbitrary dough property which was established by the German inventor, Mr. C.W. Brabender, when the instrument was developed in the 1930s. What that consistency means or how it was determined is unknown. What the test does is provide a way to compare the relative water absorption requirement of different flours to produce doughs of similar consistency. Sometimes a farinogram will have 2 peaks. The first peak which is often the highest peak and occurs within 1 to 2 minutes of mixing is a water hydration peak and not the true mixing peak. The second peak which occurs later is the true mixing peak. The correct water absorption is the level of water which causes the second peak to be centered on the 500 BU line, not the first peak. Be aware that the computer software usually reports this first peak as the mixing peak. The operator should always look at the curve and make sure the correct peak is reported. With the newer computerized models, the mixing peak can be adjusted and the software will re-read and report the correct values. For older, non-computerized models or those with older software programs, the curves can be read manually by the operator.

The time at which the peak occurs is called the mixing peak, peak time, mixing time or dough development time. This is the point where the dough has reached its maximum consistency. The true peak is often very difficult to see. Sometimes it helps to step back a few feet to look at the curve from a short distance or to look at the bottom rather than the top of the curve to see where it raises. Peak time is read by simply tracing the vertical arc from the peak down to the vertical X axis and reading the mixing time (each vertical line is 30 seconds).

Dough stability gives a measurement of the tolerance of the dough to over or under mixing and is often taken as a measure of dough strength where longer stability times indicate stronger, more desirable doughs. Dough stability is the time interval in minutes between the arrival time (time at which the top of the curve touches the 500 BU line) and the departure time (time at which the top of the curve falls below the 500 BU line).

The last measurement typically taken from the curve is the mix tolerance index or MTI. The MTI, reported in BU or FU, is calculated by multiplying the number of horizontal lines that the top of the curve drops from the peak to 5 minutes after the peak by 20 (each line is 20 BU). The MTI is a measure of rate at which the dough breaks down after mixing. Higher values indicate weaker flours which break down more quickly while lower values indicate stronger flours which break down slowly.

In order to collect all the pertinent values, the farinograph test should be run until 1) the top of the curve falls below the 500 BU line (for measuring stability) and 2) for at least 5 minutes after the peak (for measuring MTI). The minimum time to run the test should be long enough for both events to occur. They may occur in different order depending on the type of flour. For example, for a low protein soft wheat flour with a short stability of only a few minutes the run time would need to be for 5 minutes after the mixing peak occurs. A high protein spring wheat with a long stability would need to be run until the top of the curve falls below the 500 BU line. Some very strong flours may have extremely long stabilities. It is acceptable to stop these tests after 20 minutes (or another desired time) and report the stability as >20 min.

Alveograph In recent years, the alveograph has gained popularity as a dough testing instrument for all wheat classes. The alveograph measures the resistance to expansion and the extension of a thin sheet of dough as it is formed into a bubble and expanded. It was designed to mimic gas bubble expansion in fermenting dough. The Alveograph test provides information on dough strength, dough extensibility, the balance of strength and extensibility and the work input requirement to form and expand the dough bubble.

The P value is the maximum height of the curve in millimeters. It is a measure of the force required to blow and break the dough bubble. The P value is related to dough strength or elasticity where higher P values indicate stronger doughs. higher the P, the stronger the dough. The length of the curve in millimeters is the L value. This is a measure of the extensibility of the dough so longer L values indicate more extensible doughs. The P/L ratio is simply calculated from the P and L values and reports the balance between dough strength and extensibility. P/L values of 1 indicate an equal balance while values greater than 1 indicate stronger doughs with higher elasticity than extensibility and values less than 1 indicate weaker doughs having more extensibility than elasticity. The W value is the area under the curve and is reported in Joules. The W value reflects the combination of dough strength (P value) and extensibility (L value) and is also taken as a measure of dough strength where stronger doughs have higher W values.

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