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Analytical Methods:

Ash in Flour

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Analytical Methods
What is ash in flour?

Ash is the mineral or inorganic material in flour. The ash content of any flour is affected primarily by the ash content of the wheat from which it was milled, and its milling extraction rate: the amount of flour obtained from wheat after milling, when the bran and germ are removed, leaving the endosperm.1

The test for determining the ash content involves:2

  1. Incinerating a known weight of flour under controlled conditions
  2. Weighing the inorganic residue
  3. Calculating the percentage of ash based upon the original sample weight. The ash value is corrected to dry or other moisture basis for comparison.

Ash content in flour can be measured by using conventional laboratory methodologies, or rapid instrumental techniques. The selected method may be a function of cost, time and availability of qualified personnel.

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How does it work?

The ash content of wheat varies from about 1.5 to about 2.0% on a 13.5% moisture basis, but this value varies according to class of wheat, agronomic conditions and the soil type it was grown on.

The pure endosperm contains about 0.35% ash. The gradient in mineral content increases from the center to the outer layers of endosperm, reaching the non-endosperm parts of the kernel (pericarp, aleurone and germ) whose ash content is much higher than the endosperm. The ash content of bran is known to be 10–20 times that of the endosperm.3

In the cereal processing industry, there are official methods of analysis to determine the ash content of flours. The methods include:

  • AACC International Method 08–01.01 — Ash. Basic Method
  • AACC International Method 08–21.01 — Prediction of Ash Content in Wheat. Flour—Near-Infrared Method

The AACCI Method 08–01 is based on the fact that when a sample is incinerated in an oven, the high temperature vaporizes the moisture and burns away all the organic materials (starch, proteins, sugars, and fat), leaving only the ash. The residue is composed of the non-combustible, inorganic minerals that are concentrated in the bran layer.4

The AACCI Method 08–01 consists of the following steps:4,5

  1. Weighing a sample of flour or ground wheat (3–5 g) and placing it into an ash cup
  2. Heating the sample at 585°C (1,085°F) in an electric muffle furnace/oven until its weight is stable (stops decreasing). This process may take several hours.
  3. Cooling residue to room temperature
  4. Weighing residue
  5. Calculating ash content according to the following equation:5

Equation for finding the content of ash in flour.

Ash in flour is usually expressed on a common moisture basis of 14%.4

Even when using NIR technology for moisture determination, the incineration method remains  the reference in establishing the instrument’s calibration. Online systems are becoming more common in flour mills especially their ability to provide continuous monitoring during processing.2

Application

Some millers tend to overlook the impact of ash content on the flour baking performance. However, the non-endosperm parts of the wheat kernel are known to decrease the flour’s baking quality. As the ash content increases so does the level of non-endosperm material.2

Despite not being regarded as a flour quality parameter in some bakers’ specifications, ash in flour is still relevant for bakers and millers. The following are some of the reasons:3

  • Ash content is a good indicator of bran contamination in white or refined flours. As flour extraction rate is increased, the amount of contamination with non-endosperm increases and the ash content increases. For most breadmaking applications, bakers look for excellent quality flours with high protein levels and highest purity in terms of endosperm content.2
  • It is a good method to differentiate patent flours from clear flours. The ash content of patent flours is lower compared to clear flours.

References

  1. Posner, E.S. “Milling Terms.” Wheat Flour Milling, 2nd printing, AACC International, Inc., 2011, p. 464.
  2. Carson, G.R., and Edwards, N.M. “Criteria of Wheat and Flour Quality.” Wheat Chemistry and Technology, 4th edition, AACC International, Inc., 2009, pp. 97–118.
  3. Posner, E.S. “Wheat: The Raw Material.” Wheat Flour Milling, 2nd printing, AACC International, Inc., 2011, pp. 1–46.
  4. Wheat Marketing Center, Inc. “Wheat and Flour Tests: Moisture Content.” Wheat and Flour Testing Methods: A Guide to Understanding Wheat and Flour Quality, 2004, p. 13.
  5. AACC International. Approved Methods of Analysis, 11th Ed. Method 08-01.01. Ash—Basic Method. Approved November 3, 1999. AACC International, St. Paul, MN, U.S.A.