When it comes to damaged starch, the following rule of thumb applies: neither too much nor too little! Damaged starch is caused by a combination of genetics (wheat variety) and milling. It can greatly affect the behavior (hydration, consistency, fermentation) of dough during processing as well as the quality (volume, color, appearance, staling) of finished products.
As a result, damaged starch must be measured.
The first Starch Damage determination techniques used an iodine solution to detect damaged starch under a microscope. The damaged parts absorbed more of the iodine and were therefore darker. Although innovative, these techniques were complex, time-consuming, and extremely inaccurate.
Next came the polarimetric method, which made it possible to detect damaged starch in a solution of calcium chloride and relied on the fact that alpha-amylases (enzymes with the ability to break the chains of glucose that make up the starch, see “Fermentation”) digest damaged starch more easily than native granules. Once again, this method was quite complex and not very accurate.
Near-infrared spectroscopy can also be used to measure damaged starch. This method has the advantage of being extremely fast, but must be corroborated by so-called standard methods. Problems can therefore arise when a standard method fails to produce sufficiently repeatable and reproducible results (see “Measuring the reliability of a method”). Unlike moisture or protein which have specific chemical bonds that are detected at specific wavelengths, damaged starch (from a biochemical standpoint) is indistinguishable from native starch.
Most of the time, universities, research centers, and institutions use enzymatic methods to identify damaged starch. These methods often consist of five phases:
1/ An enzyme (usually alpha-amylase) is added to the flour. All requirements regarding the duration of the contact, the temperature, the pH, and the activity of the enzyme must be respected.
2/ The enzyme is denatured to stop the reaction.
3/ The solution is restored to its aqueous form by way of filtration or centrifugation.
4/ Titration (technique where a reagent of a known concentration is used to determine the concentration of an unknown chemical in a solution) or spectroscopic analysis (method for identifying a chemical element by way of the spectrum it provides after interacting with radiation such as light or X-rays) is used to determine the concentration of reducing sugars in the filtrate.
5/ This concentration is used to determine the % of damaged starch.
Enzymatic methods are extremely complex and require highly qualified personnel as well as a considerable material investment. They are unsuitable for the everyday needs of millers, especially those without large testing labs.
The amperometric method was introduced by Medcalf and Gilles in 1965 and is based on the work of Coton (1955). This method uses amperometry to measure the kinetics of iodine absorption by a suspension adulterated with flour.
In other words, it involves creating tri-iodide (I3-) ions in the solution. These ions generate an electric current (measured in μA), the power of which is directly proportional to the concentration of ions in the solution. The iodine is adsorbed (then absorbed) by the starch. If the starch is damaged, even more of the iodine is adsorbed. The method therefore consists of creating a known quantity of tri-iodide (I3-) ions and leaving these ions in contact with the flour for a fixed period of time (usually 3 minutes). The μA current is measured at the end of the test; the lower the current, the more tri-iodide (I3- ) ions have been adsorbed and the greater the damage to the flour.
CHOPIN Technologies began to market this concept in the early 1990s under the names SD4, Rapid F.T., and SDmatic. It works as follows:
- The technician prepares a solution containing 120 ml of distilled water, 3 g (+/- 0.5g) of potassium iodide, and 1.5 g (+/- 0.5 g) of citric acid.
- The solution is placed in the machine and the measurement head is lowered into the solution.
- A heating resistor heats the solution to 35°C, while a thermometer controls the temperature in real time.
- 1 g of flour (+/- 0.1 g ) is placed in the machine, on a vibrating system.
- Once the solution reaches 35°C, a pair of electrodes generate an electric current in the solution that creates free (unbound) iodine for a time that is suitable for the sample (Figure 1).
- A second pair of electrodes measure the exact electric current that is generated: IM (the amount of iodine).
- The flour is automatically introduced into the solution.
- The test continues for 180 seconds, after which time the machine measures the residual current IR.
- The machine then measures the level of iodine absorption: AI = 1-(IR/IM), which is proportional to the amount of damaged starch.
The machine is extremely easy to use and provides results in less than 10 minutes. The results can be converted into UCD (international standard), Audidier, Farrand, or AACC values on the basis of well-known flours.
Damaged starch is a normal and unavoidable byproduct of any wheat milling process. From a technical standpoint, it is a serious issue that affects the vast majority of bakery products worldwide.
Nonetheless, starch damage is not always measured as it should be.
Laboratories in the cereal grain industry have everything they need to test the quality and quantity of proteins. The same cannot be said for starch damage, despite the fact that starch accounts for 80% of all the flour that is used or produced.
At the beginning of the 21st century, this situation could be explained by a lack of quick and easy techniques for testing starch damage.
Very few millers could measure the protein content of their flour if they were still required to use Kjeldahl laboratory equipment, as this equipment is incredibly complex and can only be operated by experts. Thankfully, automated measurement devices (Kjeldatherm) and NIR methods were created which made it possible for all labs to measure protein content.
The same phenomenon is now taking place in the area of starch damage. New techniques are allowing all cereal grain labs to measure this vital parameter, which simultaneously affects the hydration of dough, machinability (stickiness, etc.), fermentation, and the characteristics of the finished product (volume, color, conservation, etc.).