Changes in starch can significantly alter qualities, such as taste,
of food made in extrusion cooking. Current testing methods take
too long to perform as they are off-line techniques. A European
consortium decided to look for a new approach. They tested three
different spectroscopic techniques to see if it was possible to
make the measurements on-line. They have since developed a prototype
test cell which one of the partners will commercialise.
Starch is a curious substance. If you mix corn starch with water, for example, you get a stir thickening mixture which appears to have the consistency of cream. That is, until you try to stir it very quickly. Then it appears to be quite hard. This popular science demonstration is explained by likening the starch polymers to miniature spaghetti-like cords; these can sometimes flow past each other and, at other times, they lock together.
Structural changes like these, and more complex ones, occur when starch is being cooked. Today, a common preparation method in the food industry is extrusion cooking where dry food, like flour, and water are mixed together and heated in a screw extruder. As the screw moves round, the mixture is pushed through a cooking zone and then through a die to produce food shapes such as pasta. Depending on the starch concentration and the extent of dispersion of the starch polymers from their native state, a range of products is possible, including snack foods.
Pushing out more food
The technology has been developed steadily over the last 50 years and now represents substantial manufacturing processes for both the human and animal food industries. As an efficient continuous process, they offer savings in energy, space and other costs compared to older technologies and these savings are passed on to the consumer.
However, we must not forget quality control is vital to get the best quality at the lowest price. Under the wrong conditions, the starch can change form and give a less desirable and less tasty product. In the food industry, present methods to assess the changes in starch during processing are quite long-winded. The tester takes a sample and then performs a paste viscosity test to see what physical state the polymer has reached. But this test is simply to slow and laborious to be used for an on-line test, a key part of modern-day factory quality control.
A new approach
In infra-red spectroscopy, infra-red radiation is shone on the sample at specific frequencies. At some frequencies, the molecules in this sample absorb the radiation. The absorption frequency depends on the type of molecule in the sample. If a molecule changes shape it will absorb at different frequencies. This spectroscopic technique or the related Raman spectroscopy could perhaps provide the answer.
The project partners, including food research organisations, spectroscopy experts, industrial specialists and an industrial end-user, decided to see if it would be feasible to use these techniques to measure changes in starch within the extrusion cooker, without taking samples. "Such a technique would offer the food industry immediate benefits in manufacturing savings through better machine control, giving good control over product quality, shorter setting times on production changes and reduced wastage," according to Dr Robin Guy, from Campden & Chorleywood Food Research Association in the UK.
The group's first task involved preparing a set of starch samples that would represent the range of starch processing used in industry. Each sample had to be analysed with the paste viscosity test, indicating the physical state of starch, as well as against the specific mechanical energy input of the extruder which indicates extent of processing. Correlating these "traditional" measurements against the spectroscopic data provided useful results.
Testing three different spectroscopic techniques with dry samples, Raman, Near Infra-red and Mid Infra-red, showed that Near Infra-red (NIR) gave the best results and would also be the most practical choice to use in a factory.
Designing a special test cell
It was not just a case of putting the spectroscopy equipment on to the production line, though. A special flow-through cell had to be designed by CCFRA and, using special optical fibres, it could be put right into the hot melt fluid inside the extruder. Rigorous performance testing of this on-line equipment was done in the factory with soft wheat flour. The test spanned typical industrial conditions and the three major variables that could affect the measurements: temperature, starch concentration and starch physical form.
The equipment and the measurement technique made the grade. From there, the group moved on to study the technique's results for wheat flour, wholemeal and maize. This gave a valuable insight into the relationship between polymers in the melt fluid and the final product.
As with many research projects, the participants are communicating their results through journal articles and conferences. This project is taking it a step further. After a special workshop to present the results to the food industry, one of the companies, Nestec, is looking at using the new technology on its production line. One of the partners, Perstorp, is preparing to market the on-line probe system, incorporating a monitoring and control package, as a new area of business.