January / February 2004
Optimization of Nutraceutical Processing
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The last two articles introduced nutraceuticals, or “functional foods,” as foods with a medicinal effect. The potentially huge market for these health-promoting and disease-preventing foods has stimulated the US food industry to produce many new nutraceutical products. One of the biggest challenges in bringing a new product to market has been the low rate of success. Typically, only one to two of every 10 new products researched make it to market. This low rate is understandable when considering the large expenditure needed for product/process/marketing research, design, and purchase of new equipment lines for new products. Only the most promising new products can be taken to the costly final production stage. To decrease the risk of failure in final production, the product must be tested thoroughly on the bench-top, and in the larger-scale pilot phase.
One job of the engineer is to ensure that research results from the lab are usable for commercial production. Therefore, engineers should have in the tool kit a knowledge of dimensional analysis, which allows scale-up. We use this tool and modeling techniques to track the history of every particle in the food (process “fingerprinting”) and to optimize processing of functional foods
Extrusion is an efficient process to produce breakfast cereals, snacks, pasta, confectionaries, and pet foods, among others. Extrusion processors are adding nutraceuticals to extruded foods, such as anthocyanins (color pigments from fruits and vegetables) added to snacks. The severe thermal and shear treatment during extrusion degrades many nutraceuticals. Because of the high cost of many nutraceuticals, processors would like to minimize this degradation. To optimize this process, the rates of degradation at various temperatures and shear rates must be know
Figure 1 shows a schematic of an extruder. |
Raw material (e.g. flour) is added to the hopper. As the two co-rotating screws move the material down the enclosed barrel, water and heat can be added. The product (e.g. dough) is mixed, kneaded, heating and compressed, and then forced through a die opening, which can be any shape. The product is cut at intervals for a desired length. Think of your favorite breakfast cereal or puffed grain snack—it’s probably extruded.
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In most continuous processes, like extrusion, the time spent by each
particle in the extruder is different. Furthermore, temperature, shear
rate and viscosity are also changing along the barrel. This type of dynamic
process poses challenges in measuring rates of degradation of a nutraceuticals,
because none of the variables is held constant, and one cannot hold the
process constant to take a measurement.
Our lab has developed methods to estimate rates of degradation for processes
where more than one variable is changing at a time continuously. Briefly,
the procedure is to establish a basic model of degradation with temperature
and shear. First, thermal experiments are performed separate from the
extruder, so that shear effects are zero. We have heated nutraceuticals
(anthocyanins and 
-carotene) and
a vitamin (thiamin) in metal containers at temperatures from 170-300ºF
to determine degradation rates without shear. Due to the changing temperature,
rates could not be solved for directly, but were determined iteratively
using a computer routine. The time each particle spends in the extruder
was measured by injecting a colored dye tracer into the extruded product.
The thermal effect on degradation was then removed mathematically from
the measured total degradation after extrusion to obtain the remaining
shear effect. This shear effect was also modeled. The final result allows
processors to see what extruder conditions (temperature, screw speed,
feed rate) minimized nutraceutical degradation, and how thermal and shear
affect the degradation.
Figures 2 and 3 show what flour with added food-grade beta-carotene looks like before, during, and after extrusion. Approximately 13% of the beta-carotene was lost during extrusion at 266ºF. Figure 4 shows a plot of anthocyanin loss when grape pomace and wheat flour were extruded. Note that the minimum measured loss was at 200 rpm screw speed, but the minimum loss due to thermal effects was at 400 rpm, and the minimum due to mechanical effects (shear) was at 50 rpm. These results make sense, because although higher screw speeds have a more severe shear effect, the particles spend less time in the extruder, so thermal effects decrease.
The modeling procedures developed and tested in our lab have application
to many food processes. For example, we can estimate rates of microbial
destruction in combination processes where high-pressure or pulsed electric
fields are used with a mild heat treatment.
In the past, researchers were reluctant to use iterative techniques because
they were labor-intensive and impractical without a computer. But now
that even personal computers have very powerful and inexpensive computer
capabilities, it is time to use more sophisticated mathematical methods
when necessary. The result will be a savings in cost, experimental time
and effort, and higher-quality and safer food.
Figure 2. Flour mixture moving through extruder barrel |
Figure 3. Flour mixture (left) versus final extruded product (right) |
Figure 4. Anthocyanin loss versus screw speed for grape pomace/wheat flour 1:3 mixture at die temperature = 257ºF total loss (blue), thermal loss (pink), mechanical loss (orange) |
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