Winning consumer preference via extrusion cooking of nutritious cereals

Winning consumer preference via extrusion cooking of nutritious cereals (©‎ Eduardo Rocha / Shutterstock.com)

Extrusion cooking is a versatile food process widely used for producing snacks, breakfast cereals and pet food. It is also used to a lesser extent for instant powders, flat breads, croutons and beverages. It is a continuous and efficient process with operational advantages such as a low cost of production or consistency in product quality. It is also a low moisture process which positively impacts the environment. A typical processing line usually consists of several units, described in Figure 1. These processing steps include dry mixing of raw materials, extrusion cooking aimed at texturising and shaping the product, and pre-drying / toasting to reduce moisture content to two to four per cent. The resulting extruded product can be coated with a sugar or fat-based spraying in a rotating drum in order to further tailor its flavour profile. In the case of water-based spraying, a final drying step may be required before cooling and packing stages. An additional milling step may be performed to deliver instant powders or porridge products.

Delivering and communicating nutrition in a strongly regulated environment

Over the last few decades, overweight and obese population issues have become a major concern for public health authorities and governments. Large communication campaigns have raised consumer awareness of the composition of their food and advantages of a healthier lifestyle. As a consequence, consumers are seeking healthier foods and increased variety in their diets. Healthy eating is mostly hindered by the lack of time, lack of availability of healthy foods or simply lack of concern regarding recommendations, which can be the case for teenagers¹. Convenient foods that can be consumed on the go such as snacks are therefore foods of choice for consumers having a busy lifestyle. They also are the most energy dense. The level of information on the nutritional composition has never been so readily available. Indeed, proactively or under pressure from governments and key opinion leaders, the level of micro and macronutrients and energy density is communicated on food packaging logos, through nutritional claims, colour-coding / traffic light systems or quick response codes.

Extruded products such as breakfast cereals, savoury or confectionery snacks or porridges are mostly composed of refined flours. As the taste of cereals is not appealing to consumers, additives such as sugar, salt or fat-based fillings are extensively used, enhancing their taste but also depreciating their nutrition. Among nutritious ingredients used to improve nutrition, those for which claims are permitted enable to provide a compiling story that can be communicated to consumers. wholegrain and dietary fibre are the most obvious health promoting ingredients. Unlike refined flour, they are rich in dietary fibres (Table 1).

Logically, they have been progressively introduced in extruded cereal products. For some type of dietary fibres, communication on their positive effects on cardiovascular health, diabetes, weight management or on the immune system was recently allowed by EU authorities, further strengthening communication credentials². Nevertheless, consumption of these healthy foods remains low with regards to the recommendations. For instance, in the United States, less than 10 per cent of the population actually consumes the three servings of wholegrain recommended per day (Marquart et al., 2007). According to EU regulations, a minimum of three grams of dietary fibre per 100 grams or at least 1.5 grams of dietary fibre per 100 kcal of product must be achieved for nutritional claims³. In the US, a wholegrain food must contain at least eight grams of wholegrain to be qualified as such⁴. Such contents can hardly be reached in portioned foods such as confectionery snacks. Additionally, achieving these quantities also significantly affects the texture, taste and appearance of extruded products, which is a barrier to their consumption. Last but not least, these nutritious ingredients may be relatively more expensive than refined flour.

Identifying consumer drivers: the route to winning preference

The use of health-promoting ingredients significantly modifies parameters driving consumer preference. These drivers include crispiness and lightness in the case of extruded confectionery snacks or sogginess for breakfast cereals. They are associated with physical or chemical parameters (e.g. size, shape or viscosity); themselves driven by the recipe composition, process parameters and physicochemical changes that occur during extrusion. Establishing the link between consumer liking, sensory profiles, physicochemical properties and processing conditions is thus fundamental to develop extruded products with nutritional benefits and strong consumer acceptance. Once consumer preference tests are performed, the reasons for liking or disliking by consumers are analysed through sensory profiling data. The resulting information guides food developers and gives direction to achieve consumer preference. Sensory analysis is an objective evaluation by trained tasters; this tool is indispensable to assess the newly developed products. Once sensory targets are clearly identified, one part of the product development consists of understanding the impact of the extrusion process on the physicochemical and sensory properties of the extruded products.

One of the main benefits of cooking extrusion is to deliver aerated textures, leading to a crispy and light product. Crispiness can be defined as a combination of sound and mechanical properties. These properties are driven by the degree of aeration and microstructure of the cellular structure. The expanded structures can be quantified by a cutting-edge method in the area of porous food products; micro-computer X-ray tomography combined with 3D image analysis⁵. This method is non-destructive and does not require any sample preparation. 3D image analysis of the X-ray pictures allows quantifying the porous structure through a variety of attributes such as e.g. porosity, cells size, walls thickness or number of cells. Figure 2 shows in 3D the surface porosity of a typical extruded cereal bead (Figure 2A) and its inner cells structure (Figure 2B). The information generated by image analysis can be used to explain the mechanical hardness of extruded products. For instance, the increased hardness of extruded cereals enriched in dietary fibre can be associated with their lower in porosity and cells size⁶. Some studies also consider the use of nuclear magnetic resonance to monitor the kinetics of liquid migration, leading to sogginess, into breakfast cereals⁷.

For extruded cereals powders which are consumed in milk or water, one of the key sensory drivers for consumer acceptance is their viscosity. Viscosity must be tailored according to the application, e.g. it must be low for cereals drinks applications while it must be medium to high in the case of cereal porridges. Rheological measurements are used to obtain a quantitative assessment of the viscosity. They allow defining targets with regards to the consumer preference data. Chemical characterisations of starch and protein and physical analysis of powder properties such at its particles size distribution, further helps identifying the main drivers of viscosity development during product preparation.

Mastering recipe composition and process conditions to deliver winning products

Mastership of the physicochemical and sensory properties of extruded products is driven by the design of the extrusion line, recipe formulation and control of the extrusion conditions. The change in size impression and organoleptic properties of extruded products, commonly observed when introducing health-promoting ingredients, can be compensated, on one hand, by changing the extrusion conditions (moisture, temperature, screw speed), and on the other hand, by changing the extruder design; e.g. its screw configuration, barrel length, die design, vapour or gas addition⁸. Parameters that can be varied depending on the targeted product properties are displayed in Table 2.

The most important ingredients in extrusion are carbohydrates. Starch is the main complex carbohydrate in human nutrition. In extrusion cooking of cereals, starch is the major component of the recipe, thus playing a role of a continuous dispersing phase for the other components such as protein, fibres or lipids⁹. Starch is composed of two molecules; amylose and amylopectin. The intensity of the thermo mechanical treatment during the extrusion process drives starch melting at low moisture condition (below 30 per cent) and degradation of these molecules. During extrusion, an intermediate fraction between amylose and amylopectin is generated. This fraction is increased under severe conditions. The shear applied to starch molecules can be controlled through the design of the screw configuration, screw speed or water content.

The viscoelastic properties of starch in the molten state are keys for the expansion of the product at the end of the die. Introducing health-promoting ingredients such as fibres or proteins mostly impact the continuity of the starch phase, its level of degradation under shear and the viscosity of the melt. Incorporation of these ingredients, dispersed in the continuous starch phase, also leads to changes in the extruder pressure and in the expansion behaviour at the end of the die. Unlike refined flours, wholegrain flours are rich in fibres and lipids (Figure 1). Incorporation of wholegrain provides lower size impression and products which are less crispy, chewier, tough and tasteless, grainy or with a mealy texture^10,11. Fibres have similar adverse effects on organoleptic properties of extruded products, although soluble fibres such as inulin deliver a higher size impression and more favourable textures compared to insoluble fibres such as cereal bran fibres¹¹.

Extruded cereal-based products, such as breakfast cereals or snacks, are usually rich in carbohydrates but may lack protein quantity and/or quality. Animal (e.g. eggs, meat, fish or milk) or vegetable (e.g. legumes, cereals) proteins can be used to enrich extruded cereals. While animal proteins contain essential amino acids, vegetable proteins lack some of them. Combination of cereal and animal proteins (e.g. wheat with milk) improves the amino acid profile and biological value of vegetables proteins. The protein content can also be increased by selecting the proper grain or pulse sources such as Bambara beans which are high in proteins. Extrusion can improve digestibility and availability by denaturing the original protein structure. Additionally it can also destroy anti-nutritional substances that might be present, such as trypsin inhibitors in soya or lecithin (Sp) in soya and pulses. It can also reduce the protein quality through the formation of non-digestible molecules generated during the Maillard reaction. The resulting browning effect, also associated with flavour release (caramel notes), is then a function of protein, sugar content as well as temperature and shear condition.

Wholegrains are richer in lipids than their refined counterparts. They are highly unsaturated and thus susceptible to rancidity development. Lipids contribute to caloric density and affect the taste and texture of extruded cereals. There is no known significant effect of extrusion on the nutritional quality of lipids. Nevertheless, amylose-complexes may be created affecting digestibility¹² as well as size impression. Lipids are often introduced in downstream equipment in frying; fresh extruded pellets are soaked into an oil bath or lipids can be sprayed during coating after an air drying step. The frying process leads to higher lipid content and therefore it is a less preferred option for developing nutritional products.

When ingredients are sensitive to the high mechanical shear and temperature delivered during cooking extrusion, their addition may happen later during the downstream processing, i.e. through a coating step or during any final dry mixing. Addition at the later stages is for instance favourable to heat and shear sensitive vitamins. Some other minor ingredients such as sodium bicarbonate enables to improved product size impression and aeration¹³.

Improving the nutrition of extruded cereal-based products is a major challenge for the food industry. This challenge can only be addressed by understanding the impact of health-promoting ingredients on consumer drivers, by designing the right recipe and by optimising the processing conditions of the full processing line.  

References

1. Croll, J.K., Neumark-Sztainer, D. & Story, M. (2001). What does healthy eating mean to adolescents. Journal of Nutrition Education, 33, 193-198
2. European Food Safety Agency (EFSA). EU Register on nutrition and health claims. http://ec.europa.eu/nuhclaims/
3. Official Journal of the European Union. Regulation (EC) No 1924/2006 of the European Parliament and of the Council of 20 December 2006 on nutrition and health claims made on foods
4. Whole Grain Council. Existing Standards for wholegrains. http://wholegrainscouncil.org
5. Trater A.M., Alavi S., Rizvi S.S.H. (2005). Use of non-invasive X-ray microtomography for characterizing microstructure of extruded biopolymer foams. Food Research International, 38, 709-719
6. Chanvrier H., Desbois F., Perotti F., Salzmann C., Chassagne S., Gumy J.-C., Blank I. (2013). Starch-based extruded cereals enriched in fibers: A behaviour of composite solid foams. Carbohydrate polymers, 98, 842-853
7. Lucas T., Le Ray D., Mariette F. (2007). Kinetics of water absorption and solute leaching during soaking of breakfast cereals. Journal of Food Engineering, 80, 377-384
8. Robin F., Dubois C., Pineau N., Schuchmann H.P., Palzer S. (2011). Expansion mechanism of extruded foams supplemeted with wheat bran. Journal of Food Engineering, 107 (1), 80-89
9. Chanvrier H., Della Valle G., Lourdin D. (2006). Mechanical behaviour of corn flour and starch-zein based materials in the glassy state: A matrix-particles interpretation. Carbohydrate Polymers, 65 (3), 346-356
10. Hamaker, B.R. (2008). Technology of functional cereal products, CRC Press, New York, pp. 548
11. Robin F., Schuchmann, H.P., Palzer, S. (2012). Dietary fiber in extruded cereals: limitations and opportunities. Trend in Food Science & Technology, 28, 23-32
12. Colonna P. and Buléon A. (1994). Transformations structurales de l’amidon. In: La cuisson-extrusion, St Paul, MN, AACC, pp. 247-319
13. Lai C.S., Guetzlaff J., Hoseney R.C. (1989). Role of sodium bicarbonate and trapped air in extrusion. Cereal Chemistry, 66, 69-73
14. Marquart, L., Miller Jones, J., Cohen, E.A., Poutanen, K., 2007. The future of whole grains. In: Marquart, L., Jacobs, D.R., McIntosh, G.H., Poutanen, K., Reicks, M. (Eds.), Whole Grains Health. Blackwell Publishing, Oxford

About the authors

Frédéric Robin works as a Project Manager at Nestlé’s Product Technology Center in Orbe, Switzerland. He has been leading innovation projects aiming at improving nutrition of food products over the last 10 years. He is the author or co-author of several scientific articles dealing with incorporation of dietary fibre or wholegrain in extruded products.

Hélène Chanvrier works as a Research Scientist at Nestlé Product Technology Center in Orbe, Switzerland. She has more than 12 years’ experience in the field of material science applied to food products, especially cereals. She has a strong expertise in structure characterisation by x-ray tomography and in starchy extruded products in general. Her main focus is now to support innovation through novel products properties, including nutrition.

Christophe Dautremont works as an Extrusion Expert, supporting project managers at Nestlé’s Product Technology Center in Orbe, Switzerland. He has 19 years’ experience related to extrusion cooking across different businesses such as breakfast cereals, snacks, infant cereals or pet food. He is the author or co-author of several patents.