As demand for fibre grows, companies are turning to a category of starches that helps make high-fibre foods more functional. Wendy Dalidowics explains their physiological benefits
Resistant starches represent an important category of ingredients because of what they offer both food manufacturers and consumers. For manufacturers, they provide the opportunity to add dietary fibre and other nutritional benefits to foods where it was previously not possible — such as white bread — while still maintaining desirable appearance, taste and texture. For consumers, they increase fibre through the foods the individuals are already eating. Therefore, the commercial availability of resistant starches and their ease of use in food applications may have far-reaching implications for both the food industry and public health.
Food starches can be classified as either glycaemic or resistant. Glycaemic starches are those that are degraded to glucose by enzymes in the human digestive tract. Resistant starches escape digestion in the small intestine but are fermented in the large intestine by bacterial microflora. Unlike glycaemic starches, most resistant starches analyse as total dietary fibre — or TDF — for labelling purposes.
Resistant starches are classified as types 1-4 based on physical and chemical characteristics that can be intrinsic to the starch or related to the processing treatment undergone by the starch.1
RS1 consists of physically enclosed starch and can be found in whole or partially milled grains, seeds and legumes. Its resistance to digestion is dependent upon the natural protection of the outer coating or seed. By this logic, whole-grain foods delivering intact grains house a significant quantity of RS1, whereas highly processed whole-grain foods do not. As foods have become more processed over the past 100 years, the amount of RS1 naturally in our diet has steadily declined.
RS2 is characterised by intact, ungelatinised starch granules. Native RS2 can be found in foods such as green bananas and uncooked potatoes and peas. There is currently only one commercially available source of natural RS2 on the market. Based on high amylose maize, it has a particularly high gelatinisation temperature, giving it substantially more process tolerance than other native sources of RS2, which typically lose all of their resistant starch content upon processing. RS3 resistant starch is non-granular starch that has been retrograded and is crystalline in structure. RS3 is present in small quantities in bread, cooked and cooled potatoes and pasta and some breakfast cereals. RS3 starches tend to have higher process tolerance than RS2s because their resistance to digestion is not dependent upon maintaining a granular structure. Some RS3s test as dietary fibre, according to AOAC methodology, while others do not.
RS4 represents a more recently developed category of resistant starches in which glycaemic starches are chemically modified to prevent digestion. The modifications typically used include esterification, etherification and cross bonding. Because they are based on chemical treatment and not properties intrinsic to the starch, RS4 products from several sources can be found.
Considerable research has been conducted to explore the health benefits of resistant starch, primarily focused on RS2 and RS3. Therefore, the benefits discussed here will be based on studies exclusive to these two types. To date, studies identifying the impact of RS4s have not been published and it cannot be assumed that the benefits proven with natural types of resistant starches apply to chemically modified RS4s.
The physiological benefits of resistant starches include their impact on the small intestine, impact on the large intestine and overall impact on health. Studies have shown that a high-glycaemic diet increases the risk of cardiovascular disease, obesity and diabetes.2,3 Modulating glycaemic response may offer more balanced energy levels and improve appetite control.2
When used to substitute for flour, RS2 has been shown to decrease glycaemic and insulin responses and increase insulin sensitivity.4 Resistant starch also helps maintain digestive health via multiple mechanisms.5 It is fermented by colonic bacteria, and as a prebiotic fibre it selectively stimulates beneficial bacteria while suppressing the viability of harmful bacteria. Higher levels of butyrate, a biomarker for colon health, result from the fermentation of resistant starch compared with other fibres.6 If the resistant starch measures as insoluble dietary fibre by AOAC methods, it also offers a lower caloric value.7
Unlike some fibres, RS2 resistant starch is slowly fermented, so it does not tend to cause discomfort.8 Best of all, resistant starch allows fibre enrichment with these health benefits in everyday foods without compromising eating quality.
Resistant starches enable the delivery of dietary fibre in a wide variety of foods. Because resistant starches have a small particle size, white appearance and bland flavour, they can be incorporated seamlessly into applications without affecting texture, taste or appearance. Resistant starches also have a low water-holding capacity. This combination of physical characteristics makes it possible to use most resistant starches to replace flour on a one-for-one basis without significantly affecting dough handling or rheology.
The amount of resistant starch used to replace flour depends on the particular starch being used, the application, the desired fibre level, and, in some cases, the desired structure-function claims. Therefore, appropriate inclusion levels can vary, but are easily tailored for each end-use. From a quality standpoint, some applications are more sensitive to flour replacement than others. For example, bread and rolls, which generally have a bland flavour, are low-fat and require a minimum amount of gluten for structure; the maximum flour replacement is typically 10-20 per cent without noticeably changing the texture. Vital wheat gluten would then be added to bring the gluten back up to its original value.
However, for chemically leavened products, flour replacement levels can be much higher and no additional vital wheat gluten is required because flour protein is not critical. Sweet goods in particular lend themselves to high resistant-starch levels because they tend to be high in fat and sugar, which helps compensate for changes in flavour and mouthfeel. In these applications, flour replacement can be upwards of 75 per cent.
Process tolerance needs to be considered when selecting and using resistant starches. The majority of commercial-resistant starches on the market will retain their dietary fibre through typical baking processes and even mild extrusion. However, extreme processing conditions may damage the resistant starch, resulting in a loss of dietary fibre. Formulators should analyse finished foods made with processes involving extremely high temperatures, pressure and/or shear to verify the level of dietary fibre.
As discussed, resistant starches are well suited for low-moisture food systems where they can be used at very high levels compared to traditional starches. The majority of the commercially available resistant starches rely on intact granules or compact crystalline regions with high melting temperatures to resist digestion. Therefore, these products generally will not swell or contribute viscosity during most processing. Essentially insoluble, resistant starch would not replace viscosifying starch in liquid applications.
Resistant starches can be a valuable resource to product developers today. Understanding the differences between the commercially available offerings will assist in choosing the one that will meet the desired goals. The manufacturer should be able to provide information regarding ingredient statement and nutritional labelling, dietary fibre content and additional health benefits.
Wendy Dalidowics is technical service engineer at National Starch Inc, based in New Jersey. www.nstarch.com
Respond [email protected]
All correspondence will be forwarded to the author.
References 1. Brown IL, et al. Hi-maize: new directions in starch technology and nutrition. Food Australia 1995; 47:272-5.
2. Higgins JA. Resistant starch: metabolic effects and potential health benefits. J AOAC Internat 2004; 87(3):761-8.
3. Ludwig D. The glycaemic index, physiological mechanisms relating to obesity, diabetes and cardiovascular disease. JAMA 2002; 287(18):2414-23.
4. Robertson MD, et al. Prior short-term consumption of resistant starch enhances postprandial insulin sensitivity in healthy subjects. Diabetologia 2003; 46:659-65.
5. Brown IL. Applications and uses of resistant starch. J AOAC Internat 2004; 87:727-32.
6. Kritchevsky D. Epidemiology of fibre, resistant starch and colorectal cancer. Europ J Canc Prev 1995; 4:345-52.
7. 21 Code of Federal Regulations 101.9 (c)(1)(i)(C)
8. Kendall CWC, et al. Assessment of resistant starch tolerance: a dose response study (abstract of preliminary results). European Nutrition Conference, Oct 2003.