Glycemic Index of Tef Food Products

In document Compositional and nutritional properties of tef and tef-based food products (Page 191-194)

CHAPTER 6: GENERAL DISCUSSION, CONCLUSIONS AND FUTURE PERSPECTIVES

6.5 Glycemic Index of Tef Food Products

Nutritionally, cereals are known as an excellent source of carbohydrates. However, an immense amount of scientific data have already shown that there is a direct relationship between the frequent consumption of easily digestible, refined or noncomplex carbohydrates and a high prevalence of diabetes, obesity, cardiovascular disease and related health complications (Akhoundan et al., 2016; Azadbakht et al., 2016; Kim et al., 2016; Luna Lopez et al., 2014). To reduce the prevalence of such chronic diseases, dietary interventions i.e. shift of consumption towards whole grain which contains high amounts of slowly digestible complex carbohydrates, and plant-based foods in general, has been widely recommended (Goff et al., 2005; Kim et al., 2016; Patel et al., 2017; Virkamaki et al., 2001). As shown in chapters 1, 2 and 3, tef whole flour has high fiber, phenolic and PA contents. Dietary fiber, PCs and PA interfere in the gastrointestinal starch digestion in such a way that it reduces its digestibility and affects the resulting glycemic index which could be considered as desirable when a lower GI food is needed. Dietary fiber can entrap starch granules and restrict the availability of water during gelatinization and then limiting the accessibility of starch granules to digestive enzymes, which results in the lowering of the GI (Kyung et al., 2014). Phenolic compounds and phytates could also be considered as amylase inhibitors which ultimately decrease the GI of starchy food products (Hoover and Zhou, 2003).

The measurement of GI has proven to be a more useful nutritional concept than the chemical classification of carbohydrates as simple or complex, as sugars or starches, or as available or

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unavailable, allowing new insights into the relation between the physiological effects of carbohydrate-rich foods and health outcomes (Foster-Powell et al., 2002). The GI category of tef based food products is missing in the international GI table (last updated in 2008) containing more than 2400 food items (Atkinson et al., 2008). As tef is the main staple cereal particularly in Ethiopia, the eGI of the tef based food products in our study will have a practical importance.

Chapter 4 discussed the digestibility and GI level of fresh injera and porridge (part 4.1) and a conventional bread (part 4.2). In part 4.1, we revealed that the GI of porridge or injera is at least medium or high according to the international GI table. However, these results could only serve as a base for further in vivo studies due to two main reasons: 1) the study was in

vitro, 2) the sampling did not take into account the confounding factor coming from the

accompaniment such as butter (fat), meat and vegetables or complex mixture of all. For example, porridge is consumed as fresh and is served with spiced butter. At least the effect of butter used as an accompaniment should be investigated as its presence could significantly affect the starch digestibility and resulting glycemic response. It is already known that adding fat to carbohydrates reduces glycemic responses by delaying gastric emptying and stimulating insulin secretion (Moghaddam et al., 2006).

The static in vitro digestion is not able to see such effects on eGI as the time of the static in

vitro is predetermined regardless of the completion of the digestion in the gastrointestinal

system. As a limitation, in vitro digestion is not able to measure the eGI of composite foods, in fact, it was already reported that GI of mixed meals is more strongly correlated with fat and protein content, than with carbohydrate content alone (Brand-Miller and Wolever, 2005). Moreover, the high eGI value resulting from the in vitro digested injera could largely deviate from in vivo GI of similar injera, as the organic acid (mainly lactic, acetic and propionic acids) (Umeta and Faulks, 1989) in injera could actually slow down the gastric emptying (Liljeberg and Bjorck, 1996, 1998) resulting in a lower GI. Therefore, follow up of in vivo studies on injera and porridge is indispensable to establish concrete conclusions whether these food products are suitable for diabetic people.

Moreover, unlike porridge always consumed as fresh, injera could be consumed as fresh or after 3-4 days storage. The results of this study only investigates GI of freshly baked injera which otherwise will change on cooling of the injera due to the starch retrogradation process.

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Therefore, future studies should also investigate the fate of GI of injera at different freshness level and also the effect of other food accompaniments with which injera is principally served. In part 4.2, effect of sourdough and storage time on eGI of conventional tef breads was investigated. We discovered that replacement of the dough with up to 30% sourdough increased the resulting eGI. The addition of sourdough increased the softness of the breads. It has been reported that food structure might have an impact on starch hydrolysis in that the increase in cell volume and/or crumb porosity renders the starch more accessible to hydrolytic enzymes and finally increases the rate of starch hydrolysis resulting high GI (Fardet et al., 2006; Hager et al., 2013).

The storage of the breads for more than one day induced a significant decrease in eGI signifying that the breads could have undergone a tremendous retrogradation during the room temperature storage. It is already established that food products with high water level exhibit high retrogradation (Carini et al., 2017; Li et al., 2017; Zeleznak and Hoseney, 1996), increasing the resistant and slowly digestible starches or decreasing the rapidly digestible starch (Li et al., 2017), also as evidenced in part 4.2. Practically, both injera and bread are consumed at different freshness levels (fresh to after 4 days of storage), however, the safety of these foods after storage should be critically taken care of. Both food products contain high moisture level (as shown in parts 4.1 and 4.2), which could facilitate proliferation of microorganisms during room temperature storage.

If storage time has to be used as a strategy to manipulate the GI of food products, maximum care should be taken not to compromise their safety and organoleptic properties. Therefore, it is worthy that future studies investigate the optimum storage time to get the lowest possible GI, without however, sacrificing their safety and much of the organoleptic properties.

When reporting GI of food products, it is worthy to clearly indicate the freshness level of the food products at the point of sampling. Most importantly, the GI of food products is better be measured as eaten for the ease of practical application, otherwise measuring GI of a frozen conventional tef bread (Wolter et al., 2014a), without however, emphasizing the need for freezing the breads could be misleading to the users of the data as breads actually are consumed either as fresh or after 1-4 days of room temperature storage. Therefore, reporting GI of a typical food product should be complemented mainly with its water level, processing conditions and freshness level. Most importantly, this type of reporting would be vital for

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traditional food products in which the way of processing is subjective unlike to conventional food products which roughly have a common way of processing.

6.5.1 Amylose content of cereals and resulting GI

It has been already proven that cereals with high amylose content result in products with a low GI due to their unique ability to retain native starches during heat processing and resilience to enzymatic digestion (Fredriksson et al., 1998; Kim and White, 2012; Klucinec and Thompson, 1999; Sandstedt et al., 1962; Van Amelsvoort and Weststrate, 1992; Van Hung et al., 2016). Passing through the same process, a food product from a waxy starch cereal will have higher GI than that of produced from a high amylose starch cereal. However, this does not mean that a high amylose content cereal always results in a low GI food product. GI is rather highly dependent on the type of process it passes (Kumar and Prabhasankar, 2014; Nayak et al., 2014) in that the GI of different foods prepared from the same cereal could be different. This have been confirmed in part 4.1 where injera and porridge showed different GI although they were prepared from the same raw material.

In the current literature, it is well documented that the resulting RS, SDS and RDS of a processed food product is affected by the nature of the type of starch and the processing treatments, however, research questions like what happens to the resulting GI, if the RDS is replaced by the same amount of RS or SDS? Which starch fraction interacts most with the rest of macro- and micro food composition in a way to reduce GI is yet not answered. Research findings to these questions will help the current efforts of artificial modification of GI of food products. Overall, the chance of getting cereals that result in a lower GI regardless of the processing steps they pass is very low, therefore, process manipulation is the key factor for investigation to achieve a desirable GI.

In document Compositional and nutritional properties of tef and tef-based food products (Page 191-194)