Impact of fermentation of tef on phytic acid and total phenolic contents

In document Compositional and nutritional properties of tef and tef-based food products (Page 113-116)

CHAPTER 3: TRADITIONAL FERMENTATION OF TEF INJERA: IMPACT ON IN VITRO IRON AND ZINC DIALYSABILITY

3.4 Results and Discussion

3.4.2 Impact of fermentation of tef on phytic acid and total phenolic contents

The pH values of the doughs used to prepare tef injera are presented in Fig. 3.1A. The pH significantly changed during the fermentation process. All varieties showed a substantial drop in pH from 5.8 (prior to fermentation) to 3.4 (after 72 h of fermentation). The PA contents of

injeras baked after 24, 72 and 120 h of fermentation are shown in Fig. 3.1B. The PA content of

the unfermented injeras of all varieties ranged between 1205 and 1552 mg/100 g dm. After fermentation, the PA content was reduced by 49-66%. Together with a substantial pH drop, all the varieties showed a drastic drop in PA content within the first 24 h of fermentation, followed by a further slight decrease in PA content, and finally reached their lowest PA levels after 72 h of fermentation, except for Quncho which showed a significant reduction in PA content observed after 120 h. Different cereals including oats, rye and wheat exhibited maximal endogenous phytase activity in the pH range of 5-6 (Konietzny and Greiner, 2002) which could explain why all the tef varieties studied here also showed the largest drop in PA content within the first 24 h fermentation in which the pH at the start of the fermentation was in the range of 5.6-5.8 as shown in Fig. 3.1A. During fermentation of cereals, endogenous phytase are reported to play the major role in decreasing PA while the importance of lactic acid bacteria was only to reduce the pH and create favorable conditions with limited phytase activity (Reale et al., 2007). Therefore, the LAB fermentation is mainly used to create an optimum pH for the phytase enzyme to act on PA degradation. This fact could explain the slight degradation of the PA as the acidity further increased or as the pH deviates from its optimum range of phytase activity, during the 24 to 120 h course of the fermentation. Studies on the backslop fermentation of tef dough showed different magnitudes of PA degradation in the range of 42%-80% (Abebe et al., 2007; Fischer et al., 2014; Urga and Narasimha, 1997). In addition to the creation of optimum pH for the phytase enzyme to act on PA, the lactic acid production could also induce a PA hydrolysis effect (Clydesdale and Camire, 1983). This could explain why most of the tef varieties attained their lowest PA contents at 72 h which

Chapter 3: Traditional fermentation of tef injera: impact on in vitro iron and zinc dialysability

98

corresponds to the lowest pH. However, the reason why the PA showed an increasing tendency in line with the increasing tendency of pH when the fermentation further progressed to 120 h could be attributed to a phosphorylation process.

The difference in the potential of backslop fermentation to reduce the PA content could be associated with the difference in microbiota and the endogenous phytase activity, owing to the differences in source of the materials, particle size of the flour, variety, harvest season, duration and temperature of fermentation. Endogenous flour phytase activity dominates the activity of sourdough microflora phytase during fermentation of cereals (Poutanen et al., 2009).

Figure 3.1 pH of dough, phytic acid and total phenolic contents of injera

(A) pH of unfermented and fermented tef dough, (B) PA content (mg 100 g-1 dm injeras) and (C) TPC (mg GAE)/100

g dm injeras made from unfermented and fermented dough of four tef varieties. The error bars indicate the standard deviation of the means. (n=3). a,b,c Values within same variety with different small superscript letters

are significantly different (p < 0.05). A,B,C Bars of same color with different capital superscript letters are

significantly different (p < 0.05). FerT-fermentation time in hour.

b c c c aAaB a bABaB b bBaB b bBaA a 0 2 4 6 8

Quncho Tsedey Zagurey Zezew

p H Tef varieties FerT 0 24 72 120 A dB cA cC cB cC bA bB bB bB aA aB aB aA aA aB aB 0 400 800 1200 1600

Quncho Tsedey Zagurey Zezew

P A Tef varieties 0 24 72 120 B aA aA aB aB bA bA aB bC cB cA bC bC cB bA aAB bC 0 200 400 600 800

Quncho Tsedey Zagurey Zezew

TP C Tef varieties 0 24 72 120 C FerT FerT

Chapter 3: Traditional fermentation of tef injera: impact on in vitro iron and zinc dialysability

99

Thus, the difference in the extent of PA destruction could largely be attributed to the difference in variety as endogenous phytase activity could be dependent on the variety of the cereal.

In cereals, minerals and PCs are mainly localized in the bran of a seed (Raes et al., 2014). The majority of these PCs exists in bound form (part 2.1) and exert a mineral inhibitory effect. The TPC of the unfermented and fermented injeras of the different tef varieties studied is shown in Fig. 3.1C. Although the TPC content in all varieties increased significantly by 42%-70% after fermentation, the proportion of the bound phenolic content in the same injeras as used in this study decreased from 83% to 68% (part 2.2) leading to a reduced inhibitory effect on the mineral bioaccessibility. As expected, a decreased content in galloyl- or catechol-bearing PCs,

e.g. gallic acid and protocatechuic acid was observed in the bound fraction of the PCs in injera

after 120 h of fermentation (part 2.2). Thus, the decrease in the mineral co-existing bound PCs could improve the mineral solubility and consequently, increase the bioaccessibility of the minerals in the small intestine.

The PA:Fe and PA:Zn molar ratios, which are frequently used to predict mineral bioaccessibility, are presented in Fig. 3.2A and 3.2B, respectively. All varieties showed a 3- to 4-fold decrease in the PA:Fe molar ratio after 120 h of fermentation. In concordance with the PA:Fe molar ratio, also a ~3-fold decrease in the PA:Zn molar ratio was observed for all varieties. Many researchers have tried to make associations between the exact amount of PA left after fermentation (or any other process) and the bioaccessibility of Fe and Zn. The adverse effect of PA on Fe bioaccessibility seems only to be eliminated by decreasing the PA content to a level below 100 mg/100 g dm (Greffeuille et al., 2011). Moreover, Hurrell (2004) suggested a degradation of more than 90% of the phytate content and/or even a complete dephytinization to reduce the PA:Fe molar ratio to a value < 1 or preferably < 0.4 for enhancing the Fe bioaccessibility. Based on the WHO (1996) recommendations, PA:Zn molar ratios with a value < 5, with a value between 5 and 15 and with a value > 15 would result in a bioaccessibility of 55%, 35% and 15% for Zn, respectively. Based on these recommended molar ratio predictions, the traditional fermentation used in this study was not able to reduce the PA content below the suggested levels to significantly increase the bioaccessibility of Fe and Zn.

Chapter 3: Traditional fermentation of tef injera: impact on in vitro iron and zinc dialysability

100

In document Compositional and nutritional properties of tef and tef-based food products (Page 113-116)