Effect of tef injera fermentation on soluble and bound phenolic contents

In document Compositional and nutritional properties of tef and tef-based food products (Page 74-78)

Part 2.2: Soluble and Bound Phenolic Contents and Antioxidant Capacity of Tef Injera as Affected by Traditional Fermentation

2.2.3 Materials and methods 1 Chemicals and reagents

2.2.5.2 Effect of tef injera fermentation on soluble and bound phenolic contents

The soluble and bound phenolic contents of injeras of different tef varieties at different fermentation times are presented in Table 2.2.2. The soluble phenolic content differed significantly at different fermentation times within each variety, as well as at each fermentation time. Unfermented and fermented injeras of the two brown seed color varieties Zagurey and Zezew showed higher soluble phenolic contents than the white varieties.

Soluble PC of the varieties increased by 92-150% after fermentation and the highest increase was observed after 72 h of fermentation. A relatively lower increase of soluble phenolic content, between 15-38% as measured in mg GAE/g dm, was observed in buckwheat, wheat and rye after fermentation with LAB and yeast (Dordevic et al., 2010) and a very high increase, up to 14-22 folds of the soluble phenolic content was also seen in wheat after fermentation (Dey and Kuhad, 2014). The increase of the soluble PCs could be owing to the action of endogenous and microbial enzymes initiated during the fermentation which leads to the release of bound PCs. Indeed several yeast and LAB, which are also involved in tef fermentation, are capable of synthesizing enzymes like esterases, xylanases, and phenoloxidases that in turn are capable of breaking down ester linkages to release bound PCs in the form of soluble PCs (Ajila et al., 2011; Jamal et al., 2011; Oliveira et al., 2012). Unlike our expectations that there would be a decrease in bound phenolic content (Table 2.2.2) due to the increased soluble phenolic content after fermentation, an increase of bound phenolic content, ranging from 13-55%, was revealed as fermentation progressed from 0-120 h. This is in agreement with studies showing an increase of both soluble and bound phenolic contents after fermentation of lentils, soy bean, black cow gram and mottled cowpea, wheat, rye and whole barley (Anson et al., 2009; Gan et al., 2016; Hole et al., 2012). Many previous studies (Acosta-Estrada et al., 2014; Bhanja et al., 2009; Dvorakova et al., 2008) showed an increase in soluble phenolic content following fermentation.

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Table 2.2.2 Phenolic and flavonoid content of soluble and bound phenolics of tef injera Phenolic compounds mg (GAE)/100 g dm

FerT Quncho Tsedey Zagurey Zezew p Quncho Tsedey Zagurey Zezew p

Soluble extracts Bound extracts

0 45.0±2.7aAB 38.3±5.6aA 52.8±2.4aB 56.1±4.7aB 0.011 226±6aA 227±3aA 307±18bB 309±11aB 0.002

24 87.1±2.0bAB 74.8±9.9bA 93.4±7.6bAB 105±8bB 0.014 265±8aAB 256±14abA 302±11bB 487±22bC < 0.001

72 101±5c 102±4c 109±8c 103±7b 0.498 356±39bB 275±19bA 461±21cC 506±10bC < 0.001

120 103±4cAB 100±2cA 118±2cC 110±6bBC 0.002 334±22bB 238±10abA 248±4aAB 427±68bC 0.002

p < 0.001 < 0.001 < 0.001 < 0.001 0.003 0.023 < 0.001 0.001

Flavonoids mg (CE)/100 g dm

Soluble extracts Bound extracts

0 19.0±2.4aA 20.8±0.3aA 35.9±1.1aB 50.6±3.3C < 0.001 243±3cB 165±25cAB 177±24A 239±7bAB 0.039

24 26.6±2.8abA 20.5±1.0aA 35.3±0.7aB 48.1±3.1C < 0.001 114±1bA 87.6±6.7aA 153±14B 163±5aB 0.002

72 33.3±2.2bA 32.7±1.5bA 42.2±2.2bB 46.9±3.1B 0.001 87.8±6.7aA 121±3abB 147±4C 160±8aC < 0.001

120 32.5±2.5bA 30.5±3.1bA 43.0±0.9bB 45.6±2.1B 0.006 117±9bA 117±8abA 135±6A 181±3aB 0.003

p 0.008 0.005 0.002 0.441 < 0.001 0.021 0.162 < 0.001

a,b,c Values within a column with different superscripts are significantly different (P < 0.05). A,B,C Values within rows with different superscripts are significantly different

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Table 2.2.3 Total (soluble + bound) phenolica and flavonoidsb content of tef injera

Total phenolic content Total flavonoid content

FerT Quncho Tsedey Zagurey Zezew p Quncho Tsedey Zagurey Zezew p

0 271±6aA 265±2aA 360±20aB 365±11aB < 0.001 262±6cB 186±12bA 213±26aAB 290±10bB 0.016

24 352±10bA 330±9bA 395±4aB 592±21bC < 0.001 141±5abAB 108±7aA 188±15aB 211±1aC < 0.001

72 457±34cB 377±15cA 569±28bC 608±9bC < 0.001 121±5aA 154±6abB 189±6aC 207±10aC 0.001

120 437±27cB 338±11bA 366±4aAB 536±74bC 0.002 150±6abA 148±11abA 178±1aA 227±3aB 0.004

p < 0.001 < 0.001 < 0.001 < 0.001 < 0.001 0.006 0.063 < 0.001

a,b,c Values within a column with different superscripts are significantly different (P < 0.05). (n=3). (n=3).A,B,C Values within a row with different superscripts are significantly

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Therefore, food processing steps such as fermentation, contribute to a better extraction efficiency of both soluble and bound PCs, resulting in a higher TPC after food processing compared to the raw material. The action of the endogenous and exogenous enzymes in the fermentation process may have improved the extractability of the bound PCs. The organic acids produced during the LAB fermentation could also have played a role in chemically rupturing the cell membranes leading to the release of extra bound PCs, which were extracted as soluble PCs in the fermented samples. This could be witnessed by the coincidence of the highest acidity of the 72 h fermented dough and the highest bound PC in their counterpart

injeras.

The (TPC) (soluble + bound) (Table 2.2.3) significantly increased in each variety as fermentation progressed from 0-120 h. Also for each fermentation time, TPC was significantly different for all varieties (P < 0.05). The TPC ranged from 265-608 mg (GAE)/100 g dm and all the varieties showed the highest TPC in injeras baked after 72 h of fermentation. Each variety demonstrated an increase in TPC by 31-54% after fermentation. The contribution of soluble PC to TPC of unfermented injeras of the varieties ranged from 14-17%, while it increased to 17-32% after fermentation. These results clearly show the importance of fermentation in the overall enhancement of soluble phenolic contents, which could be related to a possible improved bioaccessibility of PCs. The two brown tef varieties revealed higher TPC in each fermentation time compared to the white varieties. These results are also in agreement with previous studies that proved that dark pigmented seeds of quinoa varieties, buckwheat, wheat germ, barley and rye, showed higher TPC than their light colored and white varieties (Dordevic et al., 2010; Tang et al., 2016).

The soluble flavonoid content (FC) of unfermented and fermented tef injera is given in Table 2.2.2. Significant differences among varieties at all the fermentation times are observed, obtaining a maximum increase of soluble FC after 72 h fermentation. The two brown varieties showed a higher content of soluble FC than the white tef varieties in unfermented and fermented samples, which is consistent with results of soluble phenolic content.

Bound FC (Table 2.2.2) showed significant differences among the fermentation times within each variety and across different varieties, except for variety Zagurey. As fermentation progressed from 0-120 h, bound FC decreased by 18-58% compared to the unfermented

injeras. The decrease of the bound FC during fermentation could be due to the release of some

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evidenced by the increase of the soluble flavonoids. Also, the increased acidic medium could cause cleavage of proanthocyanidins into flavan-3-ols, which thereafter could be oxidized to quinones (Beta et al., 2000; Porter et al., 1985).

Additionally, in the presence of water, flavonoid compounds can undergo self-polymerization and/or interact or bind with macromolecules such as proteins and polysaccharides making the flavonoids less assayable (Beta et al., 2000). Indeed, this could be the reason why there 20- 40% decrease in the TFC (soluble + bound) (Table 2.2.3), following the fermentation.

In document Compositional and nutritional properties of tef and tef-based food products (Page 74-78)