Cassava Fermentation and Associated Changes in Physicochemical and Functional Properties

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IV. Biochemical Changes

The biochemical changes taking place during different fermentations have been elucidated. The results obtained are described.

A. pH

Reduction in pH during fermentation has been reported by almost all workers. However, the level to which the pH falls and the period taken for the pH reduction vary not only with the type of fermentation but also the conditions used for fermentation. Following the earliest scientific report by Colard and Levy,26 many others have monitored the pH during the different modes of fermentation and the values are summarized in Table 6.

Table 4

Tuber Characteristics and Extraction of Cassava Flour Through Bull Fermentation

Variety	Cooking Quality	Dry Weight (g)	SRF (g)	Waste (g)	Efficiency index of product Recovery (%)
Manukuz-Hanthan	++	48.3	42.77	0.12	87.58
Ambakadan	++ ++	40.45	38.30	0.12	94.68
S 856	+++	38.00	29.66	7.00	78.05
M4	++ ++	36.43	34.70	0.23	95.25
H 2304	+++	34.50	30.95	3.90	89.71
H 1687	+++	32.00	29.09	1.19	90.90
H 165	+	26.91	20.11	5.80	74.73

Note: SRF = sour fermented flour, ++ ++ = excellent, +++ = good, ++ = poor, + = very poor
Source: Mathew George⁵⁴.

Based on the studies on gari fermentation, Akinrele has suggested a two-stage mechanism.¹⁴ In the first step, the bacteria Corynebacterium manihot attack the starch producing lactic acid and formic acid, bringing down the pH to 4.²⁵. At this stage a mold, Geitricum candida, brings about further acidification and gari flavor. Vasconcelos reported that in gari fermentation, the pH came down in 24 h and then remained steady. However, no reduction in pH was observed in irradiated controls. The author suggests that because lowering of pH may stabilize the cyanohydrin, fermentation may not have a significant role in the detoxification. He also has compared two methods of dewatering during gari preparation. Dewatering of grated fermented cassava using either a screw press after fermentation and allowing water to continuously run off did not bring about any major difference in pH (Table 2).⁴⁶ The use of L.cellobiosus and L.plantarum reduced the lag phase during gari type fermentation. By increasing the initial acidifying microbial population, the acid level condition could be reached earlier. Although initial pH ahd no effect on final pH, addition of dextrose facilitated reduction in pH to the lowest level within 48 h.⁶¹

The effect of temperature on pH reduction during fermentation for the preparation of fufu has been studied in detail.¹⁵ The addition of legumes did not influence the fufu fermentation62 (Table 7). Examination of the pH changes at intervals of 24 h in steep water and fermenting tubes during Lafun fermentation revealed that the major drop occurred during the first 24 h in both cases and thereafter only minor changes were noticed.³⁶ There was not much difference between steep liquor and tuber pieces.

It was realized that in production of chickwangue from cassava by an improved technique, the pH reduction beyond a certain stage was not possible, which otherwise could have improved the nutritional quality.⁶³ Heap fermentation seems to be exceptional in that the pH rose during later stages56. The pH dropped from 6.3 to 5.5 and thereafter showed an increase to 8.0. The higher pH during the later s tages might be helping to break down acetone cyanohydrin. In pit fermentation, the pH initially remained high and dropped (figure 5).⁶⁴

Table 6 : pH and Titrable Acidity during Fermentation

	pH		Titrable acidity
Product	Initial	Final	Initial	Final	Period (days)	Ref.
Gari	5.9	3.7 – 3.95	–	–	4	14
	5.9	3.7 – 3.95	–	–	4	14
	7.1	4.3	0.12	0.15(%)	5	25
	6.1	4.1	0.09	0.72(lactic acid %)	3	19
	6.4	4.4	0.09	0.91(lactic acid %)	4	46
	–	4.1	–	0.68(lactic acid %)	–	29
	6.2	4.0	1.2	3.4(mg NaOH/g)	–	47
Mash	4.5	3.9	0.44	0.76 (lactic acid %)	4	29
Fufu	–	4.64	–	4.8 (lactic acid equiv)	4	15
	6.8	3.9	–	–	1	25
E. Niger	6.7	4.5	0.01	0.56	3	19
W. Niger	7.0	4.9	0.02	0.5	3	19
Lafun	7.0	4.5	0.01	0.25 (ml 0.01N NaOH)	5	10
	7.8	5.8	–	–	–	12
	6.58	3.95 – 5.25	–	–	3.5	13
Farhina	6.62	4.6 – 4.7	–	–	3	22
Heap ferm	6.3	5.5	–	–	2	56
	–	8	–	–	6	56
Starch ferm	–	3.8	0.1	1.0(%)	–	57
	6.54	5.06 – 5.17	–	–	–	58
	6.8	4.5	–	–	–	59
Extraneous Enzyme	6.0	4.7	–	–	3	60
Culture Provided	7.0	4.4	1.0	6.8 (mg/g)	3	61
Inoculum provided	6.0	4.0	–	–	3	44

Table 7: Effects of Time of Introduction of 20% (w/w) Cowpea and Soya Bean Flours to Fermenting Cassava on Final pH, Total Tirable Acidity and Overall Acceptability of Fufu.

Time of Introduction (h)	Legume	pH	Total titrable acidity	Overall acceptability ratings
0	Cowpea	3.8	0.41	2.3 (0.4)
	Soya bean	3.5	0.41	2.6 (0.3)
24	Cowpea	3.8	0.43	2.7 (0.8)
	Soya bean	4.2	0.36	2.6 (0.3)
48	Cowpea	3.9	0.43	3.6 (0.4)
	Soya bean	4.6	0.36	3.3 (0.2)
72	Cowpea	3.9	0.43	3.5 (0.6)
	Soya bean	4.3	0.36	3.3 (0.5)
Controla		3.9	0.41	3.8 (0.2)

^a No legume assumed. Values in parentheses are SD.
Source: Oyewole and Abior.

A comparison has been made between sweet and bitter varieties subjected to traditional fermentation, and it was noted that the pH dropped from 6.0 to 3.9 and 6.2 to 3.9, respectively.65 A fall in pH from 6.0 to 4.7 after 72 h with extraneous enzymes was also noticed,66 while reduction in pH to 3.9 within 24 h during lab fermentation was also reported.12 Reduction in pH was higher fro small-sized chips and the reason has been attributed to the higher surface area.60

Numfor et al. observed in a comparative study that the pH declined from 6.58 to 4.11 in natural fermentation and from 6.9 to 4.51 in inoculum provided fermentation67. Cereda observed that the pH fell from 6.2 to 3.2 in oth open and closed conditions of starch fermentation16. The most prominent fall occurred during the first 24 h in inoculum provided fermentation.17