Cassava Fermentation and Associated Changes in Physicochemical and Functional Properties

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H. Sugar Content

Sugars are generally used by the microorganisms for their growth. Reducing sugars in the fermenting mash during gari production initially increased from 3.1 to 6.2% on the first day, but subsequently fell to 4.4 and 2.87% during the second and third day, respectively.²⁴ The increase during the first day has been attributed to the break down of starch by starch-splitting enzyme, the sugar thus produced being further utilized by the organisms.

Meraz et al. compared different types of inocula in the gari type of fermentation and found that sugars are transformed into acids after an initial lag period of 8 to 18 h.61 Evolution of reducing sugars during the fermentation is given in Figure 9. I all cases, complete depletion of sugars was not observed. Two distinct patterns were evident. In the first case, initial increase in sugar concentration was followed by steady consumption and was observed for natural fermentation. The initial increase may be due to activity of antive enzymes.⁸⁶,⁸⁷ or microbial enzymes.8 In the second case, there was steady consumption of sugars, which has been attributed to high consumption rates due to a high inoculum level. Dextrose addition at different concentrations did not bring about any noticeable difference in the sugar consumption pattern, indicating that sugars are not the limiting factor in this kind of fermentation.61 Starter culture for industrial production of gari was developed and evaluated.88 These cultures led to increase in reducing sugar to very high levels in the first 24 h. The amount of sugar was 50% more than from natural fermentation. The changes in the sugar content of naturally fermented gari closely resembled those in irradiated cassava inoculated with Lactobacillus brevis, suggesting that the chemical changes are due to growth of heterofermentative lactic acid bacteria.

During fufu fermentation, the reducing sugar initially increased up to 24 h and then fell, while the total sugars registered a fall after 48 h (Table 23).⁷⁷ The initial increase in sugar concentration might be the result of starch degradation. The decline in total and reducing sugars observed at later stages might be due to conversion of sugars to organic acids, microbial utilization, or hydrolysis. The free reducing sugars decreased rapidly within first 2 d of fufu fermentation. Total sugars also decreased during fermentation, the effect being more pronounced during the third and fourth days.%48 As the reduction in glucose content was higher at 35*C, this temperature has been suggested as the optimal temperature congenial for the prganisms involved in the fufu fermentation. Further reduction in glucose content during storage of fufu has also been reported by Blanshard et al.¹⁵ A minor increase in carbohydrates was observed in Lafun fermentation.38 The reducing sugars registered an increase from an initial value of 1.1 to nearly 22% and the total sugars from 3.0 to 23% at the end of 3 d during tape fermentation.⁵⁸

Detailed analysis of reducing sugars and sucrose in different fermented products has been carried out.39 The changes in the individual sugars during preparation of gari, East Nigerian fufu, and West Nigerian fufu are presented in Table 24.¹⁹ Fructose, glucose,a nd sucrose declined to very low levels at the end of the first day itself forgari and fufu, but mannitol content exhibited a steep increase at the end of second day and fell noticeably on the third day only in case of gari. This has been explained as a result of heterofermentative lactic acid bacterial fermentation.

Ayernor has compared the carbohydrate changes in three varieties of cassava.50 The tubers were either stored in the shade or retted for the same period, and no correlation between increase in sugar content in stored roots to the product loss in fermented product was observed (Table 21). The break down of starch can be expected to increase sugar content, but this does not seem to happen. In fact, the reducing sugar content reduced to very low levels (Table 23). The sugars have probably been utilized for production of organic acids.^24,57,89 Oyenuga found that the soluble sugars increased from 4.8 to 11 g after 36 h and then reduced to 8 g. the reducing sugars increased from 3.3 to 3.6 in 24 h and then decreased to 0 at 96 h. The initial increase in sugar content has been attributed to starch breakdown and the resultant sugar is further converted to organic acids by the microorganisms. The Corynebacterium sp., although present in low numbers, led to production of lacticacid from free sugars, bringing about a reduction in the sugar content during retting.¹¹ The reducing sugar content disappeared to nondetectable levels at the end of starch fermentation both in open and closed conditions.¹⁶

Table 23 : Changes in the Starch, Total Soluble and Reducing Sugar Contents of Cassava Tubers during Fufu Fermentation

Period (h)	Starch (g/100 g)	Total soluble sugars (g/100 g)	Reducing sugars (µg/10g)
0	81 ± 0.1	4 ± 0.5	33 ± 0.7
12	80 ± 1.3	7 ± 0.3	34 ± 0.3
24	70 ± 1.1	9 ± 0.2	36 ± 1.5
36	63 ± 0.8	11 ± 0.1	25 ± 0.0
48	61 ± 0.2	11 ± 0.1	15 ± 0.4
60	58 ± 0.5	9 ± 0.7	3 ± 0.2
72	54 ± 0.4	9 ± 0.1	0 ± 0.0
84	48 ± 1.8	9 ± 0.1	0 ± 0.0
96	47 ± 1.1	8 ± 0.4	0 ± 0.0

Note : Dry weight basis
Source : Oyewole and Odunfa.⁷⁷

A reduction in the sugar content occurred during natural and inoculum provided fermentations with a higher drop in the latter (Table 18).⁶⁷ During fermentation of cassava tubers with a mixed culture inoculum, the sugar contents progressively fell or both high and low cyanide varieties (Table 22). The lower fall in sugar content observed in the high cyanide triplod variety T-300 has been explained to be due to the breakdown of cyanoglucoside during fermentation and consequent release of sugars. The later reduction in sugar content is attributed to the utilization of sugars by the organisms.^65,90

Mannitol has been implicated to cause certain health disorders if consumed in a large quantity. The mannitol content in fermented cassava products has been determined19,91,92 and it was found that fufu had the highest mannitol content, 700 gari 400 and Lafun 329 compared with 50 mg/ 100 g dry weight in dry chips (Table 25). Although fufu had highest mannitol content, still it was much lower than the potentially dangerous levels. Westby and Twiddy and Vasconcelos have also estimated the mannitol content in fermented products 19,46 (Tables 24,26). The former group found a mannitol concentration of 4.5 mg/g in gari, while the latter detected a total of 6.0 to 18.0 mg/g glucose and mannitol cochromatographed. The mannitol has been attributed to heterofermentative lactic acid bacteria. Unlike Fafunso and Bassir, they did not observe large quantity of mannitol in fufu.

Table 24 : Moisture Content, pH Value, Total Acidity and Sugar Content of Gari and Fufu Samples

Product Stage Preparation	Moisture (%)	pH (% lactic acid)	TTA	Fructose	Glucose	Sucrose	Maltose	Mannitol
Gari
After grating	65	6.1	0.09	1.4	5.0	10.7	< 0.2	< 0.2
After grating	63	6.1	0.09	0.9	3.9	10.6	< 0.2	< 0.2
Day 1 of fermentation	61	4.3	0.68	< 0.2	< 0.2	< 0.2	< 0.2	9.3
Day 2 of fermentation	60	3.9	1.10	< 0.2	< 0.2	< 0.2	< 0.2	15.3
Day 3 of fermentation	58	4.1	0.72	< 0.2	1.9	< 0.2	< 0.2	4.5
After dewatering	44	4.4	0.52	< 0.2	1.9	< 0.2	< 0.2	6.7
Exudates from dewatering	ND	ND	ND	ND	ND	ND	ND	ND
After sieving	48	4.1	0.77	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
East Nigerian Fufu
Initial Water	ND	6.7	0.01		ND	ND	ND	ND
Water after 1 day of fermentation	ND	5.1	0.05	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Root after 1 day of fermentation	ND	6.6	0.05	< 0.2	1.1	10.1	< 0.2	< 0.2
Water after 2 days of fermentation	ND	4.7	0.07	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Root after 2 days of fermentation	ND	5.7	0.07	< 0.2	0.1	5.6	< 0.2	< 0.2
After grating (day 2)	70	5.1	0.32	< 0.2	1.3	< 0.2	< 0.2	< 0.2
Grated (day 3)	69	4.5	0.56	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
After sieving	54	4.4	0.41	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
West Nigerian Fufu
Water after 1 day of fermentation	ND	5.7	0.02	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Root after 1 day of fermentation	ND	7.0	0.02	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Water after 2 day of fermentation	ND	4.4	0.07	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Root after 2 day of fermentation	ND	6.1	0.05	< 0.2	1.0	7.4	< 0.2	< 0.2
Water after 3 day of fermentation	ND	4.9	0.16	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2
Root after 3 day of fermentation	ND	4.9	0.05	< 0.2	1.3	5.7	< 0.2	< 0.2
After dewatering	65	4.0	0.32	< 0.2	< 0.2	< 0.2	< 0.2	< 0.2

Note : ND = Not Determined
Source : Westby and Twiddy.¹⁹