Dynamics of temperature and time in the bio-conversion of cassava peels collected from a local gari processing factory in Nigeria

S O Aro, O J Odejayi, V A Aletor, A A Ashimi, A A Adeboye and B Usifo

Department of Animal Production and Health, Federal University of Technology, Akure, Ondo State, Nigeria.
sambolaro@yahoo.co.uk

Abstract

The interplay of temperature and time in the bio-conversion of cassava peel was investigated in a fifteen day fermentation study during which cassava peels were fermented both naturally and through a consortium of selected micro-organisms.

The temperature of the microbially fermented cassava peel (MFCP) samples varied from 39.26±3.06^oC on day 1 to 48.00±0.17^oC on day 2 and thereafter falling to 34.40±0.20^oC on day 15 of fermentation. The temperature of the naturally fermented cassava peels (NFCP) samples varied from 35.93±2.54^oC on day 1 and rose to attain the highest temperature of 53.03±0.76^oC on day 4 and the lowest temperature of 34.33±2.25^oC on day 14 of fermentation. The temperature recorded for the fermenting cassava tuber wastes varied between the two processing methods and from the ambient temperature throughout the fermentation period. Analysis of the samples harvested from the fermenting waste at five days interval while the fermentation period lasted revealed that the moisture content decreased with increased fermentation days in the two processing methods but was more pronounced in the NFCP sample. Crude protein, ether extract and ash were improved with successive days of fermentation while the levels of crude fibre and anti-nutrients were reduced. The observation of time and temperature dynamics in the fermentation process however revealed that the interval between day 5 and day 10 could be considered as the optimum period for the bio-conversion or nutrient enhancement in the two processing methods beyond which only marginal increase or even a decrease in the nutrient enhancement could be recorded.

Keywords: Cassava wastes, fermentation, inoculation, nutrient enhancement, unconventional feed ingredients

Introduction

The use of unconventional feed ingredients as very cheap alternatives in livestock feeding has been suggested by many authors (Adebajo et al 2008; Aro et al 2008; Banjoko et al 2008). Also the use of many processing methods geared at nutrient enhancement and anti-nutrient degradation of these unconventional feed ingredients especially those of agro-industrial origin has been advanced (Ayuk et al 2008; Akinmutimi et al 2009; Yashim et al 2009).

 There is however a dearth of information on the interplay of temperature and time in the entire solid fermentation process in terms of the temperature differentials of the processing methods and the optimum period of nutrient enhancements in majority of the nutrient enhancement methods employed. The objective of this study was therefore to investigate the dynamics of temperature and time in two of the cassava peel processing methods with a view to identifying and recommending the optimum temperature and time for the bioconversion of cassava peels and possibly of other cassava waste products.  

Materials and methods

Experimental site and materials

The experiment was carried out at the Federal University of Technology, Akure which is located in the humid rain forest zone of Nigeria. The cassava peels used for the trial were collected from Ayetoro gari processing factory, located at the Federal Secretariat, Alagbaka, Akure, Ondo State. Two methods were employed in the solid fermentation technique of the cassava peels: the natural fermentation and the microbial fermentation methods.

The natural fermentation process or method was carried out by weighing fresh cassava peels of 50kg each into three polythene sacks. Their open ends were securely tied and the sacks were placed on a wooden platform in a covered shed. The temperature readings of the three samples were taken and recorded thrice daily at 6.00a.m., 12.00 noon and 6.00p.m. The ambient temperature was equally recorded at each reading of the sample temperature.

In the microbial fermentation process, two lactic acid bacteria-Lactobacillus delbrueckii and Lactobacillus coryneformis and a fungus-Aspergillus fumigatus were cultured at the Microbiology laboratories of the Department of Microbiology of the University of Technology, Akure. Fermentation trays of dimensions 4cm, 54cm and 38cm for depth, length and width respectively were constructed. One kilogramme each of dried fresh cassava peels was weighed, moistened with 1000cl of water and packed in waterproof nylon bags, sealed and steam-sterilised for 30 minutes. They were cooled to 37°C, emptied into the trays under a pre-sterilised lamina flow chamber and inoculated with 40mls of a suspension of Aspergillus fumigatus containing 1.2x10⁶ spore/ml and 30mls each of a suspension of the two lactic acid bacteria containing 1.2x10⁴ cells/ml. The trays were kept in six-tier fermentation chambers (Plate 1) and the fermentation process was monitored by taking the samples’ temperature at 6.00a.m, 12.00 noon and 6.00p.m alongside the ambient temperature for fifteen days.

Statistical analysis

All data were subjected to one-way analysis of variance (ANOVA) using the SAS (2000) statistical package. Means separation, where appropriate, was performed with Duncan’s multiple range test of the same statistical package.

Results and discussion

The result of the temperature and time dynamics in the fermentation process is shown in Table 1.

The inoculation was done when the temperature of the cassava peels was 37°C and the ambient temperature at 27°C. The fermentation process in the two methods started with a gradual rise in temperature (lag phase) and continued to rise at an increasing rate (log phase) for the first two days in the MFCP samples and the first four days in the NFCP samples such that the highest temperatures were recorded on day 2 (48.00°C) and day 4 (53.03°C) in these fermenting cassava peel samples respectively. The rise in temperature indicated the release of energy as a result of active microbial activities caused by increased microbial biomass (Plates 2 and3) due to ample availability of nutrients that resulted from primary metabolism.

This is in consonance with the work of Okafor (1987), which stated that primary metabolism is essential and is concerned with the release of energy in form of adenosine triphosphate (ATP) and other higher energy compounds which are used in biosynthetic reactions. With increase in microbial biomass, more energy was produced leading to an exothermic reaction within the substrate thus increasing the temperature of the fermenting cassava wastes. The temperature continued to rise until a decrease in temperature was observed on day 3 in the MFCP and on day 5 in the NFCP and these marked the beginning of decline in the temperature readings. The lowest temperature was recorded on day 15 in MFCP (34.40^°C) and on day 14 in NFCP (34.33^°C). The decline in temperature would have been caused by reduction in the microbial biomass as a result of the depletion of nutrients in the media and the production of toxic microbial wastes. Lee (2001) reported a decrease in temperature towards the end of fermentation as a result of the process entering the idiophase during which secondary synthesis occurred in the late logarithmic and in the stationary phase prior to the death phase. Lee (2001) also stated that microbial growth and death are influenced by availability of nutrients, accumulation of end products and environmental factors like temperature, chemicals and radiation. Figure I vividly showed the trends in the rise and fall of temperature in the fermenting cassava peels with that of the ambient temperature over the 15 day period.

Table 2 shows the result of proximate composition of the NFCP and MFCP samples on day 0, day 5, day 10 and day 15 of fermentation.

There was a progressive reduction in the moisture content of the cassava peel samples as the days of fermentation increased with the lowest moisture content recorded on the 15^th day of fermentation. This could be an indication that the micro-organisms made use of the moisture content for growth which led to the increase in the dry matter content of the peels as a result of increase in microbial biomass (Aro et al 2008b). The ash content improved in the MCFP samples throughout the fermentation period while it decreased in the NFCP sample. It would therefore mean that the selected micro-organisms under microbial inoculation had mineral element enhancing ability. The levels of crude protein and ether extracts increased as fermentation period lengthened while the level of crude fibre and NFE decreased. This is in line with the work of Aro et al (2008b) which reported that microbial fermentation of cassava starch residues led to an increase in protein, mineral and fat and decrease in crude fibre and NFE contents. The increase in protein content would have been caused by the accumulation of microbial biomass in the form of single cell proteins and also on the possible secretion of extracellular enzymes into the cassava peels by the micro-organisms in their attempt to make use of the starch content of the peel as a source of carbon (Akindahunsi et al 1999). The increase in the fat content of the cassava peels as a result of fermentation could not be categorically stated. However, there could be possible bio-transformation of excess carbohydrates to fat by the micro-organisms (Akindahunsi et al 1999).

Table 3 shows the anti-nutrient composition of the fermented cassava peels at five day intervals within the 15 day period of the experiment.

The Table revealed significant reduction in the level of the anti-nutrients as fermentation days increased. The reduction of cyanide content of the peels was similar between the two processing methods throughout the fermentation period. Similar reduction in the level of cyanide of cassava peels through solid media fermentation was reported by Oboh (2006) and Tweyongyere and Katogole (2002).  Phytate reduction was more effectively done in the natural fermentation method such that by the fifth day 55.7% of the phytate content had been degraded in the NFCP samples as opposed to 27.5% in the MFCP samples. Reduction of phytate in cassava wastes through fermentation has been reported by several authors (Ugwu and Oranye 2006; Aro et al 2008a) but differential reduction as recorded in these two processing methods is novel to this trial. A reversal of the trend was observed in the biodegradation of the oxalate component of the peels in which the MFCP samples were better degraded than the NFCP samples on the successive days of fermentation. It could be observed also that the reduction in the level of oxalate is most time and labour effective within the first five days of fermentation in the MFCP samples beyond which only minimal reduction results.

Conclusion

• An interval of 5-10 days could be the optimum time during which nutrient enhancement of cassava peels would be best ensured in the solid substrate fermentation methods described in this trial.

• Farmers wishing to adopt these fermentation techniques could therefore save labour and time while at the same time ensuring a more rapid turnover in the production of the fermented and value-added product. This is based on the fact that the period of steady increase in substrate temperature which fell within the first four days in this study usually coincides with the log phase of microbial growth during which they engage in rapid biosynthesis of major bio-molecules while a period of steady decline that follows is indicative of senescence preparatory to eventual death

• The dynamics of temperature of the fermenting substrates also revealed that the microbial inoculums recorded less fluctuation in the temperature of the fermenting substrate and therefore affords a more practical approach at moderating or controlling fermentation temperature.

• Further study is suggested at estimating the rate of nutrient enhancement and anti-nutrient breakdown at successive days of fermentation in other to determine the labour and time effectiveness of the two different processing methods.

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Received 30 January 2011; Accepted 16 April 2011; Published 1 July 2011

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