Biogas innovation offers an attractive platform for the creation of alternative source of energy if they are appropriately harnessed (Opeh and Okezie, 2011). Biogas is generated in the absence of oxygen by microorganisms in a process known as anaerobic digestion (Fang et al., 2010; Arsova, 2010). The conversion of organic matter into biogas is mainly by the action of different groups of microorganisms such as bacteria, fungi and protozoan. These microorganisms are classified into four groups such as hydrolytic, acidogenic (fermenting), acetogenic and methanogenic bacteria. They act on the different stages of the waste digestion  to bring about effective biogas production (Asikong et al., 2016). During hydrolysis, bacteria transform the organic substrate into monomers and polymers, such as proteins, carbohydrates and fatty acids. Acidogenesis involves further breakdown of the remaining components by acidogenic bacteria into short chain volatile fatty acids, ketones, alcohols, hydrogen and carbohydrate. The rest of the acidogenesis products are transformed by acetogenic bacteria into hydrogen, carbohydrate and acetic acid. Methanogenic microorganisms utilize intermediate products of these proceeding stages and convert them into methane,  carbohydrate and water. The major types of domestic anaerobic degradation system are the Chinese dome, Indian floating drum, Plug flow and the Puxin digesters (Jegede et al., 2019). They have no mechanical blenders and are unheated systems which make them cost effective and well appropriate for farmers and people living in rural areas. In sub-Sahara Africa especially Nigeria, domestic anaerobic degradation systems have had little success compared to Asia and Latin American countries due to inadequate technical application and social acknowledgment (Kalia, 2007). The realistic implementation of biogas innovation is in progress and has not acquired justifiable awareness (Opeh and Okezie, 2011). Furthermore, household digester fabricated from plastic for biogas generation from food wastes for cooking foods has not been adequately studied (Eze and Uzodinma, 2009; Nwankwo et al., 2017). Household digesters for biogas production from food wastes could be cost effective and eco-friendly energy substitute for cooking foods compared to other cooking fuel. However, this research focused on evaluating the anaerobic states of the modified bio-digester and potentials of the generated biogas in cooking foods compared to other cooking fuel.

Chemical properties of wastes before and after digestion

The result in Table 2 shows the chemical properties of wastes before and after digestion. The total solid, volatile solid, carbon, free fatty acid, chemical and biochemical oxygen demand decreased significantly (p<0.05) while moisture and protein contents of the wastes increased significantly (p<0.05) after digestion. Comparable result was reported by Okunola et al. (2018) which was attributed to activities of microorganisms for biogas production. The wastes moisture content ranged from 75.07 to 80.88% before digestion but increased up to 84.76% after digestion. Yadav  et  al. (2014) reported that high moisture content in the waste will encourage movement and contact between microorganism and organic  molecules.  The   moisture  also  increased  after digestion due to decrease in the amount of volatile and total solid (Eze and Agbo, 2010a; Yadav et al., 2014). Decrease in carbohydrate, total solid and volatile soli was more significantly (p<0.05) in WB₃ than others after digestion. This could be due to the production of organic acids (Yadav et al., 2014). The free fatty acids of wastes ranged from 0.06 to 0.10% and decreased significantly (p<0.05) after digestion. There was more significant (p<0.05) reduction in biochemical and chemical oxygen demand in WB₁ than others after digestion may due to more   conversion    of    organic    matter    in    WB₁    by microorganisms for biogas production (Utami et al., 2016).

Physical properties and volume of biogas production during waste digestion

The result in Table 3 shows that the ambient temperature ranges from 29 to 36°C and slurry temperature ranges from 31 ^oC to 37 ^oC during waste digestion. Slurry temperatures supported optimal biogas production because there was high biogas production as slurry temperature ranged from 31 ^oC to 37 ^oC. The pH of the three experimental wastes was fluctuating between 6.70 and 6.81. Fluctuation in pH may be due to higher acidogenesis and lower methanogenic activities, and vice vise (Beevi et al., 2013). Steady pH values were observed in waste WB₃ (6.81) from 12th to 17th day and WB₂ (6.75) from 12th to 14th day respectively. Steady pH values may be due to simultaneous acid production and also quick consumption of the acid by microorganisms for biogas production (Aragaw et al., 2013), Biogas production of WB₁ increased gradually until it got to the 8th day (710 L); after which its biogas production began to fluctuate with highest biogas (712 L) production on the 20th day (Table 3). This might be attributed to a positive synergetic effect on the digestion of cow dung and food waste which provided more balanced nutrients (Aragaw et al., 2013). Just as in the volume of the biogas, the biogas pressure between 12th and 20th day was higher compared to other days, which was above 30 mmHg upto 36 mmHg in WB₁. High pressure over 29-37 mmHg might affect the activities of methanogenic bacteria and biogas production (Dobre et al., 2014). Similar results were reported by Ebunilo et al. (2016) and Olorunmaiye et al. (2016) where the pressure fluctuation between 5 and 52 mmHg was recorded. The biogas production from WB₂ and WB₃ increased each day to the maximum on the 9th day (727 L) and on the 12 day (668 L) respectively, afterward biogas production began to fluctuate with corresponding biogas pressure between 10 and 36 mmHg. Similar result was reported by Aragaw et al. (2013) and Nwankwo et al. (2017). An increase in pressure may be as a result of increase in volume of biogas and temperature in the digester (Ebunilo et al., 2016; Olorunmaiye et al., 2016).

Microbial isolates and total viable count at different stages of waste digestion

The result in Table 4 shows that immediately after charging, total viable count of the three experimental wastes ranges from 2.42×10⁸ to 3.82×10⁷ cfu/ml and a total of eight morphologically and physiologically different bacteria species (Aspergillus nigar, Bacillus spp., Escherichia coli, Klebsiella, Salmonella, lactobacillus, Pseudomonas, Staphylococcus and Streptococcus) and two fungi species (Aspergillus spp and Sacchromyces) were also isolated. This might be attributed to the nature of the substrate (Ali Shah et al., 2014; Idire  et al., 2016). Immediately biogas production started, the total viable count increased significantly (p<0.05) while the isolated microorganisms reduced to six bacteria species. This may be due to the condition in the bio-degradation   system    was   thermo-dynamically unfavorable to the

microorganisms initial presence immediately after charging (Ali Shah et al., 2014) while significantly (p<0.05) increase immediately biogas production started mighty be due to high microbial populations involved in the hydrolytic and fermentative phases of biogas production. Comparable results were reported by Eze and Agbo (2010b) and Asikong et al. (2016). At the peak of biogas production, the total viable count was significantly (p<0.05) higher than the initial stage of biogas production with WB₁ (5.81×10⁸ cfu/ml) recording higher than others. This may be due to varying amount of different wastes.

The microbial loads and isolated microorganisms also decreased significantly (p<0.05) at the point of discharge. Similar result was reported by Asikong et al. (2016). This may be due to deposition of microbial metabolites and gradual exhaustion of nutrient from the wastes (Ziemiński and Frąc, 2012; Li et al., 2015; Asikong et al., 2016).

Effect of cooking with different heat sources on proximate composition and sensory characteristics of yam and rice

The result in Table 5 show that the biogas generated from different wastes cooked significantly (p<0.05) faster than kerosene but not faster than liquefied petroleum gas. Carbon dioxide and other gas (apart from methane) in biogas could reduce its cooking efficiency as reported by Eze (2012).

Abdulkareem (2005) concluded that refining biogas before using could improve its efficiency. In  terms  of  proximate  composition,  significantly (p<0.05) higher moisture content was recorded for yam and rice cooked with kerosene compared to other sources of heat may be due to  longer cooking time which allowed the foods to absorbed more water. The crude protein, fat, fibre, ash and carbohydrate contents were reduced insignificantly (p>0.05) in both yam and rice after cooking. Reduction in ash content could be that some soluble minerals dissolved in water during boiling (Assa et al., 2014). Significant (p>0.05) difference was  not  observed  in  sensory  characteristics  of yam and rice cooked with biogas, liquefied petroleum gas and kerosene (Table 6). Cooking with biogas did not affect the sensory characteristics of yam and rice differently when compared to liquefied petroleum gas and kerosene. It is quite difficult to pinpoint any of the heating sources as the best in terms of their effect on proximate and sensory characteristics of foods (Eze, 2012).