| Summary: | The need for innovative and sustainable technologies in the area of food bioprocessing and fermentation technology has brought upon a great interest in the use of cheap renewable resources in the form of raw materials available in their natural existence. Wood sawdust and rice husk are such ubiquitous lignocellulosic biomasses that can be used as novel biodegradable micro/nano-tubular cellulosic materials (TCs) after a delignification process, alone or as composites with natural microbial biopolymers such as polylactic acid (PLA) and polyhydroxybutyrate (PHB), to design natural microbial biocatalysts for food bioprocessing.
Specifically, TC from wood sawdust of Mangifera indica (mango tree) and Shorea robusta (sal tree), and husks of Oryza sativa (rice) of Indian origin were delignified and characterized in this study, and were used in various fermentation processes. Lignin was removed from the cellulose matrix by alkaline treatment to leave behind a porous, tubular structure. Surface characteristics, lignin content and other chemical components were analyzed to determine the chemical and structural differences among the cellulosic materials before and after the delignification treatment.
The proximate analysis from the leaves, sawdust and husk extracts of the plant materials included the estimation of total carbohydrates, reducing sugars, nitrogen, crude protein, ash, relative water content (RWC). The results showed higher percentage of nitrogen, crude protein, ash and RWC in the leaves of sal than its sawdust. On the other hand, lower percentage of only RWC was found in the leaves of mango and rice compared to their sawdust and husk, respectively. This study will further help to set-up certain protocols for purification of the extracts as porous biopolymer materials for various applications.
In the next part, carbonization of the TCs under nitrogen flow was done using various temperature/time combinations in order to increase their specific surface area. The materials were studied by TGA/DTA, SEM and BET analysis. Atomic Force¬¬ Microscopy, X-Ray Diffractometry (XRD) and FTIR analyses where also carried out in order to study the qualitative differences between the TCs and carbonized TCs.
The presence of tubular structure in the delignified materials at the scale of 60-100 µm was observed distinctively through SEM analyses, whereas for the carbonized TCs a bristle and more crystalline form of structures where observed at the scale of 20-100 µm. The TGA/DTA analyses showed the mass change behavior of Indian origin TCs with progressive weight loss at a temperature of 350oC. The surface area and pore diameters were found to be of 3-4 fold increase in the carbonized TCs compared to the delignified TCs. The results from XRD showed the variation in the degree of crystallinity (CI) for delignified and carbonized TCs. The FT-IR analyses were done to monitor the chemical structure by identifying the functional groups present in each sample after delignification and carbonization, confirming loss of the most amount of lignin during the delignification process and decrease in the intensity of bands during carbonization.
These cellulosic materials were evaluated as biofilters for cold pasteurization of drinkable water and for skimmed and semi-skimmed milk at 4oC, with a flow rate of 2 L/day using a one-way peristaltic pump. Regeneration of the filters was done using hot water. The microorganisms used for deliberate contamination of drinkable water were Lactobacillus bulgaricus and Saccharomyces cerevisiae AXAZ-1 and only L. bulgaricus for milk. The nano/micro-porous TC filters where packed in a nylon thin perforated fabric placed in a bioreactor with stainless steel covers. Both inflow and outflow were analyzed using the standard plate count technique and optical density (OD) determinations at 600 and 700 nm. The efficiency of the mango and sal TC was higher than the rice husk TC in all cases. The microbial removal load ranged from 80-100 % for S. cerevisiae and 70-90 % for L. bulgaricus.
Furthermore, novel biodegradable composite materials based on the mango, sal and rice TCs and microbial polymers (PLA and PHB), were prepared and characterized. The microbial polymers were encapsulated separately in sodium alginate beads forming a (PLA-alginate) matrix. TC was selected because it exhibits high purity, high mechanical strength and an ultra-fine fibrous 2D network structure, while PLA is a low cost, biodegradable matrix derived from various natural resources. PHD on the other hand is more expensive and less stable material at high temperatures - it has low mechanical strength and undergoes thermal degradation-and therefore was not further investigated.
The plain TCs and their composites were evaluated as cell immobilization carriers for use as biocatalysts in lactic acid and alcoholic fermentations. In both cases of fermentations (lactic and alcoholic) and types of biocatalysts (cells immobilized on plain or composite materials), improvements were observed compared to free cells. Lower fermentation rates, higher product yields and productivities and higher amounts of flavor-active volatile by-products were achieved. The product was assayed by various analytical techniques including GCMS, HPLC, and GC. Thus the use of composite biocatalysts for co-immobilization of different microorganisms or enzymes (in separate layers of the biocatalysts), to efficiently conduct different types of fermentations in the same bioreactor is proposed. Such biocatalysts may help avoid inhibition problems of chemical or biological nature (e.g. species competition).
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