By-Products for Solid State Fermentation


Feedstock is a term used to refer to the raw materials used for manufacturing products. What has been gaining in importance in more recent years is the broad range of bio-based materials for industrial feedstock. There is an ongoing debate about whether this should come from food crops or still from non-sustainable sources, such as trees. What has come into the picture, though, is that a number of high yield, ecologically sustainable and renewable agricultural crops commonly used for food also have myriads of by-product uses. Hemp, bamboo, coconut palm, rice, and sugar cane are good examples of bio-based materials whose by-products can be used for literally thousands of products. Others are corn, wheat, sugar beets, rutabagas, and cassava (source for tapioca).


The proven sustainability of these bio-based materials means that fewer greenhouse gasses go up into the atmosphere, there are less environmental toxins released in their processing, less energy is needed in their use, and the fact that they are readily renewable. What once was considered agricultural waste is now usable feedstock. And there are many types of these by-products. Also, in contrast to petroleum-based products, they are eco-friendly biodegradable. With crops that can easily be grown locally for industry, communities can take a more active role then in deciding about their soil, air, and groundwater.


For example, in the making of bioplastics—the common feedstock has traditionally been sugar, starch, plant oils, and natural rubber. The most important source for sugar is of course sugar cane, which already has great demand in the food industry, but the large amount of leftover matter, called bagasse, after the crushing of the cane for its juice, is high in fiber. High fiber—meaning high in cellulose—by-product utilization is what industry is looking to more and more as a source for making bioplastics. Hemp is another excellent example of a high fiber source.


Solid State Fermentation (SSF) is the process of cultivating microorganisms on solid materials rather than in a liquid nutrient broth (called submerged fermentation). The fermentation process is well known in the production of products like koji (a type of mold spore grown on rice, for making saki), miso, kefir, cheese, beer, and wine. One advantage to going with solid state is the utilization of the very by-products of the plants we have named. These plant by-products—rice husks, for example—are used as the substrate in the fermentation process. Though microorganisms have traditionally been used in liquid fermentation, it is mushrooms (fungi), which naturally feed on solid organic materials, that are the optimal agent for breaking down these substrates. Mushroom mycellium produce the enzymes that are able to break down high cellulose fibers into the complex sugars of fermentation and other nutrients. It has been discovered that the additional advantage here is that these substrates, as feedstock, can keep going, in other words, they can be used over and over, providing maximum potential out of the fermentation process. The outcome is that SSF produces a higher yield of product. The interface of mushrooms and by-product substrates for enzyme synthesis, necessary for fermentation, is clearly a more ecologically sustainable practice.


Many valued products are now available through solid state fermentation, such as antibiotics, enzymes, polyunsaturated fatty acids, the fats and oils used in baking, food flavors and colorants, organic acids, detergents, pulp and paper, and biodiesel fuels. Another product that can be produced in this manner is ethanol, which, in recent years, has been promoted as a highly important biofuel.