LOOKING DEEP INTO SOLID SUBSTRATE FERMENTATION

INTRODUCTION
Somehow, when we talk about fermentation, almost everybody conjures up the image of submerged fermentation (SF) as carried out in the big stainless steel fermentors or the fermentation vats. Not many are aware of the other type of fermentation called as solid substrate fermentations (SSF).

In the East, SSF is used in traditional food fermentations and it has always stayed in the back burners of the fermentation popularity. The ‘unpopularity of SSF does not mean SSF is inferior. In certain ways SSF has more advantages to SF depending on the situation.

It is only in the last few decades that the West is interested in exploiting SSF in the production of microbial enzymes and other microbial products. It is interesting to point here that in terms of diversity of fermentation products the SSF is more restricted compared to SF

The unpopularity of SSF compared to SF is more attributed to the poor understanding and control of the SSF fermentation compared to SF which is well established in the West. The popularity of SF in the west is more because of the rich diversity of fermentation products that can be obtained through SF compared to SSF. This does not mean that there are no SSF in the West. The diversity of cheeses produced by molds in SSF testifies to this.

SF is more established in the west is probably initiated attributed to their fascination of alcoholic beverages. This is followed by the interest in the West to scale up or carry out industrial production through which they have through science and technology to understand and control the process. They would have recognized early that in terms of fermentation products SSF is more efficient compared to the passive SF

The jumpstart in SF is perhaps single handedly contributed by the antibiotics fermentation industries during the World wars and see the legitimacy of Industrial Microbiology.

The poor understanding and limitations of SF is probably the main reason why SF always remain in the back burners of fermentation

It is comparatively, easier to carry out scale up of SF for industrial fermentations compared to SSF.

In a way it is wrong for us to regard SSF and SF as two different types of fermentations. Both types of fermentations are carried out by the microorganisms.

The main differences between SSF and SF are that:
1 SSF uses little water
2 The substrates for the microorganisms are significantly in solid
forms and not in solution

These two factors have important consequences in influencing SSF and giving SSF its unique characteristics

LITTLE WATER IN SSF
When we say SSF uses little water in fermentation it could mean literally in terms of very low volume of water used in the fermentation or very little free or available water occurred in the fermentation.

This is a significant difference compared to SF where the aqueous phase is the dominant component in the fermentation process.

In situation where little water are used in SSF, water is still required for the metabolism of the fermentative microorganisms but it occur surrounding the substrate particles as a thin film of water. A good example is in tapai fermentation.

In Sauerkraut fermentation even though large volumes of water are used for the fermentation, these water are not free or easily available to the microorganisms due to competition with the salt ions.

Where very low volume of water are used in SSF, we could regard the SSF as dry fermentation as exhibited in tempe SSF where fungal mycelial growth covered the surfaces of the wet soya beans.

In such dry SSF, there are porous spaces around the sold substrate to allow for easy mass transfer of heat and oxygen in the substrate matrix

IMPACT OF AMOUNT OF WATER IN SSF

As discussed above water is the critical issue in SSF and SSF operate in very little water environment. Water is important in SSF as
1 It is the medium where nutrients required by the microorganisms
are dissolved and transported to the microorganisms.
2 It is required by the microorganisms to carry out the various
metabolic and biochemical reactions to grow

The right amount of water is critical in any SSF. One good example is in the SSF of tempe. Higher amount of water could result in a more vigorous growth of the mycelia resulting in the clogging of the pores between the solid substrate and hampering the mass transfers of oxygen as well increase in the metabolic heat. This would result in a very hard and compact tempe which is not popular compared to the soft fluffy tempe.

With the decrease in substrate porosity and the resulting decrease in oxygen may even increase the risks of bacterial contamination. Low amount of water on the other hand may result in poor accessibility of nutrients which will result in poor growth of the mycelia.

Low amount of water in SSF means that the fermentation products in solution are not significantly diluted and will occur in high concentration. There is also the side benefits of low volume effluent generated in SSF compared to SF
SUBSTRATES IN SOLID PRESENTATIONS
In SF, the nutrients are usually dissolved, well mixed and dispersed in the fermentation broth. Good mixing will ensure good mass transfer of nutrients, oxygen and heat throughout the fermentor.

In SSF, the conditions are significantly different as the substrates occur dominantly in particle or solid forms and are not easily available to the microorganisms. These solid substrates are often static or fixed and not easily mixed or dissolved. SSF microorganisms have to actively colonized the surface of the substrates, dissolved the solid substrates powered by extracellular enzymes to release the nutrients needed for the growth

The advantages of this dry SSF are that there are large surface areas available and easy aeration. Its setback is however low heat transfer capacity. However often at times it is important in finding the correct size of solid particles in SSF. Too big a paricle might yield to surface area t volume ratio thus limiting the efficiency for microbial action. Yet at the same time using large particles will allow large void space to facilitate oxygen, water and heat mass transfer needed in aerobic SSF.

In small particles SSF even though there is a higher surface area to volume ratio to optimize microbial action, it often however hinders the efficiency of the various mass transfer processes needed to support SSF

AERATION IN SSF

We are discussing aeration in the context of aerobic SSF. Aeration serves the following main functions in SSF.
1 to maintain aerobic conditions,
2 to desorb carbon dioxide,
3 to regulate the substrate temperature and
4 to regulate the moisture level.

In SSF that involves the formation of a thin film of water around the substrate the efficiency of oxygen mass transfer is very high in the case of dry SSF such as tempe. However in tapai SSF due to the high sugar concentration resulting in the thick viscous film of liquid around the SSF, there is a very poor mass transfer of oxygen thus allowing alcohol fermentation to proceed




HEAT DISSIPATION IS CRUCIAL ISSUE IN SSF

One of the main characteristics in dry SSF such as in tempe and composting is the high generation of heat generated by the aerobic metabolism. To make the conditions worst in SSF the solid materials used in SSF are often have low thermal conductivities. This would easily result in built up of heat.

The effective dissipation of heat is often related to aeration of the SSF system. Aeration not only brings in oxygen but also help in the removal of heat from the SSF.

The amount of heat produced and managed in SSF is very crucial to the fermentation process. High uncontrollable heat is not good in SSF as it affects the composition of microorganisms, its physiology of growth and even product formation.

MICROORGANISMS IN SF AND SSF

In both type of fermentations, the microorganisms are the agents of change. Differences might differ in the type of microorganisms involved and the behaviour and physiological requirements of these microorganisms. The function of the fermentors is to support the growth of the microorganisms as such the type and configuration of SF or SSF will be dictated by the nature of the microorganisms involved

In SSF the nature of microorganisms generally differ from SF as it involves:
1 Mixed cultures or diversity of microorganisms
2 That it does not rely on single or pure culture fermentations
These two characteristics are consequences of using complex and solid substrate which can only be effectively utilized by the combined action of mixed microorganisms

Since it depend on mixed culture of microorganisms which are in most cases natural inocula, aseptic is not really required in the SSF. Exclusivity of microorganisms in SSF in most cases are imposed by the phenomena of natural protection whereby not many organisms can grow in the environment such as the very low availability of free water or very high salt or sugar concentration
This does not mean to say SSF cannot go bad or contaminated. Failure to provide the ideal conditions in SSF can result in spoilage of fermented food

MICROBIAL GROWTH FORMS

There are two main microbial growth forms commonly encountered in SSF.

1 Mycelial mat by fungi
2 Microbial film by bacteria


MYCELIAL GROWTH

In aerobic SSF mycelial growth form on the surfaces of the substrates is common. In this growth form the main unit of growth of the filamentous fungal is the hyphae. The hyphae growth is characterized by apical growth. A mass of hyphae constitute the mycelium or mat growing on the surface of SSF

The key point is that active growth occurs only at the tip of the hyphae as it actively seek new source of substrate. Extracellular hydrolytic enzymes are secreted at the apical tip to breakdown the substrate.

Its impossible for the hyphal to keep on growing deeper to search for nutrients and transport it backwards to other hyphae. Sooner the back zones of the mycelia will die or sporulation occurs.

BIOFILM GROWTH

The formation of biofilm is usually characterized in SSF where bacteria growth is dominant. The development of biofilm in SSF followed a sequential development whereby the surfaces of the SSF is colonized and development of the biofilm will result in the growth of high number of cells on the surfaces trapped within the EPS matrix

In terms of structure the biofilm offer different challenges in mass transfer compared to mycelial growth of SSF.

The function of the biofilm is in fact a barrier to mass transfers between the microorganisms and the environment. This situation would easily result in the formation of various physical and chemical gradients from the surface to the inner depth of the biofilm. Aerobic and anaerobic zones capable of supporting different physiological groups of microorganisms would be formed easily

EVOLUTION OF SSF FERMENTORS
Technologically, the SSF fermentors have not really evolved and are still stuck with the concept of racks or tray fermentors with little or no changes from the past. Many designs have been published that belong to the first category including static and agitated models but only few models are used in commercial production

It is SF fermentors with its submerged fermentation that has undergone much technological evolution in size, design and function. This was really triggered by the birth of industrial microbiology spurred by the world wars.

Even today, bioprocess engineers are coming up with newer and unorthodox design for submerged fermentation in fermentors for the production of new metabolites from different type of cells. But seriously speaking the high yield of fermentation products such as penicillin is more due to the successful account of strain development rather than significant improvement in bioreactor design

This development of SF over SSF does not mean that SSF is irrelevant. It is more a reflection of the poor understanding and limitations of the various physical, chemical and biological processes to be controlled in any SSF process. It could also mean that there is a poor appreciation of the potential of SSF in the fermentation industries.

The main limitations of engineering design of SSF are:
1 Problem of heat dissipation
2 Mixing

The main method of heat dissipation is by evaporative cooling. In this process the loss of heat is correlated with loss of water from SSF. Thus it is crucial while there is a need to cool the heat there is also the need to retain the moisture for proper SSF to procee

While good mixing is important to obtain good mass transfers in SSF in drum type fermentors, the product of SSF might not be suitable as in the case of tempe fermentation where the final presentation of the tempe is important in itself




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