Although solid-state fermentation (SSF) has been practiced for many centuries in the preparation of traditional fermented foods, its application to newer products within the framework of modern biotechnology is relatively restricted. It was considered for the production of enzymes in the early 1900s and for the production of penicillin in the 1940s, but interest in SSF waned with the advances in submerged liquid fermentation (SLF) technology. The current dominance of SLF is not surprising: For the majority of fermentation products, it gives better yields and is easier to apply. It is notoriously difficult to control the fermentation conditions in SSF; these difficulties are already apparent at small scale in the laboratory and are exacerbated with increase in scale. However, there are particular circumstances and products for which SSF technology is appropriate. For example, a desire to reuse solid organic wastes from agriculture and food processing rather than simply discarding them leads naturally to the use of SSF. Further, some microbial products, such as fungal enzymes and spores, amongst others, are produced in higher yields or with better properties in the environment provided by SSF systems.

With recognition of this potential of SSF, a revival of interest began in the mid-1970s. However, the theoretical base for SSF bioreactor technology only began to be established around 1990. Before this, there were many examples of SSF biore-actors, especially those used in the koji industry, but there was little or no information about the efficiency of heat and mass transfer processes within them. The work that has been carried out over the last 15 years is sufficient to establish a general basis of engineering principles of SSF bioreactors. This book brings together this work in order to provide this basis. It makes the key point that, given the complexity of SSF systems, efficient performance of SSF bioreactors will only be achieved through: (1) the use of mathematical models in making design and operating decisions for bioreactors and (2) The application of control theory.

Before proceeding, we must point out that we are quite aware of the potential problems that might be used by our use of the word "fermentation". In this book we use it not in its metabolic sense but rather in its more general sense of "controlled cultivation of microorganisms". Although several terms are used to denote this fermentation technique, the most common by far is "solid-state fermentation".

This book focuses on SSF bioreactors. It does not aim to introduce SSF itself. We assume that readers interested in learning about SSF bioreactors are familiar with SSF processes themselves. Even if not, a reader who understands the basic principles of SLF processes and SLF bioreactor design will be able to understand this book. In any case, readers requiring a general background regarding SSF can consult books or review articles (e.g., see the Further Reading section of Chap. 1).

Even with this focus on SSF bioreactors, the book deliberately addresses general issues and concepts. Specific examples are given to illustrate concepts, but the book neither considers all types of bioreactors that have been used nor presents all mathematical models that have been developed. We do not attempt to present all the engineering know-how so far generated for SSF bioreactors. Rather, we aim to introduce the fundamental concepts and ideas.

The main audience intended for this book is the researcher/worker in SSF who is currently developing an SSF process with the intention of eventually commercializing it. Our aim is to give this reader a broad overview of what is involved in designing a bioreactor and optimizing its performance.

We recognize that many readers may not have the necessary background to set up and solve mathematical models of bioreactor performance. This book does not attempt to teach the necessary modeling skills. Such a task would require a lengthy treatise on various mathematical and engineering fundamentals. A basic understanding of differential and integral calculus will help readers to understand various of the chapters, although it is by no means necessary to be an expert. After reading this book, the "non-engineering reader" should:

• understand qualitatively the importance of the various mass transfer, heat transfer and biological phenomena that are important in SSF systems, and the interactions amongst these various phenomena;

• understand what mathematical models of bioreactors can do. If you understand what models can and cannot do, then even if you do not have the skills to develop a model yourself, you will know when it is appropriate to seek the help of someone with such modeling skills (a "modeler");

• be able to "talk the same language" as the "modeler". In other words, you should be able to define clearly for the modeler what you wish to do, and you should be able to understand the questions that the modeler poses. In this way you can interact with modelers, even if they have no experience with SSF.

This book should also be useful for readers with modeling skills but who are working in SSF for the first time. In a succinct way, it outlines the important phenomena and the basic principles of SSF bioreactor design and operation.

We welcome comments, suggestions and criticisms about this book. Our aim is to help you to understand SSF bioreactors better. We would appreciate knowing just how well we have achieved this aim. The addresses of the editors and authors are given after the Table of Contents.

November 2005

David Mitchell

Nadia Krieger

Marin Berovic

Marin Berovic

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