Types of Meal Processing Systems

Physiologists have looked to industrial models to gain insight into the types of meal processing systems that are possible and their properties. When chemical or microbial processes are used to make commercial products in industry, the apparatus used to carry out a process is termed a reactor. Three types of reactors are recognized, as shown in Figure A. You will immediately recognize parallels in the meal processing systems of animals. Our intestines, for example, function like the type of reactor shown in Figure A2, and the rumen of a cow functions like the type of reactor in Figure A3. Box Extension 6.2 discusses these reactor types in greater depth. It focuses on the type of meal processing that occurs in ruminants, which takes place in two very different stages: a first stage in which self-replicating microbes act on the meal, and a second stage in which non-self-replicating, digestive enzymes produced by the animal act on it.

Figure A Reactor models applied to meal processing in animals Three meals (coded red, blue, and yellow) are ingested at three successive times.

In industry, when chemical or microbial processes are used to make commercial products, there is a profit motive to optimize the manner in which the processes are carried out. One important insight that comes from pondering this point is that “optimize” means different things in different contexts. For example, if raw materials are difficult to obtain, optimal processing might mean converting the raw materials as completely as possible to products; but if the raw materials are simple to obtain, optimal processing might mean converting them as rapidly as possible to products, regardless of how completely they are used. After the goal of optimization has been specified for a particular process, one can ask what type of reactor is most likely to enable the goal to be met. Mathematical models are used to answer the question, and by analogy, biologists have sought to use such models to understand how foods can be most effectively broken down in the guts of animals under various circumstances (e.g., various optimality specifications).

Among the options for reactor types, the batch reactor (Figure A1) is in many ways the simplest. In a batch reactor, the necessary reactants (raw materials and others) are added at a particular time, the reactants are allowed to work together in the reactor for a period without further additions from the outside, and finally the products are removed before new reactants are added. In a continuous-flow reactor without mixing (Figure A2), technically called a plug-flow reactor—which is tube-shaped—reactants enter continuously at one end, then flow the length of the reactor with little or no lengthwise mixing. In a continuous-flow reactor with mixing (Figure A3), technically called a continuous-flow, stirred-tank reactor, reactants enter continuously as they do in the type just discussed. However, reactants entering at different times mix with each other for as long as they stay in the reactor. When materials exit this reactor type, they are mixtures of products derived from reactants that entered at different times.

Regardless of what type of reactor is used, additional properties determine exactly how the reactor will function. These additional properties must also be taken into account in an optimization analysis. One of these is the reaction rate, the rate at which reactants are converted to products. Another is retention time, the length of time that material remains in the reactor.

Two major processes are responsible for food breakdown in animal guts. One is breakdown of food molecules by digestive enzymes that the animal produces. The other is breakdown by microbial fermentation. These processes differ in fundamental ways, one being that microbes can multiply. When conditions are good for a microbial population, the population can increase its fermentative capability intrinsically by multiplication, whereas enzyme molecules cannot replicate themselves.

In a groundbreaking application of reactor models to animal guts, Deborah Penry and Peter Jumars focused on animals that are specialized to take advantage of microbial fermentation as well as enzymatic digestion. From applying reactor-model math, they concluded that when such animals are living in situations in which it is important to extract energy substrates from foods with high efficiency, a particularly effective gut morphology is that in Figure B.

Figure B An optimal reactor configuration predicted from engineering models Models indicate that this is a particularly effective gut configuration for animals that are specialized to process food by both microbial fermentation and digestive enzymes, and in which natural selection favors high-efficiency extraction of energy substrates from food.

The gut starts with an expanded chamber that acts as a continuous-flow reactor with mixing, in which materials from multiple meals can freely mix. Then it progresses to a long, narrow, tubular chamber that functions as a continuous-flow reactor without mixing. Microbial fermentation takes place in the first chamber, whereas enzymatic digestion occurs in the second. You will note that ruminants have evolved a gut configuration that is similar to this reactor configuration predicted from the mathematics of reactor models.