Enzymes in Foods and in Feeds

Eugene F. Jansen, Arnold Kent Balls.

Enzymes are constantly changing the products in which they exist. The changes, during the growth and ripening and even after the harvesting of a plant, may be good or bad, depending on circumstances.

Once in a while it is easy to remove unwanted and destructive enzymes. For example, the sugar in sugarcane deteriorates rapidly because of the action of the enzyme invertase, which changes the cane sugar to invert sugar. The invert sugar eventually goes into the molasses. When a cane is first cut, most of the invertase is at the top. Therefore topping the cane immediately removes a great part of the harmful enzyme, and the rest of the cane can be kept many hours longer until it can be ground conveniently.

But such simple methods cannot be applied to processed foods. They are usually heated to destroy the enzymes. The process is termed blanching, scalding, pasteurization, and the like, according to the industry and the particular methods used to raise the temperature. It is ordinarily thought of as a method for reducing the population of micro-organisms, which it does; but the destruction of enzyme material by that means is also a matter of moment, particularly when other methods are to be used for minimizing the growth of bacteria as, for instance, when the product is to be kept frozen or is to be dried.

In canning foods, there is little or no enzyme problem, because the temperatures employed are usually adequate for the destruction of enzymes. The refinements required in heating technique to reduce the little to practically nothing are today the study of many food technologists.

The destruction of enzymes during the heat treatment depends on three things the time of heating, the temperature, and the nature of the enzyme. Obviously all three variables need to be considered, and the work of considering them and developing definite heating times and temperatures is now going on, but it is by no means completed. Indiscriminate heating cannot be used it would surely inactivate all enzymes, but it entails other disadvantages. The canning of orange juice is one example among many. The flavor of orange juice is imparted mainly by the suspended particles. There exists in oranges an enzyme (pectinesterase) that causes the particles to coalesce and settle, resulting in a bland juice. Pectinesterase is unusually resistant to heat. To inactivate it, temperatures higher than those needed for ordinary pasteurization are necessary. To prevent heat damage (that is, the formation of a burnt flavor) many ingenious methods have been devised to inactivate pectinesterase by a short exposure to a high temperature, followed by rapid cooling.

The importance of enzymes in the preservation of frozen fruits and vegetables was not recognized at first. In 1929 the pioneer company among frozen-food firms selected and froze thousands of pounds of fresh peas. It was a bitter experience to find that, after a few months of storage at 0 F., the peas had developed a foul odor and flavor. At that time, H. C. Diehl and C. A. Magoon, of the Department of Agriculture, suggested that scalding--blanching might improve the keeping qualities of the peas by inactivating the enzymes. Accordingly, the 1930 pack was scalded with steam before freezing. This time the peas kept well at the temperature at which the former lot had spoiled. Hence it is not enough merely to slow down enzyme reactions by holding vegetables at very low temperature the reactions must be completely stopped by a scalding process, which destroys most or all enzymic activity before freezing.

The experience gained from the peas opened up the whole question of enzyme inactivation in the freezing of vegetables. A measure of the scalding necessary had to be determined by trial and error for each vegetable.

Aside from the taste of the product, other advantages are gained from scalding. The color of green vegetables becomes brighter. The bacterial count is lowered. The bacterial count becomes a factor only during the defrosting ( thawing out) , however, whereas the enzymes keep on decomposing the material throughout the whole storage time, even though the storage temperature is so low that the decomposition goes on very slowly.

We do not know which of the enzymes are responsible for off-flavor in frozen vegetables. There is no good proof that well-blanched products deteriorate in storage because of enzyme action, except that if they are not scalded the deterioration is thousands of times faster. The destruction of various enzymes by scalding has been measured many times and in many agricultural products. It not only takes place, but the rates are known in many cases, so there is no doubt of the efficacy of scalding.

At present ( and we report on a rapidly developing subject), the situation seems to be that with unscalded fruits and vegetables the deterioration is almost all due to action of enzymes. When the same products are scalded, it may be caused by the last remnants of enzymes, or by the natural re-actions no longer catalyzed by enzymes, or a slower catalysis caused by pieces of the enzyme that have been decomposed by heat. To illustrate the last point : Some enzymes are iron compounds: others are compounds of copper. Traces of iron and copper are known to cause foodstuffs to deteriorate, though very slowly when compared with what the enzyme would do. Nevertheless, in a storage warehouse a lot of time is available to enzymes or copper or anything else.

In raw fruits and vegetables, when kept in cool storage, it may well be that the formation of off-flavor is not due to direct enzyme action but rather to the lack of it.

R. Plank advanced such a theory for the cold-storage (not frozen) injury to fruit. Grapefruit cannot be stored below approximately 50 F. without harmful results. Green bananas chilled below 46 F. will not ripen properly. Plank's explanation is that some enzymes of the extremely complicated biochemical system are retarded more than others at the low temperature, thus permitting the accumulation of the products from the enzymes of first attack. Such midproducts can cause off-flavors or other effects. It is doubtful if this is the case with processed foods, in which most enzymes have already been destroyed, but it is very likely that this sort of action is responsible for many of the troubles that develop in the cool-storage shipment and marketing of unheated agricultural products like fresh fruits and vegetables. The best-known example of a comparatively harmless change from cold is the sweetening of potatoes at temperatures slightly above 32 F. At lower temperatures the velocity of sugar formation from starch is retarded less than the rate of decomposition of sugar due to respiration. The result is a higher concentration of sugar in the potato.

Two other troublesome details cause complications.

The first is that some enzymes, notably peroxidase, are known to regenerate themselves after being heated. It must be said that this has never been demonstrated for any practical food product. But because it occurs every day in the laboratory (under rather special conditions), we suspect that other enzymes, today unknown, might do the same thing even better and thus be giving us trouble.

The second is that harmless bacteria (of which there are always a few in "commercially sterile" food products) may not grow in the product but may still be able to excrete their enzymes into the product, and thus make trouble again.

The fact remains that scalding has got rid of most of the trouble in keeping canned or frozen foods. The rest of the difficulty may be due to remaining enzymes, maybe to something else, or maybe to both. In any case, it is a slow process. Most persons associated with the frozen-pack industry believe that these slow deteriorative changes in stored processed foods are due to enzyme action; only a few believe that off -flavor formation may occur because of slower (nonenzymic) reactions.

The argument of the majority who believe in the enzyme hypothesis is that when certain enzymes, for which tests can easily be made, have been inactivated by heat, the product keeps well in the frozen state.

It is therefore of great industrial importance to have tests for the efficiency of a scalding process when it comes to inactivating enzymes.

Two enzymes have been used as an index. One is catalase, which decomposes hydrogen peroxide to water and oxygen. The other is peroxidase, which decomposes hydrogen peroxide when any one of a number of other oxidizable substances is present for example, catechol. Both enzymes are easily measured, and their disappearance on heating is taken to signify the destruction, or near destruction, of all the other enzymes as well. Packers disagree as to whether catalase or peroxidase makes the better index. Catalase is destroyed by less heat treatment than peroxidase. Testing for peroxidase therefore is a more rigorous measurement perhaps too rigorous. A need exists for research to determine the enzymes actually responsible for off-flavor formation in order that tests for them can be used, rather than an index enzyme, which apparently has little to do with the process of deterioration.

THE PROBLEM OF ENZYME CONTROL in frozen fruits is different from that in frozen vegetables. Let us first take up tree fruits, such as peaches and apples. Tree fruits contain an enzyme called polyphenoloxidase. Most of them contain a natural, tannin-like substrate, which in the presence of oxygen causes darkening at the cut surfaces. In untreated frozen-pack peaches most of the darkening occurs during the defrosting. Such peaches also develop an off-flavor.

THE CONTROL of polyphenoloxidase has taken three approaches.

The first is to scald the fruit. But if the enzyme is destroyed by scalding there is no advantage in freezing precooked peaches over canning them. Hence other methods were sought.

The second method was through plant breeding. The Sunbeam peach, although it contains polyphenoloxidase, does not darken, because it lacks tannin. Some of the new crosses with the Sunbeam peach darken only slowly. Although these new peaches have good quality, many persons think they are not equal to an Elberta.

The third method, the one most widely used on peaches and apricots, is to exclude oxygen during packing. Packaging peaches in sugar sirup also helps keep oxygen from coming in contact with the cut surfaces, but that alone is not enough.

C. W. DuBois and D. K. Tressler found that the addition of vitamin C completed the job. That vitamin functions in this manner: The first oxidation products of the tannin formed by polyphenoloxidase and oxygen are reduced by the vitamin C (which is thereby oxidized), and no darkening takes place as long as any of the vitamin remains. This method is now widely used.