Popular Science Monthly/Volume 32/November 1887/The Chemistry of Oyster-Fattening
NOT every lover of the oyster knows that the size and plumpness which are so highly prized in the great American bivalve, and which are so attractive in specimens on the half-shell or in the stew as to lead the average man to pay a considerable extra price for extra size, are not entirely natural; and even those who do know that the majority of the oysters in the market are artificially swollen by introducing water into the tissues are not all aware that the process by which this is done is closely analogous to that by which the food in our own bodies is conveyed through the walls of the stomach and other parts of the digestive apparatus and poured into the blood and lymph to do its work of nourishment.
Physiologists are, I believe, agreed that the passage of the digested food through the walls of the alimentary canal in man and other animals is, in large part, due to osmose or dialysis, and that the operation of this physical law is a very common one in the animal body. But the quantitative study of the chemical changes involved is generally rendered difficult or impossible by the very fact of their taking place in living animals where the application of chemical analysis is impossible. An opportunity is, however, offered by the oyster, which, since it lives in water and has a body so constituted as to readily permit the inflow and outflow of water and solutions of salts, may be easily used for experiments. The results of the experiments have a practical as well as scientific interest, since they confirm the common explanation of the increase in bulk of oysters by "floating," and show that it is essentially a process of watering in which the bulk is increased without any corresponding increase, but rather, if anything, a loss of nutritive material.
It is a common practice of oyster-dealers, instead of selling the oysters in the condition in which they are taken from the beds in salt water, to first place them for a time—forty-eight hours, more or less— in fresh or brackish water, in order, as the oyster-men say, to "fatten" them, the operation being called "floating" or "laying out." By this process the body of the oyster acquires such a plumpness and rotundity, and its bulk and weight are so increased, as to materially increase its selling value.
The belief is common among oyster-men that this "fattening" is due to an actual gain of flesh and fat, and that the nutritive value of the oyster is increased by the process. A moment's consideration of the chemistry and physiology of the subject will make it clear, not only that such an increase of tissue-substance in so short a time and with such scanty food-supply is out of the question, but that the increase of volume and weight of the bodies of the oysters is just what would be expected from the osmose which would naturally take place between the contents of the bodies of the oysters as taken from salt water and the fresh or brackish water in which they are floated.
If we fill a bladder with salt water, and then put it into fresh water, the salt water will gradually work its way out through the pores of the bladder, and, at the same time, the fresher water will enter the bladder; and, further, the fresh water will go in much more rapidly than the salt water goes out. The result will be that the amount of water in the bladder will be increased. The bladder will swell by taking up more water than it loses, while at the same time it loses a portion of the salt.
It does this in obedience to a physical law, to which the terms osmose and dialysis are applied. In accordance with this law, if a membranous sac holding salts in solution is immersed in a more dilute solution, or in pure water, the more concentrated solution will pass out, and at the same time the water, or more dilute solution, will pass in, and more rapidly. The escape of the concentrated and entrance of the dilute solution will be, in general, the more rapid the greater the difference in concentration and the higher the temperature of the two solutions. After the osmose has proceeded for a time, the two solutions will become equally diluted. When this equilibrium between the two is reached, the osmose will stop. If the sac which has become distended is elastic, it will, after osmose has ceased, tend to come back to its normal size, the extra quantity of solution which it has received being driven out again.
We should expect these principles to apply to the oyster. Roughly speaking, the body of the animal may be regarded as a collection of membranous sacs. It seems entirely reasonable to suppose that the intercellular spaces, and probably the cells of the body, would be impregnated with the salts of the sea-water in which the animal lives; and this supposition is confirmed by the large quantity of mineral salts which the body is found by analysis to contain, and which amount, in some cases, to over fourteen per cent of the water-free substance of the body.
It seems equally reasonable to believe that osmose would take place through both the outer coating of the body and the cell-walls of the animal's body. As long as the oyster stays in the salt water, the solution of salts within its body would naturally be in equilibrium with the water outside. When the animal is brought into fresh or brackish water, i. e., into a more dilute solution, we should expect the salts in the more concentrated solution within its body to pass out, and a larger amount of fresh water to enter, and produce just such a distention as actually takes place in the floating. If this assumption is correct, we should expect that the osmose would be the more rapid the less the amount of salts in the surrounding water, that it would proceed more rapidly in warm and more slowly in cold water; that it would take place whether the body of the animal is left in the shell or is previously removed from it; that the quantity of salts would be greatly reduced in floating; and that, if it were left in the water after the maximum distention had been reached, the imbibed water would pass out again, and the oyster would be reduced to its original size. Just such is actually the case. Oyster-men find that the oysters "fatten" much more quickly in fresh than in brackish water; warmth is so favorable to the process that it is said to be sometimes found profitable to warm artificially the water in which the oysters are floated. Although oysters are generally floated in the shell, the same effect is very commonly obtained by adding fresh water to the oysters after they have been taken out of the shell; indeed, I am told that this is a by no means unusual practice of retail dealers. Oysters lose much of their salty flavor in floating, and it is a common experience of oyster-men that, if the "fattened"' oysters are left too long on the floats, they become "lean" again.
This exact agreement of theory and fact might seem to warrant the conclusion that the actual changes in the so-called fattening of oysters in floating are essentially gain of water and loss of salts. The absolute proof, however, is to be sought in chemical analysis. In the course of an investigation conducted under the auspices of the United States Fish Commission, and which included examinations of a number of specimens of oysters and other shell-fish, I have improved the opportunity to test this matter by some analyses of oysters before and after floating. The results of the investigation are to be given in detail in one of the publications of the Commission, in which the principles involved and some side-issues of the experiments will be discussed. I give here the main results, prefacing by brief accounts of the process of "floating" oysters as actually practiced by oyster-men. The following very apposite statements are by Professor Persifor Frazer, Jr., who attributes the changes mentioned to dialytic action:
Persons who are fond of this animal as an article of food, know how much the "fresh" exceed the "salts" in size and consistency. The "Morris Coves" of this city [Philadelphia], while very insipid, are the plumpest bivalves brought to market. On the other hand, the "Absecoms" and "Brigantines," while of a better flavor (to those who prefer salt oysters), are invariably lean compared to their transplanted rivals, as also are the "Cape Mays," though, for some reason, not to the same extent.The most experienced oyster-dealers inform me that the time for allowing the salt oysters taken from the sea-coast to lie out varies, but is seldom over two or three days. At the end of this time the maximum plumpness is attained, and beyond this the oyster becomes lean again, besides having lost in flavor.
The subjoined statements by Lieutenant J. A. Ryder are interesting in this connection. They are taken from a letter to Professor Baird, United States Commissioner of Fish and Fisheries, on "Floats for the So-called Fattening of Oysters":
It is probably a fact that in all these contrivances they take advantage of the effect produced by fresher water upon oysters which have been taken from slightly Salter water. The planters of Chincoteague call this "plumping the oysters for market." It does not mean that the oysters are augmented in volume by the addition of substantial matter, such as occurs during the actual appropriation of food, but only that the vascular spaces and vessels in the animals are filled with a larger relative amount of water due to endosmose. It is a dealer's trick to give his produce a better appearance in the market, and as such I do not think deserves encouragement, but rather exposure.
Mr. Conger has actually resorted to warming fresh water to 60° Fahr. in winter, by steam-pipes running underneath the wooden inclosure surrounding the "fattening" or "plumping" float. One good "drink," as he expressed himself to me, renders the animals fit for sale and of better appearance.Conger's floats are simply a pair of windlasses supported by two pairs of piles driven into the bottom. Chains or ropes which wind upon the windlasses pass down to a pair of cross-pieces, upon which the float rests, which has a perforated or strong flat bottom, and a rim eighteen inches to two feet high. These floats I should think are about eight feet wide and sixteen feet long, perhaps twenty. These structures are usually built alongside the wharves of the packing and shipping houses, and are really a great convenience in conducting the work.
Elsewhere Lieutenant Ryder speaks of the floats thus:
Mr. F. T. Lane, of New Haven, Connecticut, writes as follows about the method of floating practiced by himself, and, as I understand, by other New Haven oyster-growers:
For the following valuable information I am indebted to Mr. R. G. Pike, chairman of the Board of Shell-fish Commissioners of Connecticut:
This "floating," as it is called, results in cleaning out and freshening the oysters, and increasing their bulk; or, as many oyster-men confidently assert, "fattening" them. If the weather is warm, they will take a "drink" immediately, if not disturbed; but if the weather is cold, they will wait sometimes ten or twelve hours before opening their valves. Good fat oysters generally yield five quarts of solid meat to the bushel; but, after floating two tides or more, they will measure six quarts to the bushel. After they have been properly floated they are taken from the shell—and as soon as the liquor is all strained off, they are washed in cold fresh water—and are then packed for market. In warm weather they are put into the water with ice, and are also packed with ice for shipping. Water increases their bulk by absorption and by mixing with the liquor on the surface of the oysters. The Salter the oyster the more water it absorbs. In twelve hours one gallon of oysters, with their juices strained out, will take in a pint of water; but when very salt and dry, they have been known to absorb a pint in three hours.
Water always thickens the natural juices that adhere to the surface of the oyster, and makes them slimy. If too much water is added, the oyster loses its plumpness and firmness and becomes watery and flabby.
Oysters that have been floated bear transportation in the shell much better than when shipped directly from their beds. Oysters, too, that are taken from their shells and packed in all their native juices spoil much sooner than when their juices are strained out and the meats are washed in fresh cold water.
Long clams are not floated, but round clams are. But both, when shucked, are washed in fresh water. This cleanses them of mud, sand, and excess of salt, increases their bulk, and improves their flavor. After washing they will keep much longer without risk of spoiling. If the salt is left in them, as they come from their native beds, their liquor will ferment, and they will quickly spoil.The above facts are gathered from the most intelligent men in the shell-fish business in Connecticut—men who have had many years' experience in gathering oysters and clams, and preparing them for home and foreign consumption. They are all agreed that by judicious floating in the shell, and by washing and soaking when out of the shell, the oyster and the clam increase in bulk and improve in quality and flavor. We will not presume to say that this increased bulk is anything more than a mechanical distention of the organs and the cellular tissues of the oyster by water; or that its improved flavor is not due simply to a loss of bitter sea-salt dissolved out by the water. Many intelligent cultivators are confident that the increase in bulk is a growth of fat; while just as many, of equal intelligence, declare that it is mere "bloat" or distention, akin to that of a dry sponge when plunged into the water. The exact nature of the change the chemist alone can determine.
The following experiments were made with oysters courteously supplied by Mr. Lane, a communication from whom was just quoted. The oysters had been brought from the James and Potomac Rivers, and "planted" in the beds in New Haven Harbor (Long Island Sound) in April, and were taken for analysis in the following November.
Two experiments were made. The plan of each experiment consisted in analyzing two lots of oysters, of which both had been taken from the same bed at the same time, but one had been "floated" while the other had not. For each of the two experiments, Mr. Lane selected, from a boat-load of oysters as they were taken from the salt water, a number, about three dozen, which fairly represented the whole boat-load. The remainder were taken to the brackish water of a stream emptying into the bay and kept upon the floats for forty-eight hours, this being the usual practice in the floating of oysters in this region. At the end of that time, the oysters were taken from the floats, and a number fairly representing the whole were selected as before. Two lots, one floated and the other not floated, were thus taken from each of two different beds. The four lots were brought to our laboratory for analysis.
The specimens as received at the laboratory were weighed. Thereupon, the shell-contents were taken out, and the shells and shell-contents both weighed. The solid and liquid portions of the shell-contents—i. e., the flesh or "solids" and liquor or "liquids"—were weighed separately, and then analyzed. We thus had, for each lot, the weights of flesh and liquids which, together made the weight of the total shell contents, and the weight of the shells, which with that of the shell contents made the weight of the whole specimens. We also had, from the analyses, the percentages of water, nutritive ingredients, salts, etc., in the flesh and in the liquids. From these data the calculations were made of the changes which took place in floating. For the details, which are somewhat extended, I may refer to the publication mentioned above. It will suffice here to give only the main results. It is assumed that the changes in the composition of the body of the animal, due to respiration, nutrition, excretion, etc., during the floating would be too small to be taken into account.
The body of the animal may be regarded as made up of water and so-called water-free substance. The water-free substance contains the nutritive ingredients or "nutrients." These may be divided into four classes: 1. Protein compounds, the so-called "flesh-formers," which contain nitrogen; 2. Fatty substances, classed as fats; 3. Carbohydrates; 4. Mineral salts. These constituents of the flesh of oysters have been but little studied. It is customary to assume them to be similar to the corresponding compounds of other food-materials, but very probably the difference, if known, might prove to be important. The mineral matters especially, which are very large in amount, appear to include considerable of the salts of sea-water. Of the nature of the ingredients of the liquids but little is known. They consist mainly of water and salts and the amounts of their ingredients which are commonly reckoned as protein, fats, and carbohydrates, are very small, so that whatever error there may be in classing them with the ordinary nutrients of food, it will not very seriously affect the estimates of nutritive values.
During the sojourn in brackish water, both the flesh (body) and the liquid portion of the shell-contents of the oysters suffered more or less alterations in composition. In order to show clearly what the principal changes, as shown by the chemical analyses, were, it may per- perhaps be permissible to cite a few of the statistics. I give more details than would perhaps be appropriate in these pages, were it not for the novelty of the results, and the importance of their bearing upon the physiology of absorption of nutritive material in our bodies as well as upon the nutritive value of the oysters.
The changes in the constituents of the body ("flesh") were mainly such as would be caused by osmose, though there were indications of secretion of nitrogenous matters and, especially, of fats, which are not so easily explained by osmose. This I will speak of later.
The amounts of gain and loss of constituents which the bodies of the oysters experienced may be estimated either by comparing the percentages found by analysis before and after dialysis, or by comparing the absolute weight of a given quantity of flesh and the weights of each of its ingredients before, with the weights of the same flesh and of its ingredients after, dialysis. For the estimate by the first method we have simply to compare the results of the analyses of the floated and the not-floated specimens. Taking the averages of the two experiments, it appears that—
|The percentages of—||Before dialysis.||After dialysis|
|Water rose from||77∙9||to||82∙4|
|Water-free substance fell form||22∙1||"||17∙6|
|Protein fell from||10∙5||to||3∙9|
|Fat fell from||2∙5||"||1∙9|
|Carbohydrates, etc., fell from||6∙9||"||5∙2|
|Mineral salts fell from||2∙2||"||1∙6|
|Total water-free substance in flesh||22∙1||17∙6|
There was, accordingly, a gain in the percentage of water and a loss in that of each of the ingredients of the water-free substance. This accords exactly with the supposition that during the floating the flesh gained water and lost salts and other ingredients.
It will be more to the point to note the absolute increase and decrease in amounts of flesh and its constituents—in other words, the absolute gain or loss of each in the floating. Estimates by this method have been made and explained in the detailed accounts referred to. They make it appear that 100 grammes of the flesh as it came from the salt water was increased by floating, in one specimen, to 120∙9, and in the other to 113∙4 grammes. This is equivalent to saying that the two specimens of flesh gained in the floating, respectively, 209 and 13∙4 percent, or, on the average, 17∙3 per cent of their original weight. By the same estimates the water-free substance in the 100 grammes of flesh before the floating weighed, on the average, 22∙1 grammes, while that of the same flesh after floating weighed only 20∙6 grammes, making a loss of 1∙5 gramme or 6∙6 per cent of the 22∙1 grammes which the water-free substance weighed before dialysis. The main results of the two experiments thus computed, may be stated as follows :
In the "floating" of 100 grammes of flesh (body) of the oysters—
|The weight of of—||Before dialysis.||After dialysis|
|Water rose from||77∙9||grammes||to||96∙6||grammes|
|Water-free substance fell form||22∙1||"||"||20∙6||"|
|Protein was assumed to remain the same||10∙5||"||"||10∙5||"|
|Fat fell from||2∙5||"||"||2∙3||"|
|Carbohydrates, etc., fell from||6∙9||"||"||6∙0||"|
|Mineral salts fell from||2∙2||"||"||1∙8||"|
In brief, according to these computations, the flesh lost between one sixth and one seventh of its mineral salts, one eighth of its carbohydrates, and one twelfth of its fats, but gained enough water to make up this loss and to increase its whole weight by an amount equal to from one seventh to one fifth of the original weight. Assuming the loss of nutritive value to be measured by the carbohydrates and fats which escaped, it would amount to about one tenth of the whole. That is to say, the total nutritive materials were one tenth less after floating than before.
In the liquid portion of the shell-contents, the percentage of water rose and that of the water-free substance fell in a very marked degree. But while the whole percentage of water-free substance was diminished, that of both protein and of carbohydrates rose slightly (the amount of fats was too small to be taken into account), so that the falling off was all in the mineral salts. The experiments do not show the exact increase or decrease in the total amounts of the liquids and their constituents, so that it is impossible to say with entire certainty whether there was or was not an actual gain of protein or fats or carbohydrates. It would seem extremely probable, however, that the liquids received and retained small quantities of these materials from the flesh (bodies) of the animals.
The apparent increase of protein and other materials belonging to the body in the liquids, though slight, is very interesting. I must refer to the detailed account of the experiments for the discussion of it and of the changes in composition of the liquids. The point is that if the changes in composition of the oysters in floating were due to osmose or dialysis alone, we should expect simply a gain of water and loss of salts (and perhaps of soluble carbohydrates). But the flesh seems to have lost a little carbohydrates and fats, and probably protein also, along with the salts, while it was absorbing water, the liquids at the same time gaining more or less of protein and carbohydrates. A way in which this may have come about is suggested by my colleague. Professor Conn, who calls attention to the fact that some mollusks, when irritated, produce an extremely abundant secretion of mucus or "slime"—so much, indeed, as to sometimes render a small quantity of water in which the animals may be confined, quite sensibly gelatinous. He suggests that the change to fresh water may induce such a secretion of mucus, and perhaps of carbohydrates and fats as well, as would account for the increase of these substances in the liquids. The observation of oyster-dealers that "water always thickens the natural juices that adhere to the surface of the oyster and makes it slimy," accords with Professor Conn's statement.
If such secretion did take place, the flesh must probably have lost a little protein during the floating. The estimates of absolute gain and loss of weight of flesh and ingredients are based upon the assumption that the quantity of protein was unaltered in floating. If protein was given off, therefore, the estimates are wrong. But the quantity of protein secreted and the consequent error must be, at most, very slight. If there is an error, its effect would be to make the quantities of nutrients after floating appear larger than they really were. In other words, if the error were corrected, it would make the loss of nutritive material in floating greater than it appears to be in the figures above given.
The experiments might have been so conducted as to decide this question of the exact gain and loss of weight of each material in the oysters. It would have been necessary to simply take a larger number in each lot before and after floating, and be certain that the number, weight, and bulk were the same in the floated and not-floated lots of each experiment. For instance, we might, in each experiment, carefully select two lots of, say, a bushel each, as taken from the beds; have the number of oysters the same in each bushel, as an additional assurance that the two lots were alike; float one bushel, and weigh and analyze both. A few experiments of this sort made under different conditions of time, temperature, kind, and age of oysters, etc., would give very reliable and valuable data. Unfortunately, the means at my disposal did not permit so thorough experiments. I am persuaded, however, that the results of such series of trials, if they could be made—and I wish they might be—would be very similar to those of the trials here reported.
It is very interesting to note that these processes of both osmose and secretion which we have been considering in the body of the oyster are apparently very similar to processes which go on in our own bodies—namely, those by which our food, after it is digested, finds its way through the walls of the stomach and other parts of the alimentary canal into the blood, to be used for nourishment. Physiologists tell us that the passage of the digested materials through the walls of the canal is in part merely a physical action, due to osmose, but that it is in part dependent upon a special activity of the cells of the villi. In like manner, the changes in the composition of the oyster, if the above explanation is correct, are caused partly by osmose, and partly by special secretive action, the cell-walls and outer coat of the body of the oyster corresponding to the walls of the alimentary canal in the human body. The forms of vital activity in the two cases are different, but osmose is concerned in both.
The main points here urged may be very briefly summarized:
1. In the floating of oysters for the market, a practice which is very general, and is also used for other shell-fish, the animals are either taken direct from the beds in salt water, and kept for a time in fresher (brackish) water before they are opened, or water is added to the shell-contents after they are taken out of the shell. When thus treated, the body of the animal takes up water and at the same time parts with some of its salts, while small quantities of the nutritive ingredients also escape. The oysters thus become more plump, and increase considerably in bulk and weight, but the quantity of nutritive material, so far from increasing, suffers a slight loss.
2. In the experiments here reported, the increase in bulk and weight was from one eighth to one fifth of the original amounts. This is about the same as is said to occur in the ordinary practice of floating or "fattening" for the market. According to this, five quarts of oysters in their natural condition would take up water enough in "floating" to increase their bulk to nearly or quite six quarts, but the six quarts of floated oysters would contain about one tenth less of actual nutrients than the five quarts not floated.
3. The gain of water and loss of salts are evidently due to osmose. The more concentrated solution of salts in the body of the animal, as taken from salt water, passes into the more dilute solution (fresher water) in which it is immersed, while a larger amount of the fresher water at the same time enters the body. But part of the exchange, and especially that by which other materials, carbohydrates, protein, etc., are given off in small quantities, is more probably due to a special secretory action.
4. The flavor of oysters is often much improved by the removal of the salts in floating, and they are said to bear transporting and to keep better. When, therefore, the oyster-man takes "good fat oysters" which "yield five quarts of solid meat to the bushel" and floats them so that "they will yield six quarts to the bushel," and thus has an extra quart, and that a quart of the largest and highest-priced oysters, to sell, he offers his customers no more nutritive material—indeed, a trifle less—in the six quarts than he would have done in the five quarts if he had not floated them. But many people prefer the taste of the floated oysters, and since they buy them more for the flavor than for the nutriment (at ordinary prices, the nutrients in oysters cost the buyer from three to five times as much as similar nutrients in the better kinds of meat), doubtless very few customers would complain if they understood all the facts. And considering that the practice is very general and the prices are regulated by free competition, the watering of oysters by floating in the shell is, perhaps, less reprehensible than at first thought it might seem. This phase of the question, however, it is not the purpose of this article to discuss.
5. From the standpoint of chemical physiology the most interesting outcome of the experiments is the very interesting parallelism they show between the processes by which the salts and other materials pass from within the body to the surrounding medium and those by which the digested materials of the food in man and other animals are conveyed through the walls of the alimentary canal into the blood and lymph to serve their purposes in nutrition. In each case the process seems to be due in part to osmose (dialysis) and in part to a special function of the organs.
To recapitulate still more briefly: The oysters in "floating" in fresher water, for some hours after they were taken from the beds in salt water, as is commonly done in preparing them for the market, gained from one eighth to one fifth in bulk and weight by taking up water, but at the same time lost about one tenth of their nutritive material. They did this by processes essentially similar to those which go on in our own bodies, and by which the digested food passes from the alimentary canal into the blood, to be used for nourishment.
- The following statements are adapted from a paper presented to the last meeting of the American Fisheries Association.
- A detailed account is also to appear soon in the "Zeitschrift für Biologie."
- "Note on Dialysis in Oyster-Culture," in "Proceedings of Philadelphia Academy of Sciences," 1875, p. 472.
- "Bulletin of the United States Fish Commission," 1884, p. 302.
- The protein is estimated in the usual way by multiplying the nitrogen by 6·25.