The curing of pork for the purpose of manufacturing bacon and ham is fundamentally a process of salting that was originally used merely as a method of preservation. A century and a half ago the curing of pork was done on the farm or in the home. To‐day, practically all the bacon and ham consumed is mass‐produced in factories, and the concern of manufacturers is to obtain the best standard products by the simplest means. The report that follows is not concerned with those factors in the quality of the product that can only be standardised by control of the conditions under which the pig is reared and of its treatment immediately before slaughter, such as the conformation and composition of the carcase. It is concerned only with those factors that are definitely due to the process of curing, namely, the cured flavour, the production of the red cured colour of the lean meat and the even distribution of salt. It is now well known that the production of the red colour is due to the interaction of a nitrite, such as potassium or sodium nitrite, with the haemoglobin of the meat, which yields a pigment, nitroso‐haemoglobin, which is more permanent than haemoglobin. On the other hand, it is known that such nitrites may have harmful effects upon animals, if taken indiscriminately into the alimentary system, so that the view generally held is that the amount of nitrite in staple foods should be as low as possible. In the traditional method of curing, nitrite is not added to the curing salt. Nitrate (saltpetre) is added. The fact that the red colour develops is due to the action of certain bacteria in reducing a portion of the added nitrate to nitrite. In this indirect method of introducing nitrite into the cure, the amount of nitrite in the product when it is finally consumed depends upon many variables, and cannot be easily or strictly controlled. The traditional procedure of adding nitrate is still the basis of commercial practice in this country ; in fact, the addition of nitrite is illegal. The traditional method of curing on the farm or in the home has, however, been modified in one very important way in the modern factory. Traditionally, dry salts were used. To‐day, the usual method employed in factories is salting by immersion in tanks of pickle, combined with the forcible injection into the carcase of a solution of common salt and saltpetre. Two advantages have been gained by this change ; first, a milder cure, containing a relatively small and fairly evenly distributed amount of salt, and, secondly, speed and economy. From the points of view both of the consumer and of the manufacturer, a number of questions emerge from this review of the present situation in this country. Thus, it may be asked : (1) Does nitrate contribute in any essential way, in addition to serving as a source of nitrite, to the process ? Does it, for example, contribute essentially to the cured flavour or to the keeping properties of the product ? (2) Similarly, do bacteria contribute in any essential way, in addition to acting as agents in the production of nitrite ? (3) How far are the temperature, time and acidity determined in tank‐curing by the need for controlling the type of bacterial flora and its activity ; and what advantages could the manufacturer obtain if the need for the bacteria and for the control of their activity did not exist ? (4) Would the elimination of the nitrate and the bacteria from the process help toward obtaining more uniform and lower amounts of nitrite in the finished product ? It has therefore been felt essential to carry out the experiments described in this report, although it may appear at first sight that they cover similar ground to that traversed as long ago as 1925 in the U.S.A. under the auspices of the Bureau of Animal Industry of the United States Department of Agriculture and of the Institute of American Meat Packers. There is, however, a basic difference in the aims of the two pieces of work. The American work, conducted on an extensive scale in a number of packing houses, was a successful demonstration that the direct use of sodium nitrite in American cures was commercially practicable, and had many operative advantages over the traditional method, which relied entirely for the production of nitrite on the bacterial reduction of nitrate. As regards the product, it was found to be at least equal in all respects to that produced by the older method. The work was not designed to elucidate the mechanism of curing, i.e., to isolate the factors responsible for the production of the cured flavour and, in fact, no steps were taken to eliminate the factor of bacterial action. The results did, however, show that the presence of nitrate, as such, was not essential for the flavour of American bacon and hams. The aim of the present work has been twofold : to determine the relative importance of the various factors responsible for the production of the cured flavour ; and to determine whether the direct use of nitrite in English cures, which are very different from those used in America, would give a satisfactory product, as judged by English standards. The first objective is not only of scientific interest, but is fundamentally related to the second. If a clearly defined factor can be shown to be responsible for the production of the cured flavour, any consequential modifications in English commercial practice that may suggest themselves can be viewed and developed on a scientific and not an empirical basis. The work therefore included a critical comparison between, first, current procedure in the factory, based on the traditional process of adding nitrate and depending on bacterial activity for the production of nitrite ; secondly, nearly sterile procedure in which both nitrate and nitrite were added in addition to sodium chloride ; and thirdly, nearly sterile procedure in which only nitrite was added. Other aspects of curing were also investigated, including the rate at which the freshly‐slaughtered pig is cooled, the action of heat on the nitrite in bacon, the minimal desirable amount of free nitrite in bacon, the bacterial flora of pork, bacon and mature tank‐pickle and the amounts of salt and nitrite in commercial bacon. The results of the experimental cures, as will be seen, establish a strong presumption that the characteristic flavour of bacon is due to the action of sodium chloride and nitrite on the flesh, and that the presence of nitrate and microbial action during pickling and maturation are not essential. The work also raises, but does not settle, other important issues. For example, what is the process by which nitrite is lost in the meat, other than by combination with haemoglobin, and what are the effects of temperature, time, acidity, etc., upon the rate and extent of this process ; does the presence of nitrate in the tissues appreciably retard or inhibit the growth of putrefactive anaerobes? These problems are now being investigated, but in the meantime it seems undesirable to defer publication of the results already obtained. What is the immediate practical upshot of the work ? The basic principles of the curing of bacon can be taken as fairly established, and they do point to the possibility of recasting current practice in this country in a way that would give the curer really effective control over the quality of his product, so far as it is determined by the actual process of curing. But it would be premature at this stage to attempt the radical changes in method that are implied. Before that could usefully be done it would be necessary to carry out a comprehensive series of experiments on a larger scale, and with an adequate range of raw material (i.e., carcases), in order to establish how far the results obtained on the small scale are reproducible in the factory. The question of this further work is under consideration.
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