Part 12

So-called "Pasteurised" milk has become during the last year or so increasingly popular in this country, whilst on the Continent it has been largely dealt in for several years past, and has commercially proved a great success. Indeed, so strong is the prejudice amongst our neighbours across the Channel against using unboiled milk that in Leipzig and other cities in Germany endeavours have been made by charitable and other societies to encourage the use of sterile milk amongst the poorer classes, whilst it has been stated that the introduction of Pasteurised milk among the poor of New York City, through the philanthropic efforts of Mr. Nathan Straus, has done much to reduce the high rate of mortality amongst infants during the hot summer months. In France, i.e. in Paris and Grenoble, in order to reduce if possible the lamentable mortality amongst infants from diarrhoea in the summer months, which was largely attributed to the use of unboiled milk, sterilised milk was distributed to the poor at the cost of the community in general. In Grenoble, according to statistics collected by Berlioz during the years 1894-6, the death-rate of infants under a year old in the months of July, August, and September fed on raw milk reached 69·3 per 1,000, whilst amongst those supplied with sterilised milk it was reduced to 27·9 per 1,000.

Just, however, as all is not gold that glitters, so all sterilised milk so-called is not necessarily free from bacteria. Indeed, according to a recent German authority, "the complete and certain sterilisation of milk is not yet to hand."

Dr. Weber examined the sterilised milk as supplied by various companies in the city of Berlin. As many as 150 bottles were tested from eight different sources, with the result that not one of these eight companies was found to be supplying milk free from bacteria, or, in other words, what it professed to be--sterile. True, the percentage of sterile bottles varied from 5 per cent. in some of the supplies to 86 per cent. in others.

Thus it may be realised how, as has been already pointed out, difficult a matter it is to devise an efficient apparatus for the reliable sterilisation of milk. So far it appears that the best results have been obtained with an apparatus devised by Flaack, a director of the Brunswick Sterilising Milk Company, and known as the Flaack apparatus. Exhaustive examinations made during the course of a whole year in the Hygienic Institute at Würzburg never once showed a failure, all the samples tested being germ-free.

Some supervision is, therefore, necessary in the case of these milk-sterilising companies to ensure that the public is obtaining what it is paying for, as it has been shown by Professor Flügge, a world-renowned authority on the subject of milk and its sterilisation, that the bacteria left over in these so-called sterilised milk samples are by no means invariably a harmless residue, but, on the contrary, may consist of individuals which he has gathered together in a class under the heading of poisonous peptonising bacteria, and which owe this unfortunate designation to the rapidity and energy with which they can engender the putrefaction of albumen. As indicating how essential it is that every detail in the sterilisation of milk should be adequately assessed, I may mention a paper recently published by H. L. Russell and E. G. Hastings, of the Wisconsin Agricultural Experiment Station in the United States, on the importance of Pasteurising milk in closed rather than in open vessels, bacteria having been found more resistant in milk when heated in contact with the air than in closed vessels, this variation being attributed to the formation of a surface pellicle, which readily forms on milk when heated in open vessels to a temperature of about 60° Centigrade or above. Experiments showed that organisms present in this pellicle or skin were capable of retaining their vitality when exposed to a temperature six degrees higher than that of the milk beneath the membrane in which they were destroyed.

Objections to the use of boiled milk have been frequently made on the grounds of its being more difficult of digestion, and hence less wholesome than the raw article. I may only point out that in this, as in most other matters where opinions may be made or unmade, and in consequence of the facts available being scanty must be more or less arbitrary in character, Dr. Duclaux, the successor to Pasteur as Director of the Pasteur Institute in Paris, has expressed himself as follows in an article on "La digestibilité du lait stérilisé." After reviewing the various special researches which have been made on the subject, he says:--

"Ceci nous amène à une conclusion qu'il faut bien avoir le courage de tirer, c'est que ces études chimiques sur la digestibilité du lait ne sont pas adéquates à la question à résoudre.... En attendant, tenons-nous-en à cette conclusion générale, que le lait pasteurisé, chauffé ou stérilisé, est encore du lait, devant la science comme devant la pratique, et que si son emploi présente parfois des inconvénients, ceux-ci sont légers et amplement compensés par les avantages."

BACTERIA AND ICE

The fate of bacteria when frozen excited the curiosity of investigators already in the early years of bacteriology, for in 1871 we find Burdon Sanderson recording the fact that water which he had obtained from the purest ice contained microzymes, or, as we now prefer to call them, micro-organisms.

It is quite possible that at the time this announcement was made it may have been received with some scepticism, for it was undoubtedly difficult to believe that such minute and primitive forms of vegetable life, seemingly so scantily equipped for the struggle for existence, should be able to withstand conditions to which vegetable life in more exalted circles so frequently and lamentably succumbs.

The tormented agriculturist realises only too well what havoc is followed by a return in May to that season

"When icicles hang by the wall, And Dick the shepherd blows his nail And Tom bears logs into the hall, And milk comes frozen home in pail."

Again, with what solicitude those of us who have gardens wait to see what will have survived the iron grip of winter in our favourite flower borders, and how frequently we have to face blanks in the ranks of some of its most cherished occupants! Numerous bacteriologists, however, have now confirmed this fact, the fields of ice and snow have been repeatedly explored for micro-organisms, and it has been shown how even the ice on the summit of Mont Blanc has its complement of bacterial flora, that hailstones as they descend upon the earth contain bacteria, that snow, the emblem of purity, is but a whited sepulchre, and will on demand deliver up its bacterial hosts. Quite apart from its general scientific interest, the bacterial occupation of ice is of importance from a hygienic point of view, and a large number of examinations of ice as supplied for consumption have been made. Thus, Professor Fraenkl and also Dr. Heyroth have submitted the ice-supply of the city of Berlin to an exhaustive bacteriological examination. These investigations showed that the bacterial population of ice as supplied to Berlin is a very variable one, and fluctuates between great extremes, rising to as many as 25,000 bacteria in a cubic centimetre (about twenty drops) of ice-water, and falling to as few as two in the same measure.

There are numerous circumstances which come into play in determining the density of the bacterial population in ice. First, of course, the initial quality of the water from which the ice is derived is a factor of great importance, for the purer the water the fewer will be the bacteria found in the resulting ice.

Again, if the ice field is wind-swept by air bearing an unduly rich complement of bacteria, as may be expected in the vicinity of populous cities, for example, then the ice will reflect in its bacterial contents the undesirable neighbourhood in which it was produced. Water in repose, again, yields purer ice than water in movement during freezing, for during rest opportunity is given for the bacteria present in suspension to subside, the process of sedimentation or deposition of bacteria which takes place under these conditions playing an important part in water-purification; when, however, the water is disturbed by swift currents, or agitated by storms, this process is interrupted, and the bacteria become entangled in the ice and frozen in situ.

The importance attaching to the physical conditions under which ice is produced in enabling an estimate to be formed of the safety or otherwise of the same for consumption may be gathered from the following extract from an American report on the subject:--

"On the whole it is evident that the conditions surrounding water when it freezes are very important factors in determining the purity of the ice formed. If there is a considerable depth of water in portions of a somewhat polluted pond or river, and the ice is formed in these portions in comparatively quiet water with but little matter in suspension, this ice will probably be entirely satisfactory for domestic use. On the other hand, ice formed in shallow portions of such ponds or rivers, even during still weather, or in any portion if there is a considerable movement of the water by currents or wind while it is forming, may be rendered by these conditions entirely unfit for domestic use."

We have learnt that ice contains bacteria, that its bacterial contents are to a certain extent dependent upon the bacterial quality of the water before crystallisation, and that an important factor in determining its purity is afforded by the physical conditions prevailing at the time of freezing.

It will be of interest to ascertain in more detail what effect the process of freezing has upon the number of bacteria present in the water--what is the degree of bacterial purification effected during the conversion of water into ice.

Now Professor Uffreduzzi, in his investigations on the ice-supply of Turin, part of which is derived from a much-polluted portion of the River Dora, found that about 90 per cent. less bacteria were present in the ice than were present in the water from which it was produced. In the making of ice, therefore, a remarkable removal of bacteria may be effected which approaches very nearly the degree of bacterial purification which is achieved during the best-conducted sand-filtration of water.

Uffreduzzi's results have been repeatedly confirmed by other researches. Thus, in regard to ice obtained from the River Merrimac, water which contained originally about 38,600 bacteria per cubic centimetre, on its conversion into ice had only from three to six. Sewage, again, containing about a million and a half bacteria per cubic centimetre after being frozen only contained under 74,000. It should be mentioned that this last figure represented the number of bacteria obtained by thawing the outside of the sewage ice-cake; inside the cake there were more found--about 121,000. The difference in these figures is due to the fact that, whereas the outer layers of ice looked quite clear, towards the centre the ice contained sewage sludge and hence more bacteria had become arrested; but in spite of this the bacterial purification effected is very striking, although not sufficient to render the use of ice from such a polluted source either palatable or desirable.

It is, of course, a well-known fact that water possesses the power of purifying itself during its transformation into ice, and that the process of crystallisation not only prevents a considerable proportion of the matters in suspension from becoming embodied in the ice, but also eliminates a large percentage of the matters in solution, the latter being driven from the water which is being frozen into the water beneath. If, therefore, ice in the act of forming can get rid of matters in solution, it is not difficult to understand how it can eject bacteria, which though so minute are yet bodies of appreciable dimension and in suspension. But that there are limits to this power of excluding bacteria, and that, as in the case of other mechanical processes, an overtaxing of the available resources is at once reflected in the inferiority of the product, is shown by the frozen sewage experiment, in which the ice, having had too large a supply of bacteria in the first instance to deal with, was unable to get rid of more than a certain proportion, and was obliged to retain a very considerable number. Hence great as is the degree of purification achieved by ice in forming, yet it must be recognised that its powers in this direction are limited, and that the fact of water being frozen does not necessarily convert a bad water into immaculate ice.

It is worthy of note that the city of Lawrence, in Massachusetts, obtains the greater portion of its ice from a river which in its raw, unpurified condition was rejected for purposes of water-supply in consequence of the numerous and severe epidemics of typhoid fever which accompanied its use. Since the application of sand-filtration to this water, however, the death-rate from typhoid in this city, instead of being abnormally high, has fallen abnormally low, and this improvement is attributed to the excellent quality of the water supplied to the city, and has taken place despite the use which still continues of ice from the polluted river. The authorities consider the city's immunity from typhoid amply justifies their sanctioning the distribution of this river-ice, the freezing of the water having rendered it sufficiently pure to remove all danger to health from its consumption.

So far we have been considering the effect on bacteria of freezing carried on under more or less natural conditions; but much interesting work of a more detailed character has been carried out with reference to the behaviour of particular varieties of micro-organisms when frozen under more or less artificial conditions.

Thus Dr. Prudden froze various bacteria in water at temperatures ranging from -1° C. to -10° C., and he found that different varieties were very differently affected by this treatment; that, for example, a bacillus originally obtained from water, and introduced in such numbers as represented by 800,000 individuals being present in every twenty drops, after four days' freezing had entirely disappeared, not one having survived. On the other hand, similar experiments in which the typhoid bacillus was used resulted in the latter not only enduring a freezing of four days' duration, but emerging triumphant after it had been carried on for more than 103 days!

In these experiments it should be borne in mind that, as the ice was frozen to a solid block or lump, there was no opportunity for the mechanical committal of the bacteria during freezing to the water beneath; all the bacteria present were imprisoned in the ice, and the fact that the typhoid bacteria were not destroyed by being frozen shows that they can withstand exposure to such low temperatures, although, as we have seen, the other variety of bacillus employed was destroyed.

Dr. Prudden, however, discovered an ingenious method by which even typhoid bacilli were compelled to succumb when frozen. In the course of his investigations he found that bacteria which had offered the stoutest resistance under freezing were extremely sensitive to this treatment if the process was carried on intermittently, or, in order words, if the temperature surrounding them was alternately lowered and raised.

In this manner the bacteria may be said to be subjected to a succession of cold shocks, instead of being permitted to remain in a continuously benumbed condition. The vitality of typhoid bacilli was put to the test under these circumstances, the freezing process being carried on over twenty-four hours, during which time, however, it was three times interrupted by the ice being thawed. The effect on the typhoid bacteria was striking in the extreme; from there being about 40,000 present in every twenty drops, representing the number originally put into the water, there were only ninety at the end of the twenty-four hours; and after a further period of three days, during which this treatment was repeated, not a single bacillus could be found. This signal surrender to scientific tactics forms a marked contrast to the stout resistance maintained for over 103 days under the ordinary methods of attack.