Outlines of Dairy Bacteriology, 8th edition / A Concise Manual for the Use of Students in Dairying

uality of milk supplies. It is therefore especially desirable from the milk-dealer's point of view that these changes should be held in abeyance as much as possible. Then too, the possibi

o taste, odor and appearance, unfortunately but little attention is paid to the possibility of infection by disease germs. The methods of control which are applicable to general milk supplies are based on the following foundations: (1) the exclusion of all bacterial

o be rejected by him as unfit for food, no further attention was likely to be given to its character. At present, however, much more emphasis is being given to the quality of milk, especially as to its germ content; and the milk dealer is beginning to recognize the necessity of a greater degree of cont

ed in one or both directions. The influence of cleanliness in diminishing the bacterial life in milk and that of low temperatures in repressing the growth of those forms which inevitably gain access has been fully dealt with in preceding chapters. It is of course not practicable to take all of these precautions to which reference has been made in the securing of large s

that all milk sold in the city shall be chilled down to 45° F. immediate

also be used with advantage in determining the sanitary character of milk. This test reveals the presence of bacteria usually associated with dirt and permits of the recognition of milks that

t regular intervals, and the herd inspected frequently by competent veterinarians. The methods of control inaugurated as to clean milking and subsequent handling are such as to insure the diminution of the bacteria to the lowest possible point. The bacterial limit set by the Pediatric Society of Philadelphia is 10,000 organisms per cc. Often it is possible to improve very materially on this standard and not infrequently is the supply produced where it contains only a few thousand organisms per cc. Where such a degree of care is exercised, naturally a considerably higher p

ON OF MILK

the milk, and preventing the rapid growth of those that do find their way therein, yet for general municipal purposes, any practical method o

d after they have once gained access is (1) by the use of

is toxic enough to destroy or inhibit the growth of bacterial life exerts a prejudicial effect on the tissues of the body. The use of chemicals, such as carbolic acid, mercury salts and mineral acids, that are able to entirely de

used may be grou

th to form inert substances. Thus bicarbonate of soda neutralizes the aci

se are nearly always sold to the milk handler, under some proprietary name, at prices greatly in excess of what the crude chemicals could be bought for in the open market. Formaldehyde has been

l. The sale of these preparations for use in milk finds its only outlet with those dairymen who are anxious to escape the exactions that must be met by all who attempt to handle milk in the best possible manner. Farrington has suggested a simple means for the detection of preservalin (boracic acid).[12

y, use of a vacuum, and increased pressure.[129] Condensation has long been used with great success, but in this process the nature of the milk is materially changed. The keeping quality in condensed milk often depends upon the action of another principle, viz., the inhibition of bacterial growth by reason of the concentration of the medium. This condit

k so that it could be shipped long distances by freezing the product, which in the form of milk-ice could be held for an indefinite period without change.[130] A modification of this process known as Casse's system has been in use more or less extensively in Copenhagen and in several places in Germany. This consists of adding a small block of milk

of normal milk showing the fat

immemorial. Heat may be used at different temperatures, and when so applied exerts a varying effect, depending upon temperatur

F.) and below the thermal death-point (130°-140° F.) will

eby stops all changes. This temperature varies, however, with the conditio

but when milk is so heated, its physical appearance is changed.[132] The methods of heating most satisfactorily used are known as s

g.

eous distribution of fat-globules. The physical change noted in comparis

o the action of heat, a number of changes in its

al milk, the butter fat for the most part is massed in microscopic clots as (Fig. 22). When exposed to 140° F. or above for ten minutes these fat-globule clots break down, and the globules become homogeneously distrib

nous elements in the milk, particularly in the albumen. Thoerner[133] has pointed out the coincidence that exists between the appearance of a cooked taste and the loss of certain gases that are expelled by heating. He finds that the milk heated in closed vessels from which the gas cannot escape has a much less pronounced cooked flavor than if

esults obtained by different investigators are quite contradictory, although the preponderance of evidence seems to be in favor of the view that heating does impair the digestibility of milk, especially if the temperature attains the sterilizing point.[135] It has been observed that there is a noteworthy increase in amount of rickets,[136] sc

are readily killed when heated above the thermal death-point of the developing cell. The destruction of the lactic forms leaves the spore-bearing types possessors of the field

is dependent upon the presence of these salts, their absence in heated milks greatly retards the action of rennet. This renders it difficult

, it can be rendered practically germ-free. Such methods are employed where it is designed to keep milk sweet for a long period of time. The treatment of milk by sterilization has not met with any general favor in this country, although it has been more widely introduced abroad. In most cases the process is carried out after the milk is bottled; and considerable ingenuity has been exercised in the construction of devices which will permit of th

ality of milk was not generally recognized until a few years ago; but the method is now growing rapidly in favor as a means of preserving milk for commercial purposes. The method does not destroy all germ-life in milk; it affects only those organisms that are in a growing, vegetative condition; but if the milk is quickly cooled,

he apparatus necessary for this purpose is not nearly so expensive as that used in sterilizing, a factor of importance when other advantages are considered. In th

is permitted, yet there are certain conditions which should be observed, and these, in a sense, fix the limi

ith ordinary raw milk. In pasteurization then, care must be taken not to exceed the temperature at which a permanently cooked flavor is developed. As previously observed, this point varies with the period of exposure. A momentary exposur

o a temperature sufficiently high to cause the fat-globule clusters to disintegrate (see Figs. 22 and 23), the globules do not rise to the surface as readily as before and the cream line remains indistinct. Where the exposure is made for a considerable period o

me reason as given under (2) cream heated above these temperatures is reduced in apparent thickness and appears to contain less butter-fat. Of course the pasteurizing process does not change the fat content, but its "body" i

ocess. The improvement in keeping quality depends upon the temperature and time of exposure, but fully as much also on the way in which the pasteurized product is handled after heating. The lowest temperature which can be used with success to kill the active, vegetative bacteria

n bacterial life in milk are of importance as indicating the foundation for the selection of the

of milk heated at di

acteria per

55° C. 60° C

22° F. 131° F. 140

-- 1,260,000 798,0

665,000 262,400 2

00 1,100,000 260,00

000 -- 87,360 --

ature did not materially diminish the number of organisms present, indicating that those forms remaining were in a spore or resistant condition. It was noted, however, that

any organisms of a disease nature that might find their way into the same. Fortunately most of the bacteria capable of thriving in milk before or after it is dr

e bacillus. On account of its more or less frequent occurrence in milk and its rep

lty in determining exactly when the organism is killed (due to its failure to grow readily on artificial media), and part from the lack of uniform conditions of exposure. The standards that previous

] If the conditions of heating are such that the surface of the milk is exposed to the air, the resistance of bacteria is greatly increased. When heated in open vessels Smith found that the tubercle organism was not killed in some cases where the exposure was made for at least an hour. Russell and Hastings[141] have

pplies. The other system has been developed primarily from the commercial point of view where a large amount of milk must be treated in the minimum time. In the first method the milk is heated for a longer period of time, about fifteen minutes at a relatively low temper

g is prolonged, the apparatus employed is built on the principle of a tank or reservoir in w

f a continuous-flow apparatus is much greater than a machine that operates on the intermittent principle; hence, for large supplies, as in city dis

ption are so materially different from where the process is used in butter-making that the type of machinery for eac

ited capacity desig

nt capacity to pasteuriz

nt can best be performed in the bottle itself, thereby lessening the danger of infection. Several different types of pasteurizers are on the market; but special apparatus is by no means necessary for the purpose. The proces

ns indicate the differe

only fr

cans are used at the same time, an inverted bowl will equalize the level. Set these in a flat-bottomed tin pail and fill with warm water t

over with a cloth or tin cover, and allow the whole to stand for half an hour. In the preparation of milk for children

home-made p

as rapidly as possible without danger

. Apparatus constructed on the reservoir or tank principle permits of the retention of the milk for any desired period of time. Therefore, a lower temperature can be employed in the treatme

un cans and immersed in water heated by steam. Ordinary tanks surrounded with water spaces can also be used successfully. The Boyd crea

narrow vat surrounded by a steam-heated water chamber. Both the milk and the wat

Pott's pa

s has a central milk chamber that is surrounded with an outer shell containing hot water. The whole machine

lk is heated momentarily as it flows through the apparatus. Most of these were primarily introduced for the treatment of cream for butter-making purposes, but they are frequent

he milk is spread out in a thin sheet, and is treated by passing it over

ossible to prevent the "scalding on" of

d over which the milk flows, while the opposite surface is heated with hot water or steam. Monrad, Lefel

n a thin sheet between two heated surfaces, thereby facilitating the heating process. In the Farrin

thin stream and then is carried back over the incoming cold milk so that the heated liquid is partially cooled by the inflowing fresh milk. In ma

recently introduced milk with that which has been partially heated. The pattern for this reservoir type is Fjord's heater, in which the milk is stirred by a stirrer. This apparatus was originally designed as

sical requirements that should be kept in mind. The greatest difficulty is the shortened period of exposure. The period which the milk is actually heated is often not more than a minute or so. Another serious defect is the inability to heat all of the milk for a uniform period of time. At best, the milk is exposed for an extremely

ow to full-cream milk, continuing at the same rate. Note the exact time of change and also when first evidence of milkiness begins to appear at outflow. If samples are taken from first appearance of milky condition a

while about two-thirds of the milk passed the machine in 40-50 seconds. This manifest variation in the rate of flow of the milk through the machine is undoubtedly the reason why the results of this

he number of organisms originally present in the same. Naturally, if mixed milk brought to a creamery is pasteurized, the num

below 1,000 bacteria per cc., and the average of twenty-five samples contained 6,140 bacteria per cc. In cream the germ content was higher, averaging about 25,000 bacteria per cc. This milk was taken from the general creamery supply, which was high in organisms, containing on an average 3,675,000 bacteria per cc. De Schweinitz[145] has reported the germ content of

lk. Black square represents bacteria of raw milk; sma

bacteria found in milk and cream when treated in machinery of this class almost always sho

's Continuous

he lower temperatures. At 158° F. the average of fourteen tests gave 15,300 bacteria per cc., with a maximum to minimum range from 62,790 to 120. Twenty-five examinations at 176° F. showed an average of only 117, with a range fr

sion. In 66 samples from one hundred thousand to one million organisms per cc. were found,

an average of 21 tests showed 12,350 bacteria remaining in the milk when the milk was pasteurized f

who found that it was necessary to heat the milk as high as 176° to 185° F.,

wing results: in some cases as many as 40 per cent. of the bacteria su

le, yet, in order to secure the best results, certain conditions must be

r clean conditions, it is likely to be rich in the spore-bearing bacteria. Old milk, or milk that has not b

mine the actual number of bacterial spores that are able to resist the hea

ncreases in a general way with an increase in age or temperature, the amount of acid present may be taken as an approximate index of the suitability of the milk for pasteurizing purposes. Biological tes

g. Where milk is pasteurized under general creamery conditions, none should be used containing more than 0.2 per cent acidity.

g.

ges in pasteurizing, and the rel

indicating rapidity of development. The solid black line shows temperature of milk during the process.

oint of any organism depends not alone on the temperature used, but on the period of exposure. With the lower limits given, 140° F., it is necessary to e

immediately chilled in order to prevent the germination of the resistant spores,

are of interest as showing the influence

and at same temperature, while another heated set was kept in a refrigerator. The unheated cultures at room temperature showed evidence of growth in thirty trials in an average of 26 ho

passing the milk through two separate coolers that are constructed in the same general way as the heater. With the first cooler, cold running water is employed, the temperature ofte

ound of water at the same temperature, if we disregard radiation, would require as much heat as would suffice to raise 142 pounds of water one degree F., or one pound of water 142° F. The absorptive capacity of milk for heat (specific heat)

ain pulp caps are best, and these should be thoroughly sterilized in steam before using. The bottling can best be done in a commercial bottling

before (A) and after (B) treatment with viscogen

ntinuous stream, the consistency of the liquid is diminished, notwithstanding the fact that the fat-content remains unchanged. Babcock and the writer[152] devised the following "cure" for this apparent defect. If a strong solution of cane sugar is added to freshly slacked lime and the mixture allowed to stan

ms can easily be determined by

h or so. Then incline piece of glass at such an angle as to cause the cream to flow down surface of glass. The cream, having the heavier body or viscosity, will move more slowly.

TNO

nts per quart is usual

pment, especially as to refrigeration; also as to the care taken on the farms. The use of

Journ. Amer. Chem.

. 58, West Va. E

h Zeit., 1

id., 189

Milch Zeit., 18

er, Chem. Ze

Chemistry of D

resumé of the work on this subject in connection with rather extensive experime

er to give them proper firmness. Marasmus is a condition in which the inge

Arch. f. Hyg.,

Journ. of Expt. M

ings, 17 Rept. Wis. Ex

d Hastings, 21 R

d Hastings, 18 R

, Bull. 44, W

Wis. Expt. Stat. R

Wis. Expt. Stat. R

itz, Nat. Med. R

Bull. 182, N. Y. (Geneva

chkunde und Milch

xpt. Stat. Rept

, Thesis, Univ

ich. Expt. Stat.,

n, Landw. Versuc

sell, Bull. 54, Wis.