« AnteriorContinuar »
isolating the different micro-organisms, several culture media are employed so that all kinds may develop. We will not, however, treat of such analyses, as they do not usually occur in practice. The questions here arising usually tend only in one direction. In the analysis of water for brewery purposes, information is generally sought for as to the micro-organisms of the water which are detrimental to the working of the brewery; the noninjurious forms are of no importance in practice, and are therefore left out of consideration. The culture medium employed in the brewery, i.e., wort and beer, is used for such investigations. This simple principle had to be insisted on at one time by Hansen, as it was neglected by many workers.
The Hygienic Analysis of Water (after Koch) is performed with meat-extract peptone gelatine as culture medium ; a certain quantity of the water is distributed in the latter and the number of colonies developed from it is determined. In analogy with this an air analysis is sometimes carried out as well by drawing the air over culture gelatine.
The object of these methods is to develop as many germs as possible. They are, therefore, also used to test the efficiency of a filter by subjecting a certain quantity of water to plate culture before and after filtering, and afterwards comparing the number of germs developed in the two cases.
Water Analysis for Brewery Purposes (after Hansen). - A technical biological analysis of water for industrial purposes is made by sowing a certain quantity of the water in sterile wort, must, etc. An example of such an analysis of water for brewery purposes as carried out by Hansen may be described here. The questions to be answered are the following: How does the water behave towards wort and beer? How rich is it in such micro-organisms as can
develop in these liquids, and are there among them such species as can cause dangerous disturbances in practical operations ?
From the analyses given below will be seen the differences which results may exhibit according as the one or the other of the above-mentioned methods is used. Hansen found by his analyses the following numbers : While cultures in beer always gave O growths and a simultaneous series of experiments with wort 0, 0, 6:6, 3 and 9 growths per 1 c.c. of water, he found when Koch's meat-extract peptone gelatine was used under the same conditions and with samples of the same water, 100, 222, 1,000, 750 and 1,500 growths per 1 c.c. of water. This shows that the Koch method is inapplicable to such brewery analyses.
The procedure is therefore as follows: When, e.g., the tap water in a brewery is to be analysed one begins by carefully cleaning the taps and tubing of the water supply, using all precautionary measures. The tap is then opened and the water allowed to run for some time, e.g., one hour, before samples are taken. This ensures the washing out of the piping. The difficulty here, as in all biological analyses, consists in getting an average sample. If the water sample is not for transmission, but is to be analysed on the spot, sterile Chamberland flasks can be used for this purpose; if, however, the water sample is to be despatched, sterile bottles with glass stoppers are employed, the sample being packed in ice.
After the sample of water has been well shaken up, a small quantity is carefully withdrawn by means of a sterile pipette. The water is inoculated into sterilised wort and sterilised beer. In order to observe with greater ease the development of organisms that may be present it is best to employ a perfectly clear wort without sediment. Freudenreich flasks are the most suitable ones to use, as the number to be used is large, and larger flasks would take up too much room.
It is obvious that the power of preventing the growth of many organisms which wort and beer possess in virtue of the constituents derived from hops, and acid, etc., fails if too great a quantity of the water to be analysed is added. Holm has determined exactly the quantity of water which can be added to lager beer wort (about 14 per cent. Balling) before this happens. According to this author 15 c.c. of wort can be treated with $ c.c. of water, and 15 c.c. of lager beer with c.c. of water, before the resisting power fails.
In many cases it would be an error to take so much water, since it may contain so many germs that the inoculation would give rise to too many growths. The water sample should on this account be diluted with a certain quantity of sterile water (or wort) or it may be added in smaller quantity to each flask. It is impossible to give a definite rule for this. In each case it is advisable to make a preliminary test in order to determine approximately the number of germs. A reliable result is usually obtained when one drop of the water is placed in each of 100 flasks with wort; these flasks are then set away at 25° C. The infecting of beer flasks can be dispensed with since Holm's investigations have shown that in his analyses organisms never appeared which were only capable of development in beer. Any growth which developed in the beer flasks was derived from such species as might easily grow in wort. In sowing a drop in each of 100 Freudenreich flasks containing wort, 5 c.c. of water in all are used, and in most cases this gives rise to not more than one growth in one flask.
If flasks inoculated in this way have stood for one week at 25° C. and during this time no development has taken
place, then there was, in the water sowed, no germ capable of development under the conditions of working. If such growths are present in some of the flasks, the contents of the latter are investigated macroscopically and microscopically, the number of growths noted and the quantity per 1 c.c. of the water calculated.
It is of some moment in practical evaluation of the water to take note of the time after sowing at which development, especially of bacteria, takes place in the wort. For it is obvious that when they develop only after four or five days they must have been so feeble as to be capable of development with great difficulty or not at all under practical conditions. In the laboratory, conditions are far more favourable for development than in practice, since the rivalry with the yeast (and the low temperature) is wanting. The result of a water analysis will therefore always be such that rather more germs are found than would have reached development under practical conditions in spite of the attempt to copy these conditions as far as possible.
Wichmann lays special stress on the importance, in evaluating the water, of noting the time when the “destruction ” of the liquid under test takes place. He proceeds by adding to each of four flasks charged with 10 c.c. of wort, 1 c.c., C.C., } c.c., and c.c. respectively of the water to be analysed. These four flasks are numbered 1, 2, 3 and 4. The destructive power of a water is taken as equal to 100, if all four flasks exhibit development after the lapse of a day; this number is got by multiplying the numbers of the flasks by certain factors and adding the four products. If development takes place in the flasks after one day this factor is 10, after two days 8, after three days 6, after four days 4, and after five days 2. Thus if all four flasks are turbid after twenty-four hours, the destructive power is: 10 x 1 + 10 × 2 + 10 x 3 + 10 × 4 = 100. If flask No. 1 shows development after two days, No. 2 after three, No. 3 after four, and No. 4 after five days, the destructive power is 1 x 8 + 2 x 6 + 3 x 4 + 4 x 2 = 40. From this it may be seen that Wichmann adds more water to the wort than Hansen and Holm in their analyses found to be advisable.
Schwackhöfer's Standard of Fitness of a Brewery Water.—Schwackhöfer has proposed the following scale as a standard of the fitness of a water for brewing purposes, twenty-five flasks being each infected with one drop of water. In those cases in which certain micro-organisms develop neither in wort nor in beer, he describes the water as specially good. If development takes place in 10 per cent. of the wort flasks at the most, but not in the beer flasks, the water is good ; if development takes place in 50 per cent. of the wort flasks and in none of the beer flasks the water is fit for use; if micro-organisms are present in more than 50 per cent. of the wort flasks, and in at most 19 per cent. of the beer flasks, the water is only to be employed in cases of necessity, and finally if the percentage of flasks of both categories exhibiting growths is higher than that above mentioned, the water is unfit for brewery purposes. It must be remembered here that chemical analysis has been entirely left out of account.
Holm's Results.—In Holm's analyses of water from the Carlsberg Breweries, bacteria, moulds and yeast-like cells (Torula, Mycoderma) appear in the wort and beer, but no species of Saccharomyces. The presence of the latter is nevertheless not precluded, but their appearance in the water is at any rate rare. The species of moulds were especially numerous, not only in wort but also in beer; the same remark applies also as regards the number of growths. Bacteria were found along with these in the wort, whereas they appeared but seldom in beer. Yeast-like cells were rarely observed. Among