SKIMMINGS.

New Zealand Mail, Issue 1291, 26 November 1896, Page 5

SKIMMINGS.

For cooling milk on farms and small dairies, Mr A. N. Pearson, the Government Agricultural Chemist, writes, in reply to a correspondent of The Leader, 24th October, that there are probably no appliances more generally suitable than the Lawrence cooler and the Laval cooler. The construction and use of the, Lawrence cooler is no doubt familiar to most of those connected with dairying. The milk flows down the outsides of two sheets of corrugated metal, between which there flows in an upward direction a stream of cooling liquid. The Laval cooler is the same in principle but different in form, the milk trickling over and under a series of hollow metal discs, like saucers placed one on the top of another, the cooling liquid passing upward slowly inside these saucer-like discs, entering at the lowest one and flowing out of a waste pipe at the top. If there be a sufficient natural supply of water cold enough for the purpose no other cooling liquid need be run through the cooler. The milk, however, is best cooled to about 55deg Fahr., and as there is but rarely a natural supply of water of this temperature, it will generally be found necessary to supplement the action of the natural by some artificially cooled liquid. The best material for artificial cooling is undoubtedly ice; lib of ice will reduce the temperature of 1 gallon of ixiik by about 17deg Fahr. If the produce from 1 gallon of uncooled milk were worth only 3d, and that from 2 gallons of milk cooled by lib of ice were worth 4d, it is evident that or even £d per lb would not be too much to pay for ice used for cooling mills. As ice can be made by large refrigerating machines at probably less than 1& per cwt, and can be easily conveyed with but little loss in large blocka packed in straw or fine shavings, it. could, no doubt, be delivered at even |d per lb on many farms where afe present it is considered unobtainable. Where, however, ice cannot, be obtained in this way, it might be possible ta make ice on the farm itself by means oi the small portable ice machine much in use at one time, and known as Carre's ■intermittent ammonia machine. The smallest of these machines is capable of making 41b of ice in an hour, at a eo' t reckoned in England at per lb. r , J machines are very simple, and tb- ."' ~ - > re is scarcely anything about them *" ". , , « i -Uau can get out or order.

_ Suppose, however, ice car x be neifcher imported on to nor made flfc the f it will then be best to ~ ,• ~ ~ ~ , c , obtain a cooling liquid by means or w , . • i rnnm „ to ~*, . J . . aat is known as a « freezing mixturr „ Tfae ch freezing mixture ducible afc * is probably t' J 8 following :—3lb of amnionic chic rMe (aal ammoniac)| 3 , b ol potash nifcrate ( sa lt p etre) and 1 gallon of water# Thig wou]d reduCQ th« tern perafcuve of i gaj | on of miJk . about 40deg Fah _ £f its fu]l cooJing elle 'ot were conveyed to the milk. In ur >ing freezing mixtures, the salts which must be quite dry and ground to a coarse powder—aro weighed out put into tiiH measured quantity of water and si it-red slowly aboufe until completely dissolved; the crystals in going into the liquid form absorb heat and form an intensely cold solution. This can be used for cooling purposes, and afterwards, by evaporation, the crystals can be re-obtained for use again. Hence with care there need be only a trifling loss, and the same portion <>i cooling salts can be

used for an indefinite period. When cooling milk by means of freezing mixtures, it will be found most economical to cool the milk first as much as possible with water only, and then to complete the cooling by means of the cool solution. Thus to begin with, the milk in the bucket as it comes from the cow will be about 95deg. ; and by immediately running it over the cooler, through which cold water was flowing, it might be cooled down to 75deg. By then passing it a second time over the cooler, through which the cold solution was flowing, it could be cooled from 75deg. down to 55deg. Thus only half the cold solution would be needed that would have to be used if the whole of the cooling were done by its means. In this way 3lb of sal ammonia, 31b of saltpetre in 1 gallon of water would then cool 2 gallons of milk to the desired temperature. At this rate, to cool 50 gallons, 751 b of each salt would be needed; and as it would be advis able to have two lots, one to be evaporating and drying while the other was in use, 1501 b of each would be required, or say Sal ammoniac is sold at 568 per cwt, and saltpetre at 345, the whole would therefore cost L 6 15s in Melbourne. The salts, which as before stated, must be powdered and quite dry, are best dissolved in a cask or wooden tub, covered with a lid at the top, provided at the bottom with a tap, and wrapped around and underneath with 6or 8 inches of straw or thatching to prevent loss of cold. The evaporation of the solution after it has been used should be done in a shallow pan or tray. A number of kerosene tins cut in half lengthwise, and provided with handles, will do if nothing else can be obtained. Then place over a slow fire, so that the water will evaporate gently without boiling, loss of salt by splashing or spirting being avoided. When the material is evaporated nearly to dryness, the hot, somewhat pasty "mass should be dug out of the pan and spread out on clean wood or iron to finish drying in the sun or in some warm dry place. When perfectly dry it should be powdered up ready for use. In recommending the use of the Lawrence or the Laval cooler, this is done because they are the most easily worked of the coolers commonly on the market, and because they completely utilise the cold in the cooling liquid employed. There is no way of completely utilising the cold so simple as that of having the cooling liquid to flow in one direction and the liquid to be cooled to flow over it in the opposite direction. This will be understood from the fact that a cooling solution will impart cold to the milk only so long as there is a difference of temperature between them. Supposing a can of milk at 75deg were dipped into a can of cooling solution at 35deg, and that eventually the milk were cooled down to 55deg and the cool solution were warmed up to 55 deg, then there would be no further exchange of temperature, the cooling cf the milk would have ceased. In that way, however, only half the cooling power of the solution would have been used. _ Yet the cooling power that was left in it could not be employed for cooling another lot of milk down to 55deg ; for suppose another can of milk at 75deg to be placed in this solution at 55deg, then after a little time the milk will have been cooled down to 65deg but the solution will have been warmed up to G">, and no further cooling effect can take place. The only way to utilise all the cooling power of the cold notation by such means would be to h*ve a series of cans of cold solution, and to dip the milk first into one, the coolin" power of which was nearly expended, then into another a little colder, then into a third still colder, finally finishing up with the freshly prepared and coldest solution. But what would be thus accomplished laboriously, the Lawrence cooler and others constructed on the same principle accomplish quite simply and automati-