Rennet

The system of manufacture and management of rennet will be throughly understood by the majority of expert chesse-makers; but there two points of importance which may not be so well known, but which deserve the closest attention and study. At the London Exhibition of 1886 prizes were offered for the best samples of rennet, and there were six competitors. The awards were made by the Society's chemist after careful analysis and experiment. One sample was found to be spiced, and another to be turbid and swarming with ferment-growth other than rennet—a by no means uncommon occurrence in home-made samples. I believe it is not yet known positively what the basis of rennet-action actually is, although the page 92 late Professor Arnold stated in a most definite manner that rennet contains a living germ, which grows and multiplies, and which constitutes its active agency. He claimed to have proved this by filtration, after which the rennet became inert, and by counting the number and size of the germs, which corresponded with the strength of the liquid. This assertion has been combated, and cannot be accepted as a true solution of the question. The samples referred to above were of the following strength :—

• No. 1. Full wood's Liquid : one part by volume coagulates 8,000 parts of milk.
• No. 2. Blumenthal Powder: one part by weight coagulates 73,780; parts of milk.
• No. 3. Hansen's Tablets: one part by weight coagulates 173,640 parts of milk.
• No. 4. Hansen's Extract: one part by volume coagulates 9,400 parts of milk.
• No. 5. Hall's Liquid : one part by volume coagulates 1,000 parts of milk.
• No. 6. Van Hassclt's Dutch Liquid: one part by volume coagulates 12,500 parts of milk.

No. 5 was a home-made rennet of which 16oz. were required to coagulate 100 gallons of milk, while only 1oz. 2dr. were required of No. 6. It was then pointed out by the judge that, as cheese-makers do not measure with extreme accuracy, they might give 2dr. more or less, in which case the result would be that the curd would come in thirty-two minutes in the one instance, or in forty-eight minutes in the other, in place of forty minutes, at which all the above samples were estimated to coagulate milk at 85° Fahr. This is suggestive of the fact that the stronger the rennet the greater care should be exercised in its use, and the greater the liability to err. For this reason water is generally added with extreme care, to minimise the liability to err in point of time of coagulation. The judge remarked that in his experiments he found weak extracts of rennet were more difficult to keep than strong extracts. At the same time, he believes that the strong extracts fail in use not because they are strong, but because cheese-makers are not sufficiently accurate and careful in their use, and next because manufacturers do not definitely state their power upon milk at a given temperature. I can thoroughly indorse these remarks, having some hundreds of records of the use of rennet of different makes, the majority of which do not correspond in the results they show with the very loose statements of their power which are printed by the makers. Blum en that's powder, which I have repeatedly used and found to be of high value, was shown by the chemist to weigh 0.555 gramme, or about 8½ grains per carefully measured spoonful, the spoon being furnished with the box of rennet A spoonful is said by the makers to be sufficient to coagulate 100lb of milk at 95° Fahr., but it actually coagulates 961b. This was a good result; and, as the powder is dissolved in water, one spoonful page 93 per 10oz., it can be regulated with the greatest nicety. The same cannot be said of the tablets, which are now known wherever cheese-making is conducted; for, from their great strength and defined form, they cannot be so well controlled, and in fact they coagulated in the competition considerably more milk than they were stated to do. They are very convenient in the hands of a thoroughly-competent man, who is practised in their use; although, when dissolved, the solution from the large tablets would be far stronger than that obtained by the use of the powder, unless they were broken up and weighed upon a chemical balance with the greatest accuracy. The cost of the powder and of the tablets are in practice found to be very similar. The powder was selected for the Society's prize.

When a calculation is made to test the strength of a particular rennet, more especially on the Continent, where the litre is used, a standard of 1 in 10,000 is adopted : that is, one part of rennet is sufficiently powerful to coagulate 10,000 parts of milk at 95° in forty minutes, the French system of weights and measures enabling the experimenter to adopt this method with little trouble. Upon the basis of these figures, my friend Dr. Jacopo Rava, of the Scale Stazione Experimentale at Lodi, in Italy, has constructed an algebraical formula, which gives the golden number of 40,000 as a key to ascertain the strength of a rennet, the quantity of milk which can be coagulated, the quantity of rennet which must be used, and the time it will be employed under certain given conditions. These formulæ I have carefully worked out and amplified, and I believe they will be found to be of considerable service in the cheese-making industry. First, in order to find the strength of a given sample of rennet, the number of litres of milk to be used (a gallon is about 4½ litres) must be multiplied by 40,000, and the result divided by the number of centimètres of rennet and the minutes occupied in coagulation. Thus, supposing 48 gallons (216 litres) of milk is coagulated in sixty minutes by the use of 18 cubic centimètres of rennet, we can ascertain the strength of this rennet as follows :—

(40,000 x 216) ÷ (18 x 60) = 8,000.

In order to ascertain how much milk can be converted into curd with a given quantity of rennet of a known quantity, and in a fixed time, it is necessary to multiply together the quantity of rennet by the time to be occupied in coagulation and by the strength of the rennet, and afterwards to divide the sum obtained by 40,000. Let us suppose that we wish to ascertain how much milk can be coagulated in sixty minutes with 18 centimètres of rennet of a strength of 8,000. We get the following formula :—

(18 x 60 x 8,000) ÷ 40,000 = 216 litres, or 48 gallons.

When it is necessary to ascertain the quantity of rennet necessary for a given quantity of milk, the number 40,000 is multiplied by the number of litres of milk, and the result divided by the sum obtained page 94 by multiplying the number of minutes the curd is setting and the strength of the rennet. Thus,—

(40,000 x 216) ÷ (60 x 8,000) = 18 cubic centimètres.

To find the time which would be employed in coagulating a given quantity of milk with a given quantity of rennet of known strength, it is necessary to multiply the number of litres of milk (4½ to the gallon) by 40,000, and to divide the result by the strength of the rennet multiplied by the number of centimètres. Thus,—

(216 x 40,000) ÷ (8,000 x 18) = 60 minutes.

It must not be forgotten that in every case the temperature of the milk is alike 95° Fahr., but there is no greater difficulty in constructing similar formulae suitable to other temperatures now that the principle is shown. With regard to powdered rennet, a graining must be accurately weighed, and dissolved in 100 times its weigh. Each cubic centimètre of the solution must be multiplied by 10 to give the equivalent starting-point of the liquid rennet. Thus, taking Dr. Blumenthal's rennet of a strength of 78,780, which for simplicity's sake may be called 80,000, we have a rennet ten times as strong as that named above. The formula will therefore be as follows :—

(40,000 x 216) (80,000 x 60) = 1.8 cubic centimètres.

This multiplied by 10 = 18 cubic centimètres, the quantity necessary to use under the same circumstances as in the above case.