Module 7. Rennet preparation and properties

Lesson 14


14.1 Introduction

Rennet is a complex of enzymes produced in any mammalian stomach to digest the mother's milk. Rennet contains many enzymes, including a proteolytic enzyme (protease) that coagulates the milk, causing it to separate into curd and whey. The active enzyme in rennet is called chymosin or rennin but there are also other important enzymes in it, e.g. pepsin and lipase. There are non-animal sources for rennet also that are suitable for vegetarian consumption.

Natural calf rennet is extracted from the inner mucosa of the fourth stomach chamber (the abomasum) of young, unweaned calves. If rennet is extracted from older calves (grass-fed or grain-fed) the rennet contains less or no chymosin but a high level of pepsin and can only be used for special types of milk and cheeses. As each ruminant produces a special kind of rennet to digest the milk of its own species, there are milk-specific rennets available, such as kid goat rennet for goat's milk and lamb rennet for sheep's milk.

14.2 Enzymes in Rennet

Rennin is an enzyme which can function very powerfully as a clotting agent at pH 6.2 to 6.4. The term chymosin is used in place of rennin to avoid confusion with another enzyme renin, which can be extracted from the kidney. The ratio of clotting to proteolytic power of chymosin/rennin is very high. Conversely, the other enzyme pepsin, functions best at high acidities (pH 1.7-2.3) and the ratio of clotting to proteolytic power is lower. All proteolytic enzymes can clot milk but the specific value of rennet in cheesemaking is that it gives rapid clotting without much proteolysis. Proteolysis by enzymes like pepsin, papain etc. can lead to bitterness in cheese.

The rennet extract from the young milk-fed calf contains 88-94% rennin and from 6-12% pepsin, while in extracts from the older fodder eating bovine, it is almost reverse i.e. 90-94% pepsin and only 6-10% rennin. So, there is variable amount of pepsin in rennets depending on the age and food of the calf from which the rennet is obtained.

14.3 Preparation of Rennet

14.3.1 Traditional method

Dried and cleaned stomachs of young calves are sliced into small pieces and then put into saltwater or whey, together with some vinegar or wine to lower the pH of the solution. After some time (overnight or several days), the solution is filtered. The crude rennet that remains in the filtered solution can then be used to coagulate milk. The enzyme present at this stage is called prorennin which on acidification, becomes rennin with an increased clotting ability. About one gram of this solution can normally coagulate 2 to 4 litres of milk. This method is still used by some traditional cheese-makers, e.g. in Switzerland, France, Romania, Italy, Sweden, United Kingdom and Alp-Sennereien in Austria.

14.3.2 Modern method

The commercial enzyme is prepared from the fourth stomach (abomasums) of the calf, known as the vell. The lining of the stomach is washed, dried, cut into small pieces and macerated in water containing about 4% boric acid at 30°C for about 5 days. Alternately, a brine extract at 15-20°C can be prepared. A common method is to dry the vells by inflation and afterwards cut them into strips and extract with sodium chloride solution (up to10%) for few days. Crude rennet extract contains active rennin as well as inactive precursor (prorennin). Addition of acid to the extract facilitates conversion of the prorennin to rennin and allows the extract to reach maximum activity. Finally, the activated rennet is standardized with respect to activity, salt concentration, pH and colour. Liquid rennet is usually preserved by a high salt content (14-20%) and by the addition of preservatives such as sodium benzoate and propylene glycol. Rennet powders are prepared from precipitates obtained by acidifying activated extracts or by saturating with sodium chloride, or both.

In one kg of rennet extract, there are about 0.7 g of active enzymes. The rest is water and salt and sodium benzoate. Typically, one kg of cheese contains about 0.0003 g of rennet enzymes.

14.4 Properties of Rennin

Rennin is a sulphur-containing protein. One part can clot about 50, 00,000 parts of milk. It is easily destroyed by heat, many chemical substances and many physical conditions. The isoelectric point of rennin is about 4.55 and it is easily destroyed by factors such as heat and high pressures. It is also digested by proteolytic enzymes and is readily adsorbed. It is very sensitive to alkali, and heating at 70°C at pH 6.8-7.0 will destroy it in 14 min.

Optimum pH for clotting of milk is 5.4 and the proteolytic power of this enzyme virtually disappears at pH 4.5.

Last modified: Thursday, 20 September 2012, 8:54 AM