Rennet Paste
Rennet paste is probably the most ancient coagulant used to clot milk. Once the shepherds used to hang the abomasa of lambs full of milk to the fireplace in order to preserve them by drying and smoking. Every day when the milk to produce cheese was ready to be clotted they used to cut a small part of the dried stomachs, to dissolve it in some water and then to add to the cheese vat to curdle the milk. Even today in some places shepherds do the same.
Rennet paste now is produced by grinding the abomasa of kid goats and lambs (which are preserved under salt or dried) with their content of clotted milk and adding a certain amount of salt. Also calf rennet paste is available on the market, but it represents only a small share. It is important that the stomachs contain the milk swallowed by the animal because it contains the lipase released from the glands in the area of the glottis. When the young animals suckle the milk, the glands excrete the lipase, which is washed down by the milk and reaches the stomach. The rennet paste preparations are therefore used to produce piquant cheeses. This is the main difference with liquid rennet or rennet powder, which don't contain any lipase and therefore are not suitable to produce piquant cheeses unless also a lipase preparation is added to the milk.
Rennet paste contains chymosin (IUBMB N° 3.4.23.4) and pepsin (IUBMB N° 3.4.23.1), which are hydrolases acting on peptide bonds (aspartic peptidases), and moreover lipase (IUBMB 3.1.1.3), which is a hydrolase acting on triacylglycerol esters, releasing fatty acids and partial glycerides. For the characteristics of these enzymes please see the related sections.
When the dairyman wants to clot the milk, he weights the requested quantity of rennet paste, dissolves it in some water and filters it to eliminate the insoluble particles before adding the extract to the milk. As this procedure is time consuming, also soluble rennet pastes are available on the market. They have to be simply suspended in some water and directly added to the milk vat, as they have already been filtered by the producer.
As rennet paste is quite a rough preparation, in the past some concerns about its hygiene arose. However many different studies about its microbiology never confirmed these concerns. Probably the low pH (about 4.5), the high content of salt (more than 20%), and the low-water activity (less than 0.88) and the fact that it is left to ripen before been used are conditions sufficient to eliminate eventual harmful bacteria.
The use of rennet paste is limited to the production of piquant cheeses, and for some of them, the production protocols request specifically its use to clot the milk. In Italy, they are represented by Caciocavallo, Caciocavallo Silano, Fiore Sardo, Pecorino di
Filiano, Pecorino Romano, Pecorino Sardo, Pecorino Siciliano, Piacentinu di Enna, Provolone del Monaco, Provolone Valpadana, Vastedda della Valle del Belice.
