The CURDLING of MILK
|chemistry secrets to obtain the curd|
|The curdling of milk occurs in different ways with
the help of chemistry let's try to understand. The methods of curdling:
It may occur thanks to milk ferments that transform lactose in to milk acid, taking the milk to 4.6 ph, or for the addition of acid substances;
RENNET: it is, generally, the sweet curdling that occurs thanks to enzymes. The substance that in the milk gets the transformation is casein, a fundamental protein that in the milk is found in a colloidal solution. The casein is a conjugate protein formed (by beside C, H, O, N, S) also phosphorus (P) in the form of ester phosphoric acid of which a part is found inside every unit of a,b,k- "salificato" casein with calcium and magnesium ions, a part is in the form of tricalcium phospote (apatite) that joins together the units of a, b, k- casein as in the picture. The whole casein "micella" is called "phospho-caseinatum of calcium". The quantity of calcium ions regulates the aggregation's state of "micelle" and the sped of their flocculation: without calcium casein can't coagulate.
In normal conditions milk has a ph of about 6.5-6.7 and at this ph value the casein is without protons, it has a negative charge and therefore the casein micelles are relatively soluble, because they repel each others. In an acid ambient milk coagulates, because casein has its isoelectric point at 4.6 ph, that is at this ph value it has a quantity of positive charges equal to the quantity of negative charges and the positive part of each "micelle" is attracted by the negative part of the others, causing the formation of ionic bonds among the "micelle" working against the dipole-dipole bonds with water, so that the protein precipitates in the form of demineralized casein and in the solution remain soluble calcium salts.
TRADITIONAL CURDLING OF RENNET
The rennet (see above) is a substance extracted by the abomasus of the suckling calf and contains natural "proteolitici" enzymes that change the structure of casein, that "qelifies" with calcium ions whose presence is guaranteed the acidification of milk to use. The fundamental enzyme is the "chimasi" that splits a particolar peptiolic bond, between methionine and phenylanine that is found in the k-casein and that keeps a hydrophyle carbohydrate's fragment. After the removal of this fragmet, the regulation factor in the watery solution is missing and the "micelle", made unstable, in the presence calcium ions, tend to join, forming the curd called "calcium para- caseinatum". Unlike the curd obtained by acidification, this keeps the most part of tricalcium phospate.
above:the casein molecule.
below:the preparation phases of "presamica" curd.
phases of "presamica" curdling (according to Mrs Emilia from Abasse)- calf abomasus containing the curd natural enzymes.