Rennet is an aspartic protease first discovered in the stomach of unweaned calves. It can specifically cut the peptide bond between Phe105-Met106 of κ-casein in milk, destroy casein micelles and coagulate milk. The coagulation ability and protein hydrolysis ability of rennet make it a key enzyme in cheese production to form texture and special flavor, and it is widely used in the production of cheese and yogurt.
Animal-derived rennet: Rennet was first derived from the fourth gastric mucosa of calves, and then rennet was extracted from the stomachs of other animals. However, due to the unstable source of animal-derived rennet, and due to considerations of race, belief, etc., researchers have turned their research direction to microbial and plant-derived rennet.
Plant-derived rennet: Many plants contain a wide range of proteases that can coagulate milk. Currently, the following are the most studied, including papain, fig protease, bromelain, ginger protease, albizin, artichoke protease, etc.
Microbial rennet: Microorganisms are the most promising source of rennet because of their short growth cycle, high yield, less time and space restrictions, low production cost, convenient extraction, and high economic benefits. Currently, more than 40 kinds of microorganisms have been found to produce rennet with a certain activity, mainly fungi, actinomycetes, bacteria, etc.
The solubility of rennet is affected by pH value, temperature and solution ionic strength. It is soluble in NaCl solution with a pH value of 5.5 and a concentration of 1 mol/L. The crystalline rennet is relatively stable at 25°C, and its solubility increases with the increase of ionic strength. The isoelectric point of rennet is pH 4.5, and it is most stable at pH 5.3~6.3, but its activity will decrease due to self-degradation at pH 3~4. Irreversible conformational changes will occur under alkaline conditions, resulting in reduced activity.
Addition amount
Usually, 20-40 ml of rennet solution is added to 100 kg of raw milk. The coagulation time is inversely proportional to the amount of rennet. If the amount is doubled, the coagulation time will be halved.
The discharge of whey is not affected by the amount of rennet. Experiments show that the amount of rennet has nothing to do with the water content and pH quality of the cheese after 24 hours. However, the amount affects the maturation of cheese. If the amount is large, the decomposition of protein during maturation will be accelerated; at the same time, because rennet decomposes protein to produce bitter peptides, a large amount will also increase the bitterness of the cheese.
The Arima method is used to determine the curd strength. Take 5 ml of 100 g/L skim milk, keep it at 35 °C for 5 min, add 0.5 ml of 1 g/L papain solution, mix it quickly and evenly, and accurately measure the time T (s) from adding the enzyme solution to coagulation. The amount of enzyme that coagulates 1 ml of 100 g/L skim milk in 40 min is defined as one Soxhlet unit. Curd strength = (2400/T) ╳(5/0.5)╳ D (dilution multiple).
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