Rennet: Definition

The name rennet should be reserved only for coagulants derived from ruminant animals and plants, according to the tradition. The coagulants obtained from microorganisms are correctly named “microbial coagulants,” while chymosin obtained from GMO is called “fermentation chymosin” or “recombinant chymosin”

Casein micelle

Figure Casein micelle.

Contents of casein micelle

Figure Contents of casein micelle.

Until about 1965, the milk for cheese production was curdled only by rennet. In those years, due to an increasing request of rennet and to a shortage in the raw material (calf abomasum), the Food and Agriculture Organization of the United Nations (FAO) invited the enzyme producers to find alternative solutions to rennet. Soon some microbial coagulants produced by fermentation of different bacteria and molds appeared on the market. Only the one produced by Rhizomucor miehei (and to some extent by Rhizomucorpusillus and Cryphonectria parasitica) are still on the market. In 1983, the production of chymosin by a GMO was announced. It took about 10 years before this enzyme was accepted by cheese producers.

In general, in Europe, Protected Designation Origin cheeses have to be produced by animal rennet as prescribed by their standard protocols. Highest-quality cheeses are normally produced using animal rennet. Most cheeses, however, are manufactured by clotting milk using fermentation chymosin obtained through genetically modified yeasts or molds. Some countries that refuse the use of products derived from genetically modified organisms (GMO) prefer to use microbial coagulants. Also, religious prescription can influence the choice of the coagulant if it has to be approved kosher or halal. All types of coagulants on the market (including animal rennet) can be found certified kosher and halal.

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