To date, the use of recombinant microbes in food production and processing has been limited to recombinant microbial enzymes and a recombinant hormone (bovine growth hormone;BGH) to boost milk production. Recombinant chymosin (rennet) is an example of a recombinant enzymes produced by microbes. The bovine chymosin gene was transferred to several fungal species via recombinant DNA technology in the 1980s. Recombinant chymosin is now widely used throughout the world in cheese making. Chymosin increases the rate of curd formation during initial fermentation of milk by lactic acid bacteria. Traditionally, chymosin was obtained from the stomach of slaughtered calves, but the supply from this source is somewhat unstable. In contrast, recombinant chymosin does not have this instability, because it can be produced through growth of recombinant yeasts in large bioreactors (vessels used for large-scale growth of cells).
Interestingly, recombinant chymosin is currently triggering hostility between the US and the EU. Proposed EU regulations governing labelling of food containing GMO’s exempt cheese made using recombinant chymosin, because such cheese, when it is sold to consumers, contains negligible amounts of recombinant chymosin. However, the US government maintains that this constitutes an unfair trade practice, because it implies that European products such as cheese are exempt from labelling, whereas foods containing small amounts (>1%) of transgenic plants (mostly imported from the US) must be labelled.
The other major use of recombinant microbes in food is recombinant bovine growth hormone (rBGH) by dairy farmers. The gene for BGH was transferred to E.coli and large-scale culture of this recombinant bacterium yields large amounts of rBGH. This is then injected into cows, which increases milk production.