**5. Conclusion**

Pasteur defined fermentation as "life without air". In lactic acid bacteria, some exogenous electron acceptors may interfere significantly with the fermentative metabolism by acting on different cellular activities. A better understanding of the adaptive mechanisms to extracellular redox is still lacking, but the results in the literature show that lactic acid bacteria may use passive or active mechanisms. A remarkable feature in lactic acid bacteria is their ability to reduce the redox environment to low Eh values.

With the prospect of food applications, changing Eh using pure or a mixture of gases has the advantage of maintaining product safety as opposed to the use of oxidizing or reducing molecules. This chapter demonstrates the importance of Eh both on the physico-chemistry of milk gels and bacterial metabolism and viability. The use of gas to modify Eh seems to be an interesting way of varying the organoleptic properties of dairy products.
