**3. Effect of Eh on a fermented dairy product: Yoghurt**

### **3.1. Reminder regarding the manufacture of yoghurt**

We chose to focus on the key steps in the manufacture of yoghurt, which are:

	- Its microbiology: to ensure consumer health, prevent the degradation of milk components that persist in the finished product and eliminate any possible competition between the starter culture and the endogenous flora that may involve bacteriophages.
	- Its chemistry: a rapid analysis of the chemical composition of the milk is necessary in order to identify any problems such as colostrum and late-lactation milk. Furthermore, these data concerning the chemistry of the milk can be useful in the standardization of the mixture.

this couple that mainly influences the stability of the Eh imposed.

**3. Effect of Eh on a fermented dairy product: Yoghurt** 

We chose to focus on the key steps in the manufacture of yoghurt, which are:

**3.1. Reminder regarding the manufacture of yoghurt** 

therefore be analysed as soon as the milk is received:

bacteriophages.

standardization of the mixture.

technology is authorised at the European level (E949).

on nitrogen and hydrogen.

food during transport.

the H+/H2 couple ( 0'

level. Of the gases used in the food industry, in this chapter we will focus more particularly

Nitrogen (N2) is odourless, colourless, tasteless, non-toxic, and non-flammable. It is used to extend the life of packaged products (authorized additive E941). It is used to expel oxygen from the packaging before it is closed, which prevents oxidative phenomena involving pigmentation, flavours and fatty acids. It is also used for rapid freezing and refrigeration of

Hydrogen (H2) has major potential in food as it is colourless, odourless and has no known toxic effects. It is already used in the food industry for the hydrogenation of liquid oils and their transformation into solid products such as margarine or peanut butter. Hydrogen is a powerful reducer in solution, even at very low concentrations. It has been used to demonstrate the effect of Eh on the heat-resistance of bacteria [7]. Hydrogen is a special reducing agent: it imposes an Eh value on the medium associated with the introduction of

With the prospect of food use, hydrogen has the advantage over chemical reducing agents of not changing the product formulation, and therefore not altering the taste. Its industrial use has been rarely seen in this context because of its low flammability limit of 4% in air at 20 °C [8], this is why N2-H2 (96%-4%) is preferred to pure hydrogen. Its use in food

 Delivery of milk: The raw material can be either fresh milk, reconstituted milk (from skim milk powder), or a mixture. In all cases, it is generally accepted that a quality product can be made from an extremely high quality raw material. With this in mind, it is essential that when the milk and other raw materials are received methods are established to detect any potential defects as early as possible. Two parameters must



 Standardization of the mixture: each component in milk plays a role. Fat has an effect on the smoothness and the feeling of softness in the mouth, lactose is the raw material used by

*<sup>h</sup> E* = -414 mV). This Eh value is highly dependent on the concentration of

lactic acid bacteria for acidification, proteins act on the texture and minerals help stabilize the gel. These components vary in cow's milk according to race, diet, stage of lactation of the animal and season, which is why, during yoghurt manufacture, it is necessary to standardize the milk fat and protein content to meet the nutritional and organoleptic characteristics of the product and obtain consistent quality throughout the year.


Heat treatment of milk also has a positive effect on enzyme activity by providing a supportive environment. The environment becomes reductive through the elimination of a high proportion of oxygen. This medium is more conducive to fermentation that takes place under anaerobic conditions.

