**4.1. Probiotic dairy products**

The use of gas to modify Eh seems to be an interesting way of varying the organoleptic properties of dairy products as well as improving the survival of oxygen sensitive strains during storage in fermented dairy products containing probiotics. Indeed, these microorganisms are mainly anaerobes. Oxygen, which is a powerful oxidant, has a drastic effect on Eh values and the viability of probiotic bacteria during manufacturing and storage [58-60]. So, many studies modify the redox potential to protect probiotics from oxygen toxicity in dairy products [1, 61-65]. However, these techniques sometimes have deleterious effects on the organoleptic properties of fermented milk. An alternative to these methods could be the use of gases. Indeed, Ebel *et al.* [18] showed that fermented dairy products made from milk gassed with N2, and more particularly those made from milk gassed with N2 – H2, were characterized by a significant increase in *Bifidobacterium bifidum* survival during storage (Figure 2).

Reprinted from Journal of J. Dairy Sci., Vol 945, Ebel B, Martin F, Le LDT, Gervais P, Cachon R, Use of gases to improve survival of *Bifidobacterium bifidum* by modifying redox potential in fermented milk, Pages No. 2185-2191, Copyright (2011), with permission from Elsevier.

**Figure 2.** Evolution of a population of *Bifidobacterium bifidum* during fermentation and storage. Different gaseous conditions were applied to the milk: control (solid line), gassed with N2 (dashed line), or gassed with N2 – H2 (dotted line).

After 28 days of storage, a difference in bacterial counts of 1.2 log and 1.5 log was observed between the control milk and after bubbling with N2 or N2 – H2 respectively. No differences were highlighted during the fermentation process. It is interesting to note that this technique was set up without affecting the fermentation kinetics and survival of *S. thermophilus* and *Lb. bulgaricus*. The use of gas is a possible way of improving probiotic survival during storage without affecting acidification properties of yoghurt strains and consequently organoleptic properties.

### **4.2. Cheese**

88 Lactic Acid Bacteria – R & D for Food, Health and Livestock Purposes

significantly decreased.

increased.

increased.

acetoin and pentane-2,3-diol [57].

**4.1. Probiotic dairy products** 

during storage (Figure 2).

**4. Impact of Eh on other dairy products** 

During the 28 days of storage, for the standard yoghurt, the quantities of acetaldehyde and dimethyl sulphide produced were relatively stable, while diketone concentrations

For yoghurts made with air bubbling, the aroma profiles remained almost constant. During storage, the concentration of acetaldehyde decreased slightly whereas that of dimethyl

For yoghurts made without oxygen (bubbling with N2), the quantities of acetaldehyde, diacetyl and pentane-2,3-dione decreased during storage while that of dimethyl sulphide

For yoghurts produced under reducing conditions (bubbling with N2 – H2), the aroma profiles during storage were the same as those made without oxygen. The concentration of acetaldehyde, diacetyl and pentane-2,3-dione decreased while that of dimethyl sulphide

Furthermore, during storage, different profiles were observed for the four aromas depending on the Eh conditions [51]. Under oxidizing conditions (+170 to +245 mV), the concentration of acetaldehyde was relatively stable during storage, which is in accordance with the literature [9, 55, 56] and the concentration of dimethyl sulphide was also stable. On the contrary, under reducing conditions (-300 to -349 mV), the concentration of acetaldehyde decreased and that of dimethyl sulphide increased. The metabolic pathways involved in the biosynthesis of sulphur compounds are still unclear. Under reducing conditions, it seems that another pathway promotes the production of dimethyl sulphide and that acetaldehyde may be reduced to ethanol. For diketones, whatever the Eh conditions, the concentration decreased during storage. Diacetyl and pentane-2,3-dione can be reduced respectively to

The use of gas to modify Eh seems to be an interesting way of varying the organoleptic properties of dairy products as well as improving the survival of oxygen sensitive strains during storage in fermented dairy products containing probiotics. Indeed, these microorganisms are mainly anaerobes. Oxygen, which is a powerful oxidant, has a drastic effect on Eh values and the viability of probiotic bacteria during manufacturing and storage [58-60]. So, many studies modify the redox potential to protect probiotics from oxygen toxicity in dairy products [1, 61-65]. However, these techniques sometimes have deleterious effects on the organoleptic properties of fermented milk. An alternative to these methods could be the use of gases. Indeed, Ebel *et al.* [18] showed that fermented dairy products made from milk gassed with N2, and more particularly those made from milk gassed with N2 – H2, were characterized by a significant increase in *Bifidobacterium bifidum* survival

sulphide increased slightly. The diketone concentration significantly decreased.

Controlling Eh in cheese seems essential in governing aroma characteristics. Indeed, a reducing Eh is necessary for the development of the characteristic flavour of certain fermented dairy products such as cheeses, notably through the production of thiol compounds [45, 66]. It has also been reported that Cheddar has a reducing Eh and is an indicator of the establishment of the conditions required for the formation of aroma compounds [67]. As shown previously, Eh can modify the metabolic pathways of aroma production by lactic bacteria [51]. Kieronczyk *et al.* [49] demonstrated that reducing Eh conditions can stimulate carboxylic acid production in cheese, while oxidative Eh conditions improve the production of volatile sulphur compounds and aldehydes. By ripening cheese under reducing Eh conditions, the production of volatile fatty acids increased [68]. Adjusting the Eh of the milk before cheese ripening could be a possible way of modifying the metabolism of lactic bacteria.

Redox Potential: Monitoring and Role in Development of Aroma Compounds, Rheological Properties and Survival of Oxygen Sensitive Strains During the Manufacture of Fermented Dairy Products 91

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