**3.2. Yoghurt strains: Lactobacillus delbrueckii ssp. bulgaricus and Streptococcus thermophilus**

The association of *S. thermophilus* and *Lb. bulgaricus* is called proto-cooperation. Each species produces one or more substances, initially absent from the culture medium, that stimulate the growth of the other species [10]. During the symbiosis observed in yoghurt, the growth phases of these two bacterial species are staggered. Initially, growth of *S. thermophilus* is observed which is then slowed by the inhibitory effect of the lactic acid produced; the growth rate of *Lb. bulgaricus* then increases [11].

*S. thermophilus* is a strain that often shows little proteolytic activity, due to general low activity or absence of a wall protease. Its growth is limited because the peptides and amino acids initially present in milk are insufficient to cover its needs. In contrast, *Lb. bulgaricus* membrane protease degrades milk caseins releasing small peptides and amino acids which can be used by *S. thermophilus* intracellular peptidases [12].

The cooperation between these two strains also involves the production by *S. thermophilus* of pyruvic acid, formic acid, and carbon dioxide (CO2 obtained from the decarboxylation of milk urea by urease) which stimulates the growth of *Lb. bulgaricus* [9, 13]. However, formic acid is released late in fermentation and in small quantities. The two bacterial species also consume the formic acid resulting from the heat treatment of milk [14].

Some authors have also demonstrated that the association of *S. thermophilus* and *Lb. bulgaricus* affects the production of volatile compounds involved in flavour development in yoghurt [15]. *S. thermophilus* produces more acetaldehyde, acetoin and diacetyl than *Lb. bulgaricus*, contrary to the rest of the bibliography concerning acetaldehyde [9, 16, 17]. Quantities of these molecules and other carbonyl compounds are not crucial per se for yoghurt flavour, but there are relationships between them that give yoghurt its distinctive flavour.

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 81

application of a shear stress and measurement of the deformation, or application of a deformation (compression, stretching or shear) and measurement of its ability to withstand this distortion. Yoghurt can be defined as a viscoelastic fluid. It therefore has both the

It has been shown that dairy products are affected by Eh [4, 19]. Delbeau *et al.* [19] showed that the use of gas to change the Eh of milk can modify the sensory properties of a fermented dairy product. However, we do not know if these modifications are due to the impact of Eh on physicochemical phenomena, lactic acid bacteria, or both. For this purpose, Martin *et al.* [20] wanted to determine to what extent chemical phenomena affect acid milk gelation under different Eh conditions. Glucono-δ-lactone (GDL) was used to acidify milk to avoid

Martin *et al.* [20] studied the effects of Eh on model acidified skim milk gels obtained using GDL and prepared under different gaseous conditions. The milk prepared in air is an oxidizing medium; nitrogen, which is a neutral gas, can be used to remove oxygen from milk - even so the milk Eh remains oxidizing in these conditions - and hydrogen leads to a reducing Eh (below 0). Martin *et al.* [20] focused on the effect of gas bubbling on gel structure

hours At t=0 At t=3.5

0.03 4.6a ± 0.0 405 ± 22 414 ± 8 0.039a ±

0.04 4.6a ± 0.0 433 ± 6 430 ± 5 0.032c ±

0.06 4.6a ± 0.0 283 ± 13 288 ± 11 0.035b ±

0.04 4.6a ± 0.0 - 349 ± 6 - 83 ± 18 0.032c ±

a-c: different letters indicate that groups were significantly different at an α risk of 5% (ANOVA test). Values in the

Reprinted from Journal of Dairy Science, Vol 92, Martin F, Cayot N, Marin A, Journaux L, Cayot P, Gervais P, Cachon R, Effect of oxidoreduction potential and of gas bubbling on rheological properties and microstructure of acid skim milk gels acidified with glucono-δ-lactone, Pages No. 5898-5906, Copyright (2009), with permission from Elsevier. **Table 1.** Characteristics of gel structure depending on the different Eh conditions (milk acidified using

• Apparent viscosity η at 500 1/s of GDL-gel at pH 4.6 and 4 °C. Measurements were carried out 24

• Evolution of average whey separation (WS) over 28 days in GDL-gels. Values are means from triplicate experiments (mean value standard deviation).

GDL-gel) At t=0 At t=3.5

hours

η (Pa.s)

WS (g/100g of

0.000 4.74a ± 1.42

0.001 1.26b ± 0.26

0.001 1.93b ± 0.33

0.001 0.59c ± 0.12

through viscoelastic properties and measurement of whey separation (Table 1).

pH Eh7 (mV)

viscous properties of a liquid and the elastic properties of a solid.

*3.3.2. Effect of Eh on a model acid skim milk gel* 

variations caused by microorganisms sensitive to Eh.

Gaseous conditions applied to milk

Air 6.80 ±

6.73 ±

Air bubbling 6.70 ±

N2 bubbling 6.8 ±

same column should be compared.

hours after addition of GDL.

N2 – H2 bubbling

GDL):

Finally, Ebel *et al.* [18] showed that during the manufacture and storage of a fermented dairy product, the populations of *Lb. bulgaricus* and *S. thermophilus* are the same whatever the Eh of the milk.
