## 3 Results and Discussion
### Incubation Time
They reported that the average half-life of a strain maintained at 25 \({}^{\circ}\)C was approximately 7 days, compared to about 43 days when stored at 4 \({}^{\circ}\)C. Additionally, [PERSON] and [PERSON] [47] carried out a study dealing with the viability of two freeze-dried strains of _Bifidobacterium_ during prolonged storage at various temperatures and concluded that the strains stored at 20 \({}^{\circ}\)C exhibited the greatest decline in the viability of bacteria, whereas those stored at \(-18\)\({}^{\circ}\)C exhibited the least. For long-term storage, the inactivation of freeze-dried starter cultures depends significantly on the storage conditions. The inactivation is correspondingly related to the storage temperature and moisture content. This is mainly due to the state of dried starter cultures. In general, components of biological materials in the dehydrated states form amorphous structures, with the typical characteristics known as the rubbery (amorphous liquid) and glassy (amorphous solid) states. The most important parameter describing the glassy state is the glass transition temperature (\(T_{g}\)), below which materials exhibit extremely great viscosity, giving them solid-like properties [48]. During storage, the \(T_{g}\) of the dried sample is an important factor affecting the viability of cultures. Thus, the moisture content becomes a key variable. When the dried starter cultures are filled in the moisture-permissible packages, the relative humidity of the storage environments will have an additional effect besides the end moisture after drying [48]. All starter cultures present in the experimental samples were conserved in the non-moisture-permissible packages (small polyethylene sealed bags) at the same relative humidity but at different temperatures. Therefore, it is believed that the dried cultures stored at different temperatures presented different amorphous states, which resulted in different storage stabilities. For instance, the cultures stored at 20 \({}^{\circ}\)C for 8 weeks showed the least viability. On the other hand, the O\({}_{2}\) and light are other critical factors influencing the stability of cultures. The main cause of the deterioration of freeze-dried starter cultures is related to the membrane lipid oxidation [28]. Research on the lipid composition of the cell membrane using gas chromatography showed that the unsaturated/saturated fatty acid index of the cell membrane changes during storage [49]. These changes largely affect the passive permeability of the membrane and contribute to cellular death [50]. In addition, rehydration of the cultures is another critical step that can influence the level of viability of freeze-dried microorganisms. The medium itself as well as its molarity and rehydration conditions can drastically affect the rate of recovery [38]. However, as all the starter cultures present in the experimental samples were packed and rehydrated in the same way, the influence of O\({}_{2}\), light, and rehydration cannot be used to explain the rheological differences obtained for the different recombined samples. Generally, as the level of viability of starter cultures decreased (Table 4), the incubation times required for the production of acid milk gels increased (Table 7) and the final gel obtained had a higher liquid-like behaviour (lower G\({}^{\prime}\) and higher tan \(\delta\)) (Figures 2 and 3, \begin{table}
\begin{tabular}{c c c c c c} \hline
**Storage temperature** & & 5 & & & 20 \\
**(\({}^{\circ}\)C)** & & & & & \\ \hline
**Storage time** & & & & & \\ (weeks) & 0 & 2 & 8 & 0 & 2 & 8 \\ \hline
**Incubation Time** & 7.2 \({}^{\circ}\) & 8.1 \({}^{\text{b}}\) & 8.4 \({}^{\text{b}}\) & 7.2 \({}^{\text{c}}\) & 8.6 \({}^{\text{b}}\) & 15.6 \({}^{\text{a}}\) \\
**(hours)** & (0.2) & (0.4) & (0.3) & (0.2) & (0.4) & (3.3) \\ \hline \end{tabular} Standard deviation is shown in brackets; means followed by various letter (a-c) are significantly different (\(p<0.05\)).  
\end{table}