This article briefly summarises the five factors to be taken into consideration to obtain good cell viability of spray dried probiotic bacteria. The factors are: (i) early stationary phase, (ii) carrier medium, (iii) additional solutes, (iv) bacteria culturing, and (ii) rehydration.
Figure 1: Bacterial growth curve
- Early stationary phase: Probiotic bacteria should be harvested in the early stationary phase (as seen in the figure above). This is where the onset of the depletion of nutrients and glucose starvation in bacterial cells cause the cells to be resistant to many stresses such as osmotic and heat stress. As a result, improved cell viability can be achieved when spray drying.
- Carrier medium: The concentration of carrier medium (e.g. Arabic gum, maltodextrin) can also affect the survival of bacteria after spray drying. Introducing carrier medium increases survival of bacteria but only up to a certain threshold, as too high a solid content of carrier medium can lead to larger particle/droplet size. Larger size requires longer drying times, which increases duration of time being subjected to heat stress that can trigger cell death prior to complete drying.
- Additional solutes: Additional solutes such as amino acids, quaternary amines (e.g., glycine, betaine, carnitine), and sugars (glucose, fructose, lactose, mannose or sorbitol) can increase the viability of probiotic bacteria during the drying process. These solutes will be accumulated by microorganisms to maintain osmotic balance as water evaporates away. As a result, proteins and cell membranes are stabilised even when cells are dried.
- Some sugars (sorbitol, maltose and mannitol) are also effective protectants against lipid oxidation damages of membrane fatty acids. This attribute is especially important during the powder storage phase (after spray drying)
- Bacteria culturing: During the culturing of bacteria, the introduction of adverse conditions or stresses (duration, heat, salt and/or pH) during microbial growth (exponential growth phase, see above figure) can lead to better tolerance responses to spray drying.
- Rehydration: Slow rehydration is recommended to limit osmotic shock, hence help to maintain high cell viability after rehydration.