Table 1.

Various techniques used to encapsulate probiotics

Encapsulation techniques	Encapsulate structure(s)	Typical size range	Critical parameters	Merits	Demerits	References
Conventional approaches
Spray drying		The diameter of particles about 5–20 µm (varies with the size of the nozzle)	Temperature -optimization of temperature is required to maintain the viability of probiotics as well as product with less residual moisture content. Approximately inlet temperature ranges between 110–160 °C and outlet temperature ranges between 55–85 °C. The concentration of wall material—normally in the range of 15–50% (w/v). Low concentration (less viscous) is preferable to avoid clogging and for easy atomization. The feed flow rate in a range of 2–12 ml/min (vary based on chamber capacity, nozzle size, atomization air flow rate, and aspirator rate)	Rapid process, continuously operatable single process unit for particle formation, high reproducibility, low operational/production cost, easily scalable without major modifications, and adaptability to most common industrial equipment; The liquid feed system can operate at relatively low pressure; Powders obtained by spray drying have better flow properties; Suitable for the production of monodisperse powders with particle size in the micrometer scale; Particle size can be optimized by changing nozzle size, design, and operation parameters; The smooth skin-forming ability of the spray-dried particles offer a protective environment to the entrapped probiotic cells	High capital cost, expensive maintenance, low yield due to the loss of product in the walls of the drying chamber; The feed solution has to be pumpable for the atomization process (low viscous solution/ slurry/ suspension). Hence, spray drying is not suitable for highly viscous feed material; Thermal inactivation of probiotics due to high inlet temperature and evaporation rates causes cellular damage to probiotic cells; High temperature does not have a direct impact on the viability of probiotics; rather, it depends on the time–temperature combination that decides the extent of microbial inactivation during spray drying. Short-time exposure might avoid thermal inactivation; Shear force/ stress acting on the core and air occlusion in the atomized droplet during atomization have a direct impact on the viability of probiotics; The loss of viability also depends on the type of carrier/wall material used	[106–108, 121, 126, 128, 129, 241, 242]
Freeze-drying		Irregular in shape, particle size > 1 mm, polydisperse (broad particle size distribution)	Condenser temperature should be less than the product temperature during the sublimation process of freeze-drying Vacuum pressure: 0.1- 0.5 Torr. Lower pressure is preferred for the sublimation process	Highly porous structure, better rehydration, and solubility; Sublimation of moisture under vacuum avoids water phase transition and oxidation	Longer drying time, high energy consumption, and high operational/ production costs limit their commercial-scale application; More expensive (30–50 times higher than the spray drying process); Amorphous and hygroscopic irregular porous structure leads to stability loss of the product (core instability due to large air–solid interface); Probiotic cells are entrapped close to the surface of freeze-dried powders, which affects the stability of probiotics during transit through the acidic conditions of the upper GI tract	[121, 134, 242, 243]
Extrusion technique		Microbeads around 0.5- 3 mm diameter (varies with needle gauge size)	Concentration of gelling agent (0.5–2.5% w/v), solution pH and viscosity, ionic strength and concentration of cross-linker solution (concentration 2–5% w/w), and reaction time	Simple and inexpensive method; Gentle operation—provides better probiotic viability and protects the cells from damage; It does not involve deleterious solvents and can be done under both aerobic and anaerobic conditions	Difficult to scale-up at the industry level due to slow process and very low production capacity; Particles with larger size distribution (polydispersity); Limited choice of wall material	[151, 241, 244]
Emulsion technique		Emulsion size about 25 µm – 2 mm	Speed of agitation, phase-volume ratio, emulsifier type, solution pH, and viscosity	Probiotics entrapment in the oil phase of protein-stabilized emulsions protected the cells when exposed to GI tract enzymes/ acids; High survival rate of encapsulated probiotics; The emulsion methods produce capsules sized from a few micrometers to 1 mm	Particles with extremely large size distribution (polydispersity) and low yield; Controlled stirring and homogenization are required to achieve a narrow particle size distribution; High-shear process—prolonged shear forces may cause damage to cells which affects the viability of probiotics during processing	[118, 158, 243]
Emerging approaches
Spray-freeze-drying		The diameter of particles is about 20–80 µm (varies with the size of the nozzle)	Concentration of feed solution, viscosity, feed flow rate, nozzle size, atomization air flow rate, aspirator rate, and type of cryogenic medium. Also during freeze-drying, the conditions such as shelf temperature, vacuum pressure, and drying time	Spray-freeze-dried particles exhibits controlled size and large specific surface area than spray-dried particles; Excellent reconstitution capacity; Improved yield as compared to the spray drying process; Low-density particles with porous nature; Spray-freeze-dried probiotic microcapsules showed high cell viability and stability	High energy consumption, more expensive and requires additional coating/ capsule for protection against adverse environmental conditions; It requires cryogenic medium (liquid nitrogen/ liquid hydrogen/ liquid argon); Dual stress (thermal and osmotic stress) to the probiotic cells; To prevent viability loss during quick freezing, stabilizing additives are required	[121, 138, 167, 241, 245]
Refractance window drying		Flaky structure with a preferred thickness, after blending the particle size > 1 mm, polydisperse (broad particle size distribution)	The temperature of hot water (for probiotics preferably 40–60 °C), concentration/ total soluble solids of the feed solution, feed layer thickness, and drying time	Dried products are of high quality due to the self-limiting dehydration method; Suitable for heat-sensitive materials; A simple and inexpensive method with less energy consumption; Rapid drying at atmospheric pressure; A suitable method for drying low viscous liquids, high viscous slurries, purees, pastes, wet solids/ slices of fruits and vegetables	Inconvenient in handling powder with high sugar content; exhibits high stickiness due to their hygroscopic nature and high ˚brix	[171, 246, 247]
Electrohydrodynamic processes		The average diameter of electrospun probiotic fibers is around 100–150 nm with a probiotic bead size of 300–800 nm; The diameter of electrosprayed particles around 200–800 nm	Solution parameters such as molecular weight of polymer, concentration, viscosity, conductivity, and surface tension. Process parameters are applied voltage, flow rate, and tip-target distance	Encapsulation without application of heat with relatively high encapsulation efficiency; Monodisperse electrosprayed particles with a high surface area; Electrospun fibers with high reproducibility and yield	Low throughput technology—difficult to scale-up; Challenge of mass production—low yield (typically in the range of milligrams/hour); High voltage electrohydrodynamic processes can be injurious to cells and can affect probiotics cell viability	[147, 248, 249]