STABILITY AND RESISTANCE TO PH AND TEMPERATURE

The effectiveness of probiotics in supporting poultry health and immunity is largely determined by the stability and resistance of microorganisms to environmental conditions, particularly the pH of the digestive tract and the temperature during feed processing [22]. The poultry digestive tract exhibits significant pH variation, ranging from an acidic environment in the proventriculus (pH 2–4) to neutral or slightly alkaline conditions in the small intestine (pH 6–7.5) [238]. To remain viable until reaching the intestine, probiotics must be able to survive gastric acid, adhere to the epithelium, interact with the GALT, and modulate the poultry immune system [239].

Regulatory approval often requires documented stability, safety, and strain identification. In the European Union, probiotic strains used as feed additives must undergo formal safety and efficacy assessment before authorization, whereas regulatory frameworks in parts of Asia and developing regions may vary in stringency and enforcement [240]. Consequently, product quality, labeling accuracy, and viable cell counts may differ across markets, influencing both efficacy and producer confidence.

Besides pH, temperature tolerance is also a crucial factor, especially when probiotics are added to feed that is heated during pelletization (70–90°C) [241]. Probiotic microorganisms, including Lactobacillus spp., Bifidobacterium spp., and Bacillus spp., have varying heat tolerances [242]. Spore-forming bacteria, such as Bacillus spp., are more resistant to high temperatures than non-spore-forming bacteria [243]. Meanwhile, non-spore-forming probiotics typically require additional protection, such as encapsulation or a protective matrix, to maintain their viability during feed processing and storage [244].

Strategies for stabilizing probiotics include methods such as freeze-drying, microencapsulation, and the use of prebiotics as a protective matrix, allowing bacterial cells to remain viable during storage and passage through the digestive tract [245]. Probiotics that are resistant to gastric pH and feed processing temperatures have a greater chance of reaching the intestine in a viable state, establishing colonization, and providing optimal immunomodulatory effects, including stimulation of innate and adaptive immune cells, increased antibody production, and regulation of the gut microbiota [13]. Compliance with regional regulations regarding microbial safety, absence of transferable ARGs, and accurate strain declaration is increasingly important for international trade and consumer acceptance, particularly in antibiotic-reduction programs [202].