## 4 Microbial Fermentation for Commercial Production of C\({}_{30}\) Caroenoids
### Abiotic Stresses for the Optimization of C\({}_{30}\) Carotenoid Production  
Various methods have been used to enhance carotenoid yield, including the application of stress factors, progressive nutrient deprivation, and recombinant techniques for the production of high-yield strains (Table 2) [44]. The processes employed in biotechnological applications depend on the stability and synthesis requirements of the desired products. Carotenoid production and cell growth are affected by the nutrient composition of the culture medium and culture conditions. For industrial production, the costs associated with substrates and the running costs of large-scale fermenters must be minimized while optimizing final product yields. Optimization studies that use response surface methodology often analyze the main factors for scaled-up production and maximize desired parameters, such as carotenoid production, while mitigating undesirables, such as high cost. An example of this is the optimization of nitrogen and carbon sources, particularly when standard media components are substituted by natural compounds or agricultural waste. This optimization reduces costs while also increasing sustainability, yielding environmentally friendly production methods [72]. Microbial growth or carotenoid synthesis can be maximized within a specific range of conditions. Environmental conditions optimal for cell growth are not necessarily optimal for carotenogenesis. In certain bacterial species, high temperatures are ideal, whereas in others, lower temperatures provide better carotenoid synthesis. This is also true for pH, salinity, and carbon and nitrogen sources. However, with biotechnological advancements, the effects of these modulating parameters on carotenoid production can be adjusted. Metabolically engineered strains of _E. coli_ have been compared to determine the effect of recombinant hosts and carotenoid structure on carotenoid yield and ascertain whether heterologous carotenoid formation was strain-dependent. The initial pH of the culture media influences bacterial cell growth and carotenoid production, but as the bacteria grow, the acidity/alkalinity levels of the media change due to by-products of bacterial metabolism [83], as is the case with LAB. In yeast cells, the initial decrease in pH is not permanent and the pH rises as the yeast proliferates and intensively produces carotenes, followed by stabilization of the pH values [84]. Acid or alkaline tolerance is highly strain-dependent, with certain microorganisms accumulating more carotenoids at neutral pH, while others exhibit optimum productivity in slightly acidic or alkaline conditions [85; 86]. Temperature is an important factor in bacterial growth and carotenoid synthesis and affects biosynthetic pathways, stability, reactivity of substrates, enzymes, and the products involved in these pathways [87]. Temperature also plays a role in enzyme concentrations within biosynthetic pathways and in internal temperature regulation [88; 89]. Aeration has a known effect on aerobic microorganisms, with agitation increasing the movement and availability of oxygen and nutrients within the culture. For aerobic microorganisms, culture aeration is important for the efficient supply of oxygen to the growing biomass, which in turn promotes carotenoid synthesis. Intensive aeration and agitation stimulate carotenoid production as long as the threshold at which the mechanical shear forces could cause cell damage is not crossed [90; 91]. The substrate source and composition in the culture media also influence carotenoid production and yield. The most common carbon sources for carotenoid biosynthesis are sucrose and glucose. Cell growth maintenance requires available carbon, and there