**4. Limitations and future challenges**

Faced with environmental and societal problems such as pollution, global warming, and overpopulation, crop yields are increasingly challenging to sustain. Moreover, while demand is increasing in developed countries, poor populations are struggling to feed themselves, and undernutrition is high in these countries. This is why the food and agriculture industry must find solutions to meet the needs of all. Among these, better use of plant resources and better exploitation of their byproducts are two solutions of interest. In addition, consumers are looking for more natural and healthy products, and industrials are looking for economically viable bio-based solutions. Fruit and vegetable waste and cereal by-products are likely to be reused because of their quantity and richness in nutrients and bacterial strains suitable for lactic fermentation.

When lactic acid bacteria ferment the nutrients in them, the functional and nutritional properties increase, representing significant opportunities for the agrifood, biotechnology, medical, nutraceutical, and cosmetic industries. As presented in this chapter, the fermentation of plant products by *Lactobacillus* allows the production of numerous bioactive molecules for the development of many applications. Nevertheless, to meet the demand, lactic acid fermentation by *Lactobacillus* requires optimization. First of all, the use of plant by-products requires a crucial design of the fermentation process according to the raw material (solid, liquid, semiliquid fermentations). This design could lead to the development and emergence of new processes that should be able to meet industrial viability, economic returns, and consumer needs. Therefore, much work is still needed on these processes to increase the commercialization of new bio-based products from plant by-products. On the other hand, *Lactobacillus* strains are fastidious bacteria in their nutritional requirements and are not necessarily well adapted metabolically for growth from any substrate, and the use of GMOs is a very limiting criterion for many applications (food, cosmetics, etc.). The growth parameters and enzymatic activities of *Lactobacillus* strains have a major impact on applications, particularly when the fermentation substrate is complex. It is therefore necessary to work on the culture conditions and metabolic adaptation of these strains in order to maximize the enzymatic activities and production rates of the molecules of interest. Therefore, many constraints exist, such as the lack of scientific data and hindsight, the control of culture conditions, and the separation and purification processes to recover bioactive compounds. Further efforts are urgently needed to overcome these problems. Nevertheless, one of the advantages of production with *Lactobacillus* is its ability to produce several types of molecules simultaneously, typically lactic acid and other molecules (derived or transformed from the substrate), which makes the fermentation process industrially interesting. Such multiproduct strategies should be promoted in the near future up to industrial scale.
