**1. Introduction**

The world's population of 7.6 billion people is still growing and is expected to reach 8.3 billion by 2025 and almost 10 billion by 2050 [1]. Concomitantly, the Earth's resources are depleting. According to the different scenarios, global food demand is expected to increase by 40–68% by 2050 [2]. Among food resources, plants are of particular interests, as the global production of plant-based products is constantly increasing while producing significant waste. In this context, recycling or revalorizing these by-products is a priority [1].

The main objectives of using plant by-products are to revalorize wastes, reduce pollution, and limit resource depletion. Fermentation is one of the least polluting methods. Plant by-products fermentation contributes to sustainable development; in fact, this type of valorization is part of some objectives of the United Nations 2030 Agenda, notably the third objective: good health and well-being and the twelfth objective: responsible consumption and production. The consumption of fermented plant by-products therefore allows responsible consumption. The fermentation of plant by-product leads to bioactivities related to human health such as antioxidant, anti-inflammatory, or antimicrobial activities that contribute to good health and well-being [3]. Plant-based foods are sources of many bioactive compounds such as fibers, vitamins, minerals, or phenolic compounds. These nutrients are necessary for the survival and growth of organisms [4]. In many countries, the health benefits of certain plant and their traditional use have been recognized for decades [5]. Since industries have been exploiting plant-based foods, many agro-industrial by-products that still contain valuable compounds have been generated. Many companies are now seeking to recycle waste from their fruit and vegetable activities in order to address environmental and economic issues. For example, cereal waste reached about 40,000–45,000 tons per year in Europe [6]. The by-products are mainly used for livestock feed or methanisation but have great potential to generate food or dietary supplements for human use [6, 7]. Another example concerns the waste from the citrus industry, which amounts to 50 million tons per year and is the most important waste from fruits exploitation. The management of by-products represents a real food waste problem and raises major issues [8]. Therefore, in recent years, there has been a growing interest in the valorization of plant by-products.

In China, for 9000 years, humans have empirically exploited the fermentation process for numerous applications [9]. Studied since the nineteenth century, lactic acid fermentation has been an essential process for food processing and preservation for many millennia [10]. Humans took advantage of it for their food, notably by developing bread, beer, wine, cheese, or vinegar. Subsequently, fermentation with lactic acid bacteria has been largely studied to improve the nutritional and functional properties of plants. Due to their richness in nutrients, water, and natural ferments, plants such as fruits and vegetables represent an optimal substrate for *Lactobacillus* [11]. Lactic acid bacteria constitute a diverse group of Gram-positive, catalase-negative bacteria producing lactic acid as the main end product. Many food products fermented by lactic acid bacteria are obtained with organisms belonging to the genus *Lactobacillus* [12]. With more than 200 species of *Lactobacillus* bacteria [11], this genus is certainly the main and most diverse group of lactic acid bacteria. A study published in 2020 re-evaluated the genetic relatedness and phylogeny of *Lactobacillus* species. Based on a polyphasic approach such as whole-genome comparison, core genome phylogeny, physiological criteria, and ecology of the organisms, the genus *Lactobacillus* was reclassified into 25 genera (2 preexisting genus and 23 new genera). This work showed the great and extensive diversity of the *Lactobacillaceae* family [13]. *Lactobacillus* species are commonly used in fermented food. Depending on the species, their enzymatic activities including amylase, lactate dehydrogenase, peptidase, proteinase, α- and β-glucosidases, decarboxylase, lactate dehydrogenase, peptidase, phenolic acid decarboxylase, phenol reductase, proteinase or tannase are very useful in food fermentation [14]. These enzymes can degrade the plant cell wall matrix, resulting in the release of many bioactive compounds, which may or may not be modified structurally by the action of other enzymes in the bacteria.

Today, several ecological and economic issues are at the heart of lactic fermentation research. The optimization of yield, cost, and energy consumption and the valorization of plant-derived products represent challenges for the industry [15]. To meet this demand, the use of new substrates and the genetic engineering of

Lactobacillus *Use for Plant Fermentation: New Ways for Plant-Based Product Valorization DOI: http://dx.doi.org/10.5772/intechopen.104958*

fermentation strains are being studied as potential solutions [11]. Moreover, it is now known that lactic fermentation increases the content of bioactive compounds. Indeed, this fermentation process is well known to strengthen the immune and antioxidant (AO) effect of medicinal plants by increasing the bioavailability of active compounds, but also through the production (or the bioconversion) of plant metabolites into new bioactive molecules [16]. To increase the bioactivities and organoleptic characteristics of fermented products, *Lactobacillus* converts metabolizable molecules with their enzymes, in particular *L. plantarum,* which is one of the most used *Lactobacillus* as a fermentation starter. This degradation increases the bioavailability of molecules and improves their absorption [17]. A fermentation starter is usually a consortium of bacteria that helps the fermentation process to start. Today, the use of starter cultures in food fermentation is one of the necessary ingredients for good production. In addition, LAB used as starter in the food industry provide safe product with good nutritional and organoleptic qualities. LAB are used as starter for many products, including fruit, vegetables, and cereal [18]. As illustrated in **Figure 1**, the production of biomolecules by lactic fermentation of plant by-products can induce other bioactivities. This chapter refers to antioxidant (AO), anti-inflammatory (AI), antimicrobial, prebiotic activities, and others. These can be used in human food and beverage, livestock feeding, or biotechnology mainly to produce lactic acid. Those activities and applications will be detailed in this chapter.
