**Abstract**

Agro-industrial by-products have not been efficiently valorized. *Lactobacillus* used to transform these by-products into interesting metabolites is a way to increase the adding-value of these residues and to contribute to the circular economy. These lactic acid bacteria (LAB) metabolize the available substrate produced by enzymes that are responsible for breaking complex carbohydrates into glucose and subsequently obtaining lactic acid through glycolysis in a homofermentative process. By-products used like substrates to produce lactic acid must be rich in carbohydrates e.g. whey, cassava peel, pineapple peel, and molasses, among others. In addition, from lactic acid obtained, it is possible to develop functional foods such as easily-assimilated beverages and to be antagonists to pathogenic microorganisms such as *E. coli*, improve the quality of final products and extract compounds of interest like pigments.

**Keywords:** *lactobacillus*, lactic acid, agro-industrial by-products, circular economy, lactic acid bacteria (LAB)

### **1. Introduction**

Agribusiness is a productive activity that combines the industrial and agricultural processes to obtain products with added value, which can have food or non-food applications [1]. As a result of the agro-industrial activity, waste is generated in significant quantities that represent an environmental problem, due to the inefficiency of its use or the lack of knowledge about appropriate methods for its treatment [2]. Agro-industrial waste is defined as a solid or liquid material product of the processing of primary products, which are not used in the production process. However, these can be used to generate another product of greater economic or ecological value [2].

The use of agro-industrial by-products obeys the guidelines of the circular economy, where the concept of waste disappears, and this element will be used as a resource for nature, society, and industry. The circular economy aims to avoid the creation of waste that can have a negative impact on the environment, climate, and health [3]. In addition, the use of agro-industrial by-products may contribute to the Sustainable Development Goals (SDGs) of The United Nations Development Program (UNDP), in particular, "no poverty" and "zero hanger".

The use of LAB to valorize food by-products through the production of lactic acid, has generated interest in recent years. LAB are microorganisms that are widely used in the food industry since they allow obtaining fermented products with pleasant sensory characteristics. In addition, to favor the intestinal biota, they have been attributed properties such as the ability to remove toxic metals from aqueous solutions [4], generate unfavorable environments for the growth and development of pathogenic microorganisms, such as *Escherichia coli*, *Listeria monocytogenes*, *Pseudomonas*, etc. [5]. Also, LAB as *Lacticaseibacillus* and *Lactobacillus acidophilus* strains have a positive in vivo response to reduce the bioavailability of methyl mercury (CH3Hg) in the human being [6].

LAB have been classified as Gram-positive bacteria. These microorganisms are facultative anaerobes, they do not form spores and have the shape of bacilli. The main metabolite generated for LAB in the fermentation process is lactic acid, an organic acid very interesting in industry [7]. LAB are demanding microorganisms in terms of nutritional requirements, they need certain amino acids and vitamins of the B complex for their development. The most common media used in the fermentation of LAB include, sugar (glucose, lactose, or sucrose), calcium carbonate (used as a buffer for the medium), and malt germ (for the contribution of nitrogenous elements and growth factors). The optimal fermentation temperature for *Lactobacillus* is between 15 and 55° C [8] and the fermentation time should be around 6 days, to achieve around a yield of 90% in lactic acid [9].

Lactic acid production may result from homolactic or heterolactic metabolic routes. The homolactic route consists of the conversion of glucose to pyruvic acid, which through the presence of lactate dehydrogenase, acts as an electron acceptor in the oxidation of NADH, becomes lactic acid. Heterolactic fermentation is carried out through the pentose pathway where there is the formation of xylulose-5-phosphate, an intermediate product in the formation of lactic acid [10]. Finally, other products, in addition to lactic acid, are formed such as ethanol, acetic acid, and carbon dioxide [9].

Lactic acid is composed of two functional groups: carboxyl and alcohol, obtaining an asymmetric carbon that provides it the optical activity. This acid has two optical isomers: lactic L (+) and lactic D (−) (**Figure 1**). However, only the L (+) isomer, which is considered a GRAS substance by the FDA, is considered a food additive e.g., as acidulant and preservative. However, it has other industrial importance in cosmetic, pharmaceutical, and chemical applications [12].

Lactic acid has been widely used in recent decades, for example as the precursor of polylactic acid (a biodegradable biopolymer to manufacture packaging material for the food industry), 3D printing applications, and for medical uses, among others. Currently, the production of lactic acid is achieved through fermentation, however,

**Figure 1.** *Isomeric configurations of lactic acid [11].*

#### *Use of* Lactobacillus *for Lactic Acid Production from Agro-Industrial By-Products DOI: http://dx.doi.org/10.5772/intechopen.106697*

the use of raw materials as sources of carbon and energy represents an environmental problem with a high cost. For this reason, environmentally friendly alternatives have been evaluated from agro-industrial by-products, as raw materials to obtain this acid [13]. For example, the use of whey cassava bark and pineapple bark, using immobilized *Lactobacillus delbrueckii subsp. delbrueckii* [14], Bioconversion of agroindustrial co-products into lactic acid by *Latilactobacillus sakei* [15]. Efficient conversion of agro-industrial waste into D (−) lactic acid with *L. delbrueckii subsp. delbrueckii* [16], the use of *Lactobacillus*, in bakery by-products, meat, whey, etc. [17]. In this chapter, the use of agro-industrial by-products is exposed through the use and application of LAB such as *Lactobacillus* in order to provide adding value to these interesting substrates.
