Agroindustry is an activity that incorporates primary agricultural, livestock, and forestry production without neglecting the aspects of administration, marketing, and financing. It is an economic activity combining the agricultural production process with the industrial one to generate food or semi-finished raw materials destined for the food market [1]. Approximately 76 million tons of organic waste are generated annually in Mexico, of which 79% is primary waste (during harvest) such as corn husks and stalks, sugar cane tops and husks, wheat straw, barley straw, and beans, among others, the remaining 21% is a secondary waste obtained from post-harvest processing, among which are: sugar cane bagasse, corn cobs, maguey or agave bagasse, as well as coffee pulp [2].
Using waste or by-products reduces the exploitation of resources and pollution and degradation of the ecosystem, avoiding inadequate final disposal such as burning, use in landfills, or dumping into water sources [3]. Mexico is the seventh largest sugar producer in the world and consumes 47 kg per capita in raw form. Sugar cane as a raw material is cultivated by 184,000 growers in 227 municipalities in 15 of the 32 states of Mexico, covering more than 777,078 ha; this places sugarcane as the second most valuable agricultural product in the country, only preceded by grain corn. Likewise, sugarcane constitutes one of the ten most important crops, covering 3.9% of the total area harvested in the annual agricultural cycle [4].
Sugar cane is used for the production of sugar and fuel alcohol, during the production process, it generates 250 kg of bagasse, 30 kg of filter cake, 6 kg of ashes, and 45 kg of molasses per ton of cane; in addition, for each liter of alcohol produced, up to 15 L of vinasse is generated [1]. Sugarcane residues have been used in multiple branches as food for animal production, raw material for the production of cellulose, paper, and compost, among others [3, 5, 6].
Agro-sugarcane waste (ASW) is generated during the manual harvesting of sugarcane, of which 28% corresponds to straw and the top of the cane, which are not used since, during harvest, the crop is burned to eliminate weeds and damage to the stems, causing a strong environmental impact. Cane tips have around 58% dry fiber, 4.3% protein, and 34% carbohydrates, making them ideal as an alternative feed for cattle [7].
Agroindustrial waste has a high potential for use due to its varied chemical composition, which is reflected in the diversity of existing alternatives for its revaluation [8]. Depending on the source and composition, some treatment can be done to make it accessible for the growth of microorganisms and to produce the desired product of interest ecologically and economically [9]. Bioprocessing technology has opened a gap to revalue agricultural residues for microorganisms to produce valuable products, such as biomass, Lactic Acid (LA), and other valuable metabolites that have various applications in various fields. The bioconversion of residues into biomass is the most appreciated application of agricultural residues, especially those that are lignocellulosic [10].
Microbial fermentation is a process in which the strain, culture medium, reactor, and culture conditions combine and interact. Probiotic culture optimization strategies usually focus on these four aspects to obtain higher biomass or metabolites [11]. Probiotics are recognized as food supplements for humans and farm animals. They are also used in aquaculture to inhibit the growth of pathogens and reduce the presence of viruses. These can increase the number of nutrients, enzyme activity, and immune response and improve water quality [12]. They are non-toxic, non-pathogenic living microorganisms that the digestive route can administer to benefit the host [13]. Lactic Acid Bacteria (LAB) are commonly used in the food industry because they provide characteristics of fermented products such as texture, flavor, and smell. They also produce biomass with high probiotic content and LA [14].
L. acidophilus is one of the best-known species of the Lactobacillus genus in the LAB group and exists in the gastrointestinal tract and vagina of humans and animals [15]. It can inhibit the invasion of pathogens and modulate the immune response in vitro and in vivo; it can produce bacteriostatic compounds similar to bacteriocin and acids that lower intestinal pH. Adding L. acidophilus to poultry feed helps prevent the proliferation of pathogenic bacteria and regulates the intestinal flora through competitive exclusion and antagonism [16]. It is a probiotic widely used in fermentation processes; it adapts easily in the intestine due to its tolerance to the acidic conditions of the intestinal tract and its ability to form colonies in the host. It has a great capacity to absorb and catabolize complex carbohydrates that are difficult to digest [17].
Estimating microbial growth kinetics and yield factors is essential to improving microbial analysis [18]. Kinetic models provide advanced insights into microbial growth behavior using detailed experiments and accurate and repeatable mathematical models [19]. Consequently, kinetic modeling becomes a vital tool for optimizing design and control parameters, media reformulation, modifying operating specifications, and developing large-scale fermentation processes for the target metabolite [20].
Considering that ASW has a high content of carbohydrates and nutrients, this research aimed to develop an anaerobic fermentative bioconversion using L. acidophilus to produce biomass rich in lactobacilli and LA. For this reason, the fermentative process was monitored and mathematically modeled (logistic model, Gompertz and Gompertz generalized) at three different agitation speeds (100, 150, and 200 rpm) in a laboratory-scale stirred reactor with a capacity of 5 L for three days, at constant temperature (37 ± 0.5°C).