**2.** *G. max* **(L) Merrill "soy": general characteristics**

The scientific name for soybeans is *G. max (L.) Merrill.* It is of Asian origin, native to north and central China [1]. It belongs to the *Fabaceae* family and the subfamily *Faboideae.* Depending on the country, it is popularly known by different names: soya (Portugal, France and England), soia (Italy) and sojabohne (Germany) [2]. It is an annual, herbaceous, shrubby species and its vegetative cycle ranges from three to seven months, depending on environmental conditions [3]. The optimal circumstances for its growth are the subtropical regions due to their permanently humid weather [4]. Morphologically, it is an erect and branched plant, reaching 80 to 100 cm in height. Almost all varieties show pubescence on the stems, leaves and pods. The basal leaves are unifoliate, oval, with a short half-life; from the second pair of leaves their folial development is alternate, trifoliate with oval or lanceolate leaflets, narrow or wide, depending on the variety, green in vegetative state and at maturity they turn yellow-brown.

After the vegetative growth period, the duration of which depends on the cultivar, latitude and environmental conditions (length of day and temperature) in addition to cultural practices, the soybean plant enters the reproductive phase. The flower is perfect (hermaphrodite), with axillary budding, developing a grouping of 2 to 35 flowers, which can be white or purple [5]. Palmer et al. [6] and Takahashi et al. [5] established that the color of soybean flowers is controlled mainly by six genes (W1, W2, W3, W4, Wm and Wp). Under the W1 genotype, the combination with W3W4 results in deep purple flowers, W3w4 has pale purple flowers or with purple coloration at the base of the petal, w3W4 produces purple flowers and w3w4 has almost white flowers [7].

The seed develops in pods 4 to 6 cm long; each pod has between 2 to 3 seeds. The seed has a shape that varies from round to subtly oval and can present different colors depending on the variety; they can be essentially yellow, black or green. The root system is pivotal and can reach a depth of 15–30 cm; it is capable of nodulation in symbiosis with bacteria of the genus *Rhizobium* [8].

Nutritionally, soy is an excellent biotype; since it contains almost everything the human being needs for his diet. It has between 38 and 40% protein, 18% fat due to its polyunsaturated nature, 15% carbohydrates and 15% fiber. It supplies most of the amino acids needed for protein synthesis, predominantly Lysine [9]. It is the only protein of vegetable origin with an amino acid score of 100%, when compared to proteins of animal origin, although it is limited in the amino acid methionine, so it is important that it is combined with a cereal (rice, quinoa, oats) or with animal proteins to be able to form complete proteins [10, 11]. It has a high concentration of potassium and is a good source of magnesium, phosphorus, iron, calcium, manganese and phosphates. It also provides vitamins such as vitamins E and B6 [9, 12].

Soy contains isoflavones, which are mainly found in roots and seeds [13]. The concentration ranges between 1.2 and 4.2 mg/g dry weight, depending on the characteristics of the soil in which it is grown, the climate, and the plant maturation at the time of harvesting the seeds [14]. Like other phenolic compounds, soy isoflavones are mainly found as glycosylated conjugates (> 80%), which are absorbed in the intestine and have low estrogenic activity [15]; only after hydrolysis do they acquire their maximum bioavailability and biological activity [16].

## **2.1 Soybean cultivation characteristics**

The cultivation of soy (*G. max*) worldwide is one of the most important due to its profitability and high demand for the product and its derivatives [17]. To improve soybean crop yields, a set of variables such as genetic aspects, nutrient availability, as well as other factors (crop rotation) must be analyzed, with the aim of achieving sustainability in production.

Pérez [18] assures that in order for this variety to obtain adequate growth and yield, it needs a significant amount of available nutrients so that it is able to yield good grain production. The nutrient requirements per ton of harvested soybean grain exceeds that one needed by other field crops such as corn or wheat.

Among the macronutrients and micronutrients necessary for its development are: phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), zinc (Zn), sulfur (S), boron (B), copper (Cu) and organic matter (OM) [17]. According to Ortiz [19], soy contains high concentrations of nitrogen (N) in the grains and in the plant. Biological nitrogen fixation (BNF) increases as the crop develops, in this way the root nodules transfer between 30 and 50% of the fixed N to the vegetative stages, 80–90% between flowering and the formation of the fruit, reaching the maximum contribution during the filling of the grains [20].

#### **2.2 Industrial use of soy**

Soy is industrially used for the production of oil for both human and animal consumption. From the processed grain, 20% crude oil and 75% soy flour are obtained. Most of the soy flour is used as raw material for animals. Small percentages of protein are processed for human consumption in the form of soy milk, soy flour (SF), soy protein concentrate (SPC), tofu, and many retail food products [21].

Soy is also used for the production of industrial chemicals such as biodiesel, bio-composites, candles, ink, and wood. It is also used to produce synthetic wood floors and interior plywood. Soy products and other materials obtained from natural resources can be used as ingredients for the production of adhesives for wood due to their ability to bond with different materials [21]. Biodegradable soy-based adhesives are considerably less polluting for the environment than petrochemical adhesives [22].

Soybean residual biomass (a low-cost residue resulting from oil extraction) was functionalized with an industrial sulfur-based chelating agent, which is a precipitation agent used in industrial wastewater treatment. This biomass combination and chelating agent is used for removing heavy metals from aqueous solution, such as Pb (II), Ni (II) and Cu (II) [23].

Various derived by-products from the soy processing industry are used for the manufacture of food, cosmetics and cleaning products. Also, soy is one of the most widely used crops for the production of biodiesel [24].

Soy production represents a very important fraction of the PBI of the entire Southern Common Market (Mercosur) agribusiness, with great economic importance for these countries. Practically, the Mercosur countries make up 42% of

the total soy planted in the world, compared to 33% planted in the US, satisfying together the growing world demand [25].
