# Environmental Impact of Corn Tortilla Production: A Case Study
## 2 Materials and Methods
### LCA Approach and Assumptions  
The process studied included only the corn-nixtamalisation-dough-tortillas production. The functional unit is one kilogram of tortillas packed in kraft paper. In the limits of the system (Figure 2), it was considered from the obtaining of raw materials for the production of corn, to the elaboration of the already packaged product, with a focus on the cradle to the door, i.e., \"cradle to gate\". The stages of obtaining the rest of the raw materials, the use of the product, and the disposition of the residues at the end of life were excluded. In this study no comparisons were made between systems. A producer in Quiroga, Michoacan was surveyed in the year of 2017 to understand the process of corn production. A tortilla shop, equipped with a nixtamal mill and an automatic tortilla maker was studied, which is representative of the business class in Mexico that complies with the official standard for dough, tortillas and prepared flours [26].  
Figure 1: Automatic machine or “tortilladora”. Source: TAPSA [22].  
Figure 2: Limits of the system in the life cycle of the tortilla.  
The stages considered in the LCA are:
* Production of raw material. The production of white corn is considered because it is the raw material for the production of tortillas. It includes the preparation of the earth up to the drying of the seed. The preparation of the land uses the conservation tillage technique with the use of an agricultural tractor, which includes a ploughing phase, two narrowing phases and sowing. Mechanical planners are used to plant the seed and the first fertilisation of the soil is carried out, adding the total amount of the required phosphorus and 40% of the total nitrogen. The second fertilisation consists of adding the remaining nitrogen to the corn. Pest control is carried out by sprinklers coupled to the tractor. Harvesting is done with agricultural harvesters to later go through a grain drying process. Once dry, the grain is packed in sacks and distributed to the tortilla shops.
* Transport of raw material. This includes the distribution of the pesticides, fertilisers and supplies necessary to produce corn, as well as those needed for the production of tortillas, such as corn and calcium hydroxide in the nixtamalisation stage.
* Nixtamalisation. The technique is based on the cooking of the corn grains in a calcium hydroxide solution (Ca(OH)\({}_{2}\)) or lime. For each kg of corn, 15 g of lime are used in 3 L of boiling water (85-100 \({}^{\circ}\)C) for at least 45 min, depending on the hardness of the grain [27]. The mixture is left to rest for 8-24 h in the cooking solution (knowing as nejayote), then washed two or three times with clean water to remove the nejayote, the pericarp of the grain and any impurities. The result of alkaline cooking is the nixtamalised or nixtamal corn. Subsequently, it is passed to a wet milling process to obtain the dough (with 48-55% humidity).
* The cooking of the dough. The dough is baked in an automatic tortilla machine which moulds, cuts and bakes. The machines are fuelled by natural gas or propane as a fuel source and the mechanical system is powered by electricity. The dough is fed through a hopper where there are two rollers (kneading) that rotate in opposite directions, directing the dough to the extruders which, in turn, feed the moulders, where it is shaped flat and then cut out by some blades. The discs of dough move along a hot iron caterpillar to be cooked at temperatures of 280-300 \({}^{\circ}\)C with a residence time of 20 to 40 s. Each side is cooked twice, at the end the tortilla is obtained [28]. Approximately 30 g of dough is needed for each tortilla. The complete process of the production system is shown in Figure 3.