  Citation: Choque-Quispe, D.; Ramos-Pacheco, B.S.; Choque-Quispe, Y.; Aguilar-Salazar, R.F.; Mojo-Quisani, A.; Calla-Florez, M.; Solano-Reynoso, A.M.; Zamalloa-Puma, M.M.; Palomino-Malpartida, Y.G.; Alcarraz-Alfaro, T.; et al. Storage Conditions and Adsorption Thermodynamic Properties for Purple Corn. Foods 2022, 11, 828. https://doi.org/10.3390/ foods11060828 Academic Editor: Laura Gazza Received: 7 February 2022 Accepted: 9 March 2022 Published: 14 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). foods Article Storage Conditions and Adsorption Thermodynamic Properties for Purple Corn David Choque-Quispe 1, * , Betsy S. Ramos-Pacheco 1 , Yudith Choque-Quispe 2 , Rolando F. Aguilar-Salazar 3 , Antonieta Mojo-Quisani 4 , Miriam Calla-Florez 4 , Aydeé M. Solano-Reynoso 5 , Miluska M. Zamalloa-Puma 6 , Ybar G. Palomino-Malpartida 7 , Tarcila Alcarraz-Alfaro 7 and Alan Zamalloa-Puma 6 1 Department of Agroindustrial Engineering, Universidad Nacional José María Arguedas, Andahuaylas 03701, Peru; bsramos@unajma.edu.pe 2 Department of Environmental Engineering, Universidad Nacional José María Arguedas, Andahuaylas 03701, Peru; ychoque@unajma.edu.pe 3 Department of Basic Sciences, Universidad Nacional José María Arguedas, Andahuaylas 03701, Peru; raguilar@unajma.edu.pe 4 Department of Agroindustrial Engineering, Universidad Nacional de San Antonio Abad del Cusco, Cusco 08000, Peru; antonieta.mojo@unsaac.edu.pe (A.M.-Q.); miriam.calla@unsaac.edu.pe (M.C.-F.) 5 Department of Environmental Engineering, Universidad Tecnológica de los Andes, Andahuaylas 03701, Peru; ayma_21@hotmail.com 6 Department of Physics, Universidad Nacional de San Antonio Abad del Cusco, Cusco 08000, Peru; miluska.zamalloa@unsaac.edu.pe (M.M.Z.-P.); alan.zamalloa@unsaac.edu.pe (A.Z.-P.) 7 Department of Chemical Engineering, Universidad Nacional de San Cristobal de Huamanga, Ayacucho 05000, Peru; ybar.palomino@unsch.edu.pe (Y.G.P.-M.); tarcila.alcarraz@unsch.edu.pe (T.A.-A.) * Correspondence: dchoque@unajma.edu.pe Abstract: Adsorption isotherms provide insight into the thermodynamic properties governed by food storage conditions. Adsorption isotherms of purple corn of the Canteño variety were evaluated at 18, 25, and 30 ◦ C, for the equilibrium relative humidity (ERH) range between 0.065 and 0.95. The equilib- rium moisture (X e ) was determined by the continuous weight-change method. Seven mathematical models of isotherms were modeled, using the coefficient of determination R 2 , mean absolute error (MAE), and estimated standard error (ESE) as the convergence criterion. Thermodynamic parameters such as isosteric heat (q st ), Gibbs Free Energy (ΔG), differential entropy (ΔS), activation energy (E a ), and compliance with the isokinetic law were evaluated. It was observed that the adsorption isotherms presented cross-linking around 75% ERH and 17% X e , suggesting adequate storage conditions at these values. The GAB and Halsey models reported better fit (R 2 > 97%, MAE < 10%, ESE < 0.014 and random residual dispersion). The reduction of X e from 17 to 7%, increases q st , from 7.7022 to 0.0165 kJ/g, while ΔG decreases considerably with the increase in X e , presenting non-spontaneous endergonic behavior, and linear relationship with ΔS, evidencing compliance with the isokinetic theory, governed by q st . E a showed that more energy is required to remove water molecules from the upper layers bound to the monolayer, evaluated using C GAB . The models predicted the storage conditions, and the thermodynamic parameters show the structural stability of the purple corn grains of the Canteño variety during storage. Keywords: purple corn; adsorption isotherm; isosteric heat; Gibbs free energy; differential entropy; activation energy; isokinetic theory 1. Introduction Pigments in the grains of purple corn (Zea mays L.), in addition to being used as natural colorants, are attributed biological functions as antioxidants [1,2], and are found in mainly the pericarp, aleurone, endosperm, and embryo of corn [3–6]. These compounds present nutritional interest for their contribution to human health due to their beneficial properties [7,8]. Similarly, purple corn, due to its color, is used in food, cosmetic and Foods 2022, 11, 828. https://doi.org/10.3390/foods11060828 https://www.mdpi.com/journal/foods