The use of bio-nanocomposites for liberation of active substances is one of the popular alternatives for safely preserving food. In this context, the objective of this paper was to evaluate the interaction between montmorillonite sodium clay nanoparticles (MMT) and citric acid (CA) on whey protein isolate (WPI) bio-nanocomposites, developed by casting. CA was added to aid in the dispersion of MMT layers. The thermal analysis revealed that the addition of MMT, alone, increased the thermal stability of the bio-nanocomposites, but there was a reduction when combined with CA. The elastic modulus (EM), tensile strength (TS), elongation (E) and puncture strength (PS) increased with increasing MMT content from 0 to 3% by weight, with the consequent reduction of the puncture deformation. The combination of MMT and CA caused a decrease in TS and EM and increase in elongation making the films less rigid, weaker and more extensive due to the plasticizing effect. The improvement of these properties can be attributed to good dispersion/exfoliation of MMT in the WPI matrix, as seen in X-ray diffraction and transmission electron microscopy. The interaction between MMT and CA enabled to obtain bio-nanocomposites with thermal, structural, morphological and mechanical properties for use as packaging materials.48 179188Adhikari, B., Howes, T., Bhandari, B.R., Langrish, T.A.G., Effect of addition of proteins on the production of amorphous sucrose powder through spray drying (2009) Journal of Food Engineering, 94 (2), pp. 144-153Alves, V.D., Mali, S., Beléia, A., Grossmann, M.V.E., Effect of glycerol and amylose enrichment on cassava starch film properties (2007) Journal of Food Engineering, 78 (3), pp. 941-946Anker, M., Berntsen, J., Hermansson, A.-M., Stading, M., Improved water vapor barrier of whey protein films by addition of an acetylated monoglyceride (2002) Innovative Food Science & Emerging Technologies, 3 (1), pp. 81-92Aouada, F.A., Mattoso, L.H.C., Longo, E., New strategies in the preparation of exfoliated thermoplastic starch-montmorillonite nanocomposites (2011) Industrial Crops and Products, 34 (3), pp. 1502-1508(1999) Standard test method for enthalpies of fusion and crystallization of polymers by differential scanning calorimetry (DSC), , ASTM D 3417, American Society Standard Testing and Materials, Philadelphia(2000) Standard practice for conditioning plastics for testing, , ASTM D618-00, American Society Standard Testing and Materials, Philadelphia(2002) Standard test method for tensile properties of thin plastic sheeting, , ASTM D 882-02, American Society Standard Testing and Materials, Philadelphia(2003) Standard test method for transition temperatures of polymers by differential scanning calorimetry, , ASTM D3418-03, American Society Standard Testing and Materials, PhiladelphiaBarreto, P.L.M., Pires, A.T.N., Soldi, V., Thermal degradation of edible films based on milk proteins and gelatin in inert atmosphere (2003) Polymer Degradation and Stability, 79 (1), pp. 147-152Cagri, A., Ustunol, Z., Ryser, E.T., Antimicrobial edible films and coatings (2004) Journal of Food Protection, 67 (4), pp. 833-848Canevarolo, S.V., (2003) Técnicas de caracterização de polímeros, , Artliber, São Paulo, SPChabrat, E., Abdillahi, H., Rouilly, A., Rigal, L., Influence of citric acid and water on thermoplastic wheat flour/poly(lactic acid) blends. I: thermal, mechanical and morphological properties (2012) Industrial Crops and Products, 37 (1), pp. 238-246Chivrac, F., Pollet, E., Schmutz, M., Averous, L., New approach to elaborate exfoliated starch-based nanobiocomposites (2008) Biomacromolecules, 9 (3), pp. 896-900Coupland, J.N., Shaw, N.B., Monahan, F.J., O'Riordan, E.D., O'Sullivan, M., Modeling the effect of glycerol on the moisture sorption behavior of whey protein edible films (2000) Journal of Food Engineering, 43 (1), pp. 25-30Dias, M.V., Machado Azevedo, V., Borges, S.V., Soares, N., de Barros Fernandes, R.V., Marques, J.J., Development of chitosan/montmorillonite nanocomposites with encapsulated α-tocopherol (2014) Food Chemistry, 165 (0), pp. 323-329Djordjevic, I., Choudhury, N.R., Dutta, N.K., Kumar, S., Synthesis and characterization of novel citric acid-based polyester elastomers (2009) Polymer, 50 (7), pp. 1682-1691Fornes, T.D., Paul, D.R., Modeling properties of nylon 6/clay nanocomposites using composite theories (2003) Polymer, 44 (17), pp. 4993-5013Ibanoglu, E., Effect of hydrocolloids on the thermal denaturation of proteins (2005) Food Chemistry, 90 (4), pp. 621-626Kaczmarek, H., Podgórski, A., The effect of UV-irradiation on poly(vinyl alcohol) composites with montmorillonite (2007) Journal of Photochemistry and Photobiology A: Chemistry, 191 (2-3), pp. 209-215Kim, S.J., Ustunol, Z., Sensory attributes of whey protein isolate and candelilla wax emulsion edible films (2001) Journal of Food Science, 66 (6), pp. 909-911Kokoszka, S., Debeaufort, F., Lenart, A., Voilley, A., Water vapour permeability, thermal and wetting properties of whey protein isolate based edible films (2010) International Dairy Journal, 20 (1), pp. 53-60Kumar, A.P., Depan, D., Singh Tomer, N., Singh, R.P., Nanoscale particles for polymer degradation and stabilization-trends and future perspectives (2009) Progress in Polymer Science, 34 (6), pp. 479-515Lavorgna, M., Piscitelli, F., Mangiacapra, P., Buonocore, G.G., Study of the combined effect of both clay and glycerol plasticizer on the properties of chitosan films (2010) Carbohydrate Polymers, 82 (2), pp. 291-298Ma, X., Chang, P.R., Yu, J., Stumborg, M., Properties of biodegradable citric acid-modified granular starch/thermoplastic pea starch composites (2009) Carbohydrate Polymers, 75 (1), pp. 1-8Ma, X.F., Yu, J.G., Ma, Y.B., Urea and formamide as a mixed plasticizer for thermoplastic wheat flour (2005) Carbohydrate Polymers, 60 (1), pp. 111-116Martínez-Camacho, A.P., Cortez-Rocha, M.O., Ezquerra-Brauer, J.M., Graciano-Verdugo, A.Z., Rodriguez-Félix, F., Castillo-Ortega, M.M., Chitosan composite films: thermal, structural, mechanical and antifungal properties (2010) Carbohydrate Polymers, 82 (2), pp. 305-315McHugh, T.H., Huxsoll, C.C., Krochta, J.M., Permeability properties of fruit puree edible films (1996) Journal of Food Science, 61 (1), pp. 88-91Mulvihill, D.M., Ennis, M.P., (2003) Functional milk proteins: Production and utilisation, , Kluwer, New York NY, USANijdam, J., Ibach, A., Eichhorn, K., Kind, M., An X-ray diffraction analysis of crystallised whey and whey-permeate powders (2007) Carbohydrate Research, 342 (16), pp. 2354-2364Park, H.R., Chough, S.H., Yun, Y.H., Yoon, S.D., Properties of starch/PVA blend films containing citric acid as additive (2005) Journal of Polymers and the Environment, 13 (4), pp. 375-382Park, S.I., Zhao, Y., Incorporation of a high concentration of mineral or vitamin into chitosan-based films (2004) Journal of Agricultural and Food Chemistry, 52 (7), pp. 1933-1939Prakash Maran, J., Sivakumar, V., Sridhar, R., Prince Immanuel, V., Development of model for mechanical properties of tapioca starch based edible films (2013) Industrial Crops and Products, 42 (0), pp. 159-168Ramos, O.L., Reinas, I., Silva, S.I., Fernandes, J.C., Cerqueira, M.A., Pereira, R.N., Effect of whey protein purity and glycerol content upon physical properties of edible films manufactured therefrom (2013) Food Hydrocolloids, 30 (1), pp. 110-122Ramos, O.L., Silva, S.I., Soares, J.C., Fernandes, J.C., Poças, M.F., Pintado, M.E., Features and performance of edible films, obtained from whey protein isolate formulated with antimicrobial compounds (2012) Food Research International, 45 (1), pp. 351-361Ryan, M., McEvoy, E., Duignan, S., Crowley, C., Fenelon, M., O'Callaghan, D.M., Thermal stability of soy protein isolate and hydrolysate ingredients (2008) Food Chemistry, 108 (2), pp. 503-510Shi, R., Zhang, Z., Liu, Q., Han, Y., Zhang, L., Chen, D., Characterization of citric acid/glycerol co-plasticized thermoplastic starch prepared by melt blending (2007) Carbohydrate Polymers, 69 (4), pp. 748-755Sobral, P.J.A., Menegalli, F.C., Hubinger, M.D., Roques, M.A., Mechanical, water vapor barrier and thermal properties of gelatin based edible films (2001) Food Hydrocolloids, 15 (4-6), pp. 423-432Sothornvit, R., Hong, S.-I., An, D.J., Rhim, J.-W., Effect of clay content on the physical and antimicrobial properties of whey protein isolate/organo-clay composite films (2010) LWT - Food Science and Technology, 43 (2), pp. 279-284Sothornvit, R., Rhim, J.-W., Hong, S.-I., Effect of nano-clay type on the physical and antimicrobial properties of whey protein isolate/clay composite films (2009) Journal of Food Engineering, 91 (3), pp. 468-473Souza, D.H.S., Borges, S.V., Dias, M.L., Andrade, C.T., Structural evaluation and thermal properties of poly(lactic acid) and different synthetic micas nanocomposites (2012) Polymer Composites, 33 (4), pp. 555-561Stojanović, Ž., Katsikas, L., Popović, I., Jovanović, S., Jeremić, K., Thermal stability of starch benzoate (2005) Polymer Degradation and Stability, 87 (1), pp. 177-182Tunç, S., Duman, O., Preparation and characterization of biodegradable methyl cellulose/montmorillonite nanocomposite films (2010) Applied Clay Science, 48 (3), pp. 414-424Vartiainen, J., Tammelin, T., Pere, J., Tapper, U., Harlin, A., Biohybrid barrier films from fluidized pectin and nanoclay (2010) Carbohydrate Polymers, 82 (3), pp. 989-996Wakai, M., Almenar, E., Effect of the presence of montmorillonite on the solubility of whey protein isolate films in food model systems with different compositions and pH (2015) Food Hydrocolloids, 43 (0), pp. 612-621Wang, K., Chen, L., Kotaki, M., He, C., Preparation, microstructure and thermal mechanical properties of epoxy/crude clay nanocomposites (2007) Composites Part A: Applied Science and Manufacturing, 38 (1), pp. 192-197Wang, S.F., Shen, L., Tong, Y.J., Chen, L., Phang, I.Y., Lim, P.Q., Biopolymer chitosan/montmorillonite nanocomposites: preparation and characterization (2005) Polymer Degradation and Stability, 90 (1), pp. 123-131Wang, N., Zhang, X., Han, N., Bai, S., Effect of citric acid and processing on the performance of thermoplastic starch/montmorillonite nanocomposites (2009) Carbohydrate Polymers, 76 (1), pp. 68-73Yang, J.P., Yang, G., Xu, G., Fu, S.Y., Cryogenic mechanical behaviors of MMT/epoxy nanocomposites (2007) Composites Science and Technology, 67 (14), pp. 2934-2940Zhang, Q.-X., Yu, Z.-Z., Xie, X.-L., Naito, K., Kagawa, Y., Preparation and crystalline morphology of biodegradable starch/clay nanocomposites (2007) Polymer, 48 (24), pp. 7193-7200Zhong, Y., Song, X., Li, Y., Antimicrobial, physical and mechanical properties of kudzu starch-chitosan composite films as a function of acid solvent types (2011) Carbohydrate Polymers, 84 (1), pp. 335-342Zhou, P., Labuza, T.P., Effect of water content on glass transition and protein aggregation of whey protein powders during short-term storage (2007) Food Biophysics, 2 (2-3), pp. 108-116