Pure zein films are known to be very hydrophobic, but are inappropriate for edible coating applications because of their brittle nature. In an attempt to improve the flexibility of these coatings, the influence of low concentrations of oleic acid (OA) as a plasticiser on mechanical, topographical and wetting properties of zein thin films was evaluated. Films were first obtained by casting from aqueous ethanol solutions with 4.0% in mass of zein and additions of 0%; 0.25%; 0.50%; and 1.0% (w ⁄ w) of OA. The results indicate an improvement in mechanical properties with increasing plasticiser leading to a reduction in the elastic modulus. An increase in the elongation at break has been observed, but with minor influence on tensile strength. All plasticised zein films have similar initial contact angle (approximately 69°) with a timedependent receding drop behaviour. An increase in plasticiser concentration increases film's affinity towards water. As measured by atomic force microscopy, a consistent linear relation (R 2 = 0.991) was estimated between film composition and surface adhesion and consequently on the hydrophilicity. Surface topography also varied with plasticiser addition, becoming smoother as the OA concentration increases. When tested as an edible coating on pears (Pyrus communis L.), a formulation with 0.25% wt of plasticiser achieved the best results in preserving the pear mass as measured during the evaluated storage time (12 days) at room temperature. A 0.5% concentration of plasticiser had no influence and higher amounts resulted in a reduction in fruit protection.
IntroduçãoPor serem baratas, resistentes, versáteis e apresentarem alta flexibilidade, as embalagens de polímeros sintéticos como o PET, o poliestireno, o polietileno e demais olefinas têm sido utilizadas na indústria de alimentos por mais de 50 anos. A quase totalidade desses materiais é contudo, de descartabilidade rápida e já ocupam de 15% a 20% do volume do lixo urbano [1] , causando impactos ambientais de diferentes magnitudes e de difícil remediação. Embora haja ultimamente uma conscientização e motivações de ordem pública para a recuperação e o reaproveitamento de embalagens plásticas, sua reciclagem ainda é incipiente e não é superior a 15% do total descartado [2] . Nos últimos anos o incentivo ao uso de materiais biodegradáveis e ambientalmente amigáveis, tais como plásticos de origem bacteriana como os da família dos polihidroxialcanoatos (PHA), os derivados de proteínas ou aqueles base em amido termoplástico e demais polissacarídeos têm crescido e tendem a substituir gradualmente e, em determinadas aplicações, os polímeros sintetizados a partir do petróleo.Dentre os chamados polímeros naturais, um material de interesse para a confecção de filmes para emprego em embalagens para contato com alimentos são os derivados de zeínas. As zeínas, ou prolaminas, são proteínas de reserva compostas por vários polipeptídeos que representam mais de 50% da massa total das proteínas presentes no endosperma do milho (Zea mays). Essas proteínas têm baixa qualidade nutricional e são indicadas para usos em aplicações diversas que não alimentícias, principalmente em farmacêutica como material de encapsulamento de drogas [3] , na impermeabilização de papeis e embalagens cartonadas [4] e associadas a polímeros sintéticos para minimizações de contato e deteriorações em alimentos [5] . Além disso, essas proteínas apresentam alto grau de polimerização, superiores aos encontrados nas poliamidas e no poliéster [6] . Zeínas são consideradas proteínas globulares e constituídas de frações classificadas de acordo com sua massa relativa (Mr) e solubilidade, como zeínas α, γ, β e δ. As zeínas α são as mais abundantes constituindo aproximadamente 80% de todas as zeínas e correspondente as proteínas com Mr = 19 kDa (Z19) e Mr = 22 kDa (Z22) [7] . Essas proteínas contêm 10 segmentos helicoidais sucessivos dispostos de forma antiparalela que são estabilizados por ligações de hidrogênio [8] . Por serem ricas em resíduos de aminoácidos apolares (leucina, prolina, alanina, glicina, valina e outros), as zeínas α são altamente hidrofóbicas, portanto insolúveis em água, mas solúveis em meios alcoólicos [9] . Por conseguinte filmes produzidos a partir da diluição de zeínas puras apresentam também caráter hidrofóbico, o que pode ser potencialmente interessantes para aplicações como revestimentos ou barreiras à umidade e ao vapor de água. De um modo geral, filmes à base de zeínas apresentam também uma boa barreira ao transporte de oxigênio, dióxido de carbono e demais compostos voláteis [10] . Os filmes puros de zeínas são extremamente frágeis have...
The effectiveness of corn‐zein edible coatings in blocking oil oxidation on macadamia nuts was studied by using high‐resolution Hydrogen Nuclear Magnetic Resonance (1H NMR) spectroscopy. Formulations with a zein concentration of 4.0% by weight and oleic acid (OA) as a plasticizer in different proportions were evaluated. Accelerated oxidation was conducted by using an O2‐reactor in a 60C water bath. The oil extracted from the nuts at the beginning of the study and after 10 and 30 days into the experiment were analyzed. Results from the 1H NMR analysis exhibited that the major products formed by lipid oxidation were conjugated dienes. The coatings were imperceptible to the naked eye, but textural changes were visible under scanning electron microscopy. The best coating formulation was a combination of zein with 0.25% OA, which completely inhibited oxidation. Plasticizer addition of 0.50 and 1.00% wt, however, had a negative effect on the coating's property as a barrier against oxygen, resulting in higher rates of oxidation than in the uncoated samples. PRACTICAL APPLICATIONS Hydrophobic zein‐based edible coatings, as used in this study, are capable of acting as barriers against oxygen and water vapor by reducing, or even eliminating, undesirable oxidation in unshelled macadamia nuts during long‐term storage. The results of our study have demonstrated the ease of forming stable coatings to reduce oxidative fatty rancidity. The films are invisible to the naked eye and do not produce visible changes in the texture and the color of the coated nuts. This study indicates that zein/oleic acid edible coatings are feasible to use for large‐scale applications, providing an alternative way to improve the oxidative stability and eventually to lengthen the shelf life and quality of macadamia nuts.
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