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- Effect of microwave heating with different exposure times on physical and chemical parameters of olive oilPublication . Malheiro, Ricardo; Oliveira, Ivo; Vilas-Boas, Miguel; Falcão, Soraia; Bento, Albino; Pereira, J.A.This study reflects the effect of different microwave heating times (1, 3, 5, 10 and 15 min) on physical and chemical characteristics of three Portuguese olive oils from three protected designation of origin (PDO), "Azeite de Tras-os-Montes PDO", "Azeites da Beira Interior PDO", and "Azeite de Moura PDO". The parameters evaluated were free acidity, peroxide value, specific extinction coefficients (K-232 and K-270), color and chlorophylls and carotenoids content. A differential pulse voltammeter was also used to monitor the changes in a-tocopherol content. The results showed that microwave heating produce losses in the quality of the different analysed olive oils. The heating time did not promote the occurrence of hydrolysis in the samples since no changes in free acidity values were found. All other parameters were affected by exposure time in a similar way: in the firsts 3 min no marked changes were observed, after that the quality of the oil decrease significantly. The microwave heating time also affects the total chlorophylls, carotenoids and alpha-tocopherol contents which clearly decreased as long as the exposure time increases. After 15 min of heating the electrochemical signal, due to the alpha-tocopherol, disappear completely in the voltamogram.
- Effect of olive leaves addition during the extraction process of overmature fruits on olive oil qualityPublication . Malheiro, Ricardo; Casal, Susana; Teixeira, Hélder; Bento, Albino; Pereira, J.A.The harvest period is one of the most important factors influencing olive oil quality. This period is extended for several months and the late-extracted olive oils are characterized by quality loss and reduced resistance to oxidation. The aim of this work was to verify the effect of olive leaves addition during the oil extraction process in the olive oils quality and composition. In two consecutive years (2009 and 2010), different olive leaves amounts (1%, 2.5%, 5% and 10% w/w) were added during the extraction process of cv. Cobrançosa olive fruits, collected in the late season. Standard quality parameters, oxidative stability, fatty acids profile, tocopherols, chlorophylls, and carotenoids contents were evaluated. Olive leaves addition induces a slight increase in acidity, peroxide value, K232, and K270 without compromising olive oils classification, but the resistance to oxidation was significantly improved. Vitamin E increased nearly 30% with 10% of leaves added mainly due to the considerable increase in α-tocopherol. A similar effect was observed in the contents of chlorophylls (chlorophyll a and pheophytin a) and carotenoids (lutein and β-carotene), that attributed a more intense greener pigmentation and enhanced nutritional attributes. Significant correlations were observed for several parameters with the amounts of leaves added. Moreover, leaves addition modified the characteristics and composition of the olive oils in a way that was possible to discriminate and to classify each group.
- Can tea extracts protect extra virgin olive oil from oxidation during microwave heating?Publication . Malheiro, Ricardo; Casal, Susana; Lamas, Hugo; Bento, Albino; Pereira, J.A.Olive oil is a key ingredient in worldwide cuisine. Microwave cooking is also increasingly used as a timesaving technology but it induces severe changes in the lipids, such as hydrolysis and oxidative reactions that lead to quality and nutritional losses. In this sense, the present work intends to verify the potential increased olive oil stability under microwave cooking induced by white and green tea aqueous extracts, rich in antioxidants. Quality parameters (free acidity, peroxide value, K232, K270 and ΔK), composition (fatty acids profile, tocopherols and total phenols content), antioxidant activity and oxidative stability were studied during different microwave heating exposure times (0, 1, 3, 5 and 10 min). Until the first 3 min of heating the tea extracts protect olive oil from the oxidative process. However, with higher heating periods the extracts were pro-oxidants, mainly green tea extracts. Globally, and independently from the addition of tea extracts, the olive oils suffered drastic changes in their quality and composition. PUFA content decreased with exposure time as well as antioxidant activity, total phenols and total vitamin E content, and oxidative stability, with higher losses in the olive oils with green tea extracts. Positive correlations were established between the quality parameters and the increasing heating time, and negative correlations with the composition and bioactive and stability indexes.