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- Olive oil stability under deep-frying conditionsPublication . Casal, Susana; Malheiro, Ricardo; Sendas, A.; Oliveira, Beatriz; Pereira, J.A.The suitability of different commercial olive oil categories for domestic frying was investigated. Oil samples were taken every 3 h of frying and evaluated for free acidity, peroxide and p-anisidine values, specific extinction coefficients, oxidative stability, fatty acids, vitamin E, b-carotene and total phenols, until the total polar compounds achieved the maximum legal value (25%). All olive oils were fried during more time than the commercial vegetable oil blend taken for comparison (from 24 to 27 h, against 15 h). The extra-virgin Protected Designation of Origin (PDO) olive oil was characterized by reduced levels of oxidation and hydrolysis, and superior amounts of minor antioxidant compounds. The ‘‘olive oil” commercial category behaves similarly, but ‘‘Cobrançosa” olive oils performance was slightly worse, and clearly different between years, highlighting the importance of blending different cultivars. The vegetable oil, despite containing significantly higher amounts of vitamin E, was highly susceptible to oxidation under frying conditions when compared to all olive oils. The results also show that the chemical composition of olive oils, particularly the amount of natural antioxidants, are important parameters in their predictive behavior along the frying process, but mostly that olive oil is clearly resistant to frying conditions, independently to the commercial category chosen.
- 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.
- Unexplored olive cultivars from the Valencian Community (Spain): some chemical characteristics as a valorization strategyPublication . Salazar-García, Domingo C.; Malheiro, Ricardo; Pereira, J.A.; López-Cortés, Isabel; Salazar-García D.The olive processing industry has till date been dominated by a small group of cultivars, leading to the possibility of some olive cultivars becoming extinct in the near future. In this study, we determined the composition of some chemical components in the olive oils from 31 minor olive cultivars of the Valencian Community. Our main aim was to identify suitable cultivars, which could produce differentiated olive oils, thus aiming towards their valorization. The average oil content of minor olive cultivars was found to be good, with some of them reporting approximately 60% (dry basis). On average, the total phenolic content was 229 mg kg− 1, with cv. Mas Blanc reporting the highest content (570 mg kg− 1). Among the various tocopherols found in olives, α-tocopherol was the main constituent, with a maximum concentration of 290.6 mg kg− 1. Linoleic acid was the main polyunsaturated fatty acid and varied between 3.4% (cv. Del Pomet) and 16.9% (cv. Blanqueta Enguera). Special attention needs to be paid to the composition of sterols, since some olive oils exceeded the limits established for some sterols by the current European legislation. Some of the cultivars studied were highly productive, and originated differentiated olive oils with a rich composition of antioxidants and essential fatty acids. In some cases, these beneficial compounds were higher than those of commercial oils obtained from the most common cultivars worldwide. These results could contribute to the commercial exploitation of some of the studied cultivars.
- Improvement of sensorial and volatile profiles of olive oil by addition of olive leavesPublication . Malheiro, Ricardo; Rodrigues, Nuno; Bissaro, Camila Andressa; Leimann, Fernanda Vitória; Casal, Susana; Ramalhosa, Elsa; Pereira, J.A.The changes caused by the addition of olive leaves (0, 5, and 10%) during the extraction of olive oil and malaxation time (20, 30, and 30 min) in the volatile profile and sensory attributes of olive oil from cv. Cobrançosa were studied. To investigate such transformations, a central composite designs from the Response Surface Methodology (RSM) was used, retrieving 13 runs combining leaf percentages and malaxation times. Each run was extracted in triplicate (39 olive oils overall). Sensory attributes were improved to leaves addition, mainly green and fruitiness attributes in olfactory and gustatory-olfactory sensations, but high malaxation times ( > 30 min) reduced pungent and bitter notes. Leaves addition increased the amounts of total volatiles, particularly the GLV's (green leaves volatiles) (E)-2-hexenal, (Z)-3-hexenal, and (Z)-3-hexenyl acetate, directly correlated with the improved sensory attributes. RSM models a showed positive linear effect with leaves addition, but a negative effect with malaxation time. These results suggest the use of olive leaves as effective odorants for the olive mill industry, while enabling the reduction of malaxation times and by-product amounts. Practical applications: The results obtained clearly open new lines of research to use olive leaves, a sub-product of olive oil extraction, in a valuable way. Olive leaves can be used as natural sources of odorants for olive oils. Furthermore, their use during the extraction of olive oils from overmature olives may also lead to an improvement of the volatile fraction and provide enhanced sensory properties to the consumers, thus conferring an added value to these oils. Another important practical application is the extraction process. In our work, we advise to optimize both the percentage of leaves and the malaxation time as much as possible, as they facilitate both sensory and volatile fractions of the extracted olive oils. Sensory attributes are improved to leaves addition, mainly green and fruitiness attributes in olfactory 17 and gustatory-olfactory sensations, but high malaxation times ( > 30 min) reduced pungent and bitter 18 notes. Leaves addition increase the amounts of total volatiles, particularly the GLV's (green leaves 19 volatiles) (E)-2-hexenal, (Z)-3-hexenal, and (Z)-3-hexenyl acetate, directly cor related with the improved 20 sensory attributes. RSM models a show positive linear effect with leaves addition, but a negative effect 21 with malaxation time. These results suggest the use of olive leaves as effective odorants for the olive mill 22 industry, while enabling the reduction of malaxation times and by-product amounts.