The cost of starting materials for the production of biodiesel is typically 75 % of the final retail price. Oils previously used for frying, waste frying oils (WFO), are a very suitable resource. Repetitive use of oil for frying foods involves high temperature, moisture and aeration for extended periods. The most important deterioration processes triggered by these conditions are hydrolysis and oxidation. In this study, 24 WFO samples of different origins were analyzed and classified as potential starting materials for biodiesel production using three quality parameters representing the main factors that affect the conversion of WFO. These parameters were: acid value, content of polar compounds and content of polymers, which varied in the ranges from 0.2 to 7.6, 14.9 to 43.2 and 0.9 to 15.2 %, respectively. Ester content obtained using conventional transesterification (TE) for WFO conversion decreased with increased levels of WFO deterioration determined by any of the three parameters noted above. TE products obtained had ester content between 81.4 and 95.7 %. Total ester content of a WFO sample with relatively low %AV could be increased to 96.5 % using a twostage base catalysis TE. Finally, conversion of WFO samples resulted in ester contents of 89.0 and 91.3 %, respectively, when transesterified by conventional TE. After blending up to 10 % with refined oil, the ester content achieved was near 96.5 %. Thus, the blending represents an alternative for obtaining a product with suitable ester content.

RESUMENDeterioro del aceite durante el almacenamiento de los hígados de merluza en comparación con el del aceite extraído de ellos.Los hígados de merluza (Merluccius hubbsi) son un subproducto de la industria fileteadora y se utilizan como fuente de aceite. En este trabajo se estudia el aceite contenido en dichos hígados durante su almacenamiento a 4 o C y a -20 o C frente a la del aceite extraído de ellos y guardado en iguales condiciones. El aceite contenido en los hígados almacenados a -20 o C, se oxida en un par de semanas; el aceite extraído de ellos y almacenado en iguales condiciones se conserva durante varios meses. El problema principal radica en que, una vez que el aceite se deteriora en los hígados, no es posible estabilizarlo por el agregado de antioxidantes.Para obtener un aceite de buena calidad es imprescindible que los hígados se almacenen a una temperatura menor de -20 o C, que su aceite se extraiga lo antes posible, que se le adicione de antioxidantes adecuados y que se le almacene refrigerado. PALABRAS-CLAVE: Aceite de hígado de merluza -Deterioro oxidativo -Enranciamiento SUMMARYComparative study of oil deterioration during the storage of hake liver and its oil.Hake liver (Merluccis hubbsi) is a by-product of the fillet industry and is used as a source of oil . In the present paper the oil content of livers stored at 4 o C and -20 o C is studied compared to that of the oil extracted from livers and stored under the same conditions. Oil contained in livers stored at -20 o C was oxidized in two weeks; the oil extracted from livers and stored under the same conditions was preserved for several months. The problem is that once the oil becomes deteriorated in livers it is not possible to stabilize it by adding antioxidants. Thus it is necessary to store livers at temperatures lower than -20 o C, to extract the oil as soon as possible, to add the correct antioxidants and to store it under refrigeration, in order to obtain a high quality oil. KEY-WORDS: Hake liver oil -Oxidative deteriorationRancidity. INTRODUCCIÓNUno de los recursos pesqueros más explotados en el Uruguay es la merluza (Merluccius hubbsi), la que se encuentra distribuida en el Río de la Plata y en el Océano Atlántico Sudoccidental. La industria del fileteado de merluza es importante: los filetes congelados se comercializan en el mercado interno y se exportan. Los hígados son un subproducto que se suele desechar.Los hígados de esta especie de merluza son relativamente grandes ya que representan un 7% del peso del pescado entero. Como en la mayoría de los peces, el hígado es el órgano de depósito de los lípi-dos y éstos contienen entre 35% y 60% en peso de un aceite muy rico en ácidos grasos poliinsaturados de la familia omega-3 (Rodríguez et al., 1993;Grompone, 1992). De acuerdo con Méndez y Grompone (1993), los pesos de los hígados de merluza adulta sufren importantes variaciones según la estación del año (entre 33 y 58 gramos), las que se deben, fundamentalmente, a cambios en su contenido lipídico (entre 14 y 35 gramos de aceite po...

Beef tallow is a byproduct of the slaughter industry. As a consequence, meat producer countries obtain a high amount of this low value-added fatty material. In Uruguay, it is generally used for food purposes or for biodiesel production. Globally, around half of the beef tallow produced worldwide is used for the manufacturing of food. To the best of our knowledge there are no published studies concerning the stability of beef tallow when exposed to high temperatures. The aim of this work was to study some Uruguayan beef tallow brands and compare its stability with that of the most frequently used frying oils in Uruguay (sunflower high oleic, rice bran and sunflower oil) to assess its suitability for frying. Stability was assessed by the oxidative stability index and thermoxidation in absence of food. Even though beef tallow's inherent stability indicated that it should be highly stable to oxidation, the majority of the analyzed samples exhibited a similar or lower stability than sunflower high oleic. This might be explained by a different composition in pro-oxidants and/or antioxidants between the beef tallows and the oils. According to the thermoxidation assays, which are carried out in similar conditions to those of a frying process, three of the beef tallow samples, sunflower high oleic and rice bran oil would be similarly suitable for frying, while sunflower oil and the other two samples of beef tallow evidenced a lower thermoxidative stability, thus not being recommended for this use.

Extra virgin olive oil is recognized as a very stable oil because of its composition in fatty acids and its content in natural antioxidants (tocopherols and polyphenols). In the bibliography are works that address different aspects of this stability, from the duration of its useful life to its performance in the frying of foods. Some works also link their stability with the content of natural antioxidants. For example, Franco et al. (2014) studied the content of phenols and their antioxidant capacity in olive oils of seven different varieties. Baccouri et al. (2008) found a good correlation between the oxidative stability (measured in Rancimat) of the oils studied and the concentration of total phenols and tocopherols.