Citrate is a normal constituent of milk that affects milk-processing characteristics. It is an intermediate in the tricarboxylic acid cycle and plays an indirect role in fat synthesis by providing reducing equivalents in the form of NADPH. The objective of this study was to investigate variation in citrate with stage of lactation and de novo fatty acid synthesis, without confounding dietary effects. Twenty-four cows were fed the same diet, and milk citrate and fatty acids were determined over a 10-d period. Eight cows were in early lactation [13 +/- 1.8 d in milk (DIM; mean +/-standard error], 8 in midlactation (130 +/-4.6 DIM), and 8 in late lactation (283 +/-3.4 DIM). For cows in early, mid, and late lactation, milk yield was 34.4, 34.4, and 21.4 L/d [standard error of difference (SED) 1.78]; milk fat was 50.4, 40.3, and 41.4 g/L (3.68); milk citrate was 11.3, 9.7, and 10.1 mmol/L (0.64); the ratio of 4-14 C:18-20 C fatty acids was 0.9, 1.3, and 1.2 (0.07). Activity of the fatty acid synthase enzyme system (EC 2.3.1.85) was calculated as acetate used for chain elongation (ACE); ACE (mol/d) for cows in early, mid, and late lactation, was 7.3, 11.1, and 8.1 (SED 1.05). For individual cows, citrate (mmol/L) = 14.3 -0.44 xACE (r2 = 0.58). We propose that ACE provides a more accurate indication of synthase activity than do fatty acid ratios or yields. This study confirms the hypothesis that variation in milk citrate with stage of lactation is related to de novo synthesis of fatty acids and that the relationship is independent of diet and milk yield.

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RESUMENComportamiento de aceites poliinsaturados en la preparación de patatas fritas para consumo inmediato: Formación de nuevos compuestos y comparación de méto-dos analíticos.En este estudio se aplican diferentes métodos analíticos -compuestos polares, distribución de compuestos polares, acidez, índi-ce de peróxidos, dienos y tríenos conjugados, ácidos oxidados insolubles en éter de petróleo, viscosidad y ensayo oxifrit-a muestras de aceites de soja, girasol y mezclas de soja y colza, obtenidas en establecimientos públicos, antes y después de ser usadas en la fritura de patatas. Los dos objetivos principales fueron: conocer los compuestos formados durante el proceso de fritura de patatas y evaluar la posibilidad de sustitución de determinaciones complejas por métodos analíticos más simples y rápidos.Los resultados indicaron que, aunque los compuestos de polimerización y oxidación son los más representativos entre los nuevos compuestos originados durante el proceso, un 20% de las muestras contenían cantidades anormalmente elevadas de compuestos de hidrólisis cuya presencia es difícil de explicar. A partir de las correlaciones existentes entre métodos analíticos se puede deducir que, en este sector, la acidez libre y el porcentaje de dienos conjugados (o viscosidad), son útiles para predecir las cantidades de los principales grupos de nuevos compuestos presentes en los aceites de fritura poliinsaturados. PALABRAS-CLAVE: Aceite de fritura (caiidad) -Aceite poliinsaturado -Compuestos polares -Método analítico rápido -Patatas. SUÍVIMARY Performance of polyunsaturated oils during frying of potatoes in fast food shops: Formation of new compounds and correlations between analytical methods.In this study, different analytical methods, i.e. polar compounds, polar compound distribution, free fatty acids, peroxide value, conjugated dienes and trienes, oxidized fatty acids, viscosity and oxifrit-test, are applied to samples of both initial polyunsaturated oils and their counterparts after being used in the frying of potatoes in fast food shops. Two main objectives were established: the study of formation of new compounds during the frying process and the evaluation of possibilities to substitute complex methodologies for simpler and more rapid techniques.Results indicated that, although new compounds comming from oxidation and polymerization stand out, 20% of the samples contained ^jexsj high amounts of hydrolytic products i.e. diglycerides and fatty acids, whose formation is difficult to explain.From the analysis of correlation coefficients between analytical methods, the utility of free fatty acids and conjugated dienes (or viscosity) to predict the amount of the main groups of new compounds in used polyunsaturated frying oils, was deduced.

Oleoresin of rosa mosqueta (Rosa rubiginosa) was encapsulated with starch or gelatin by spray-drying. Stability of the powders was studied at 25, 40, and 55°C in the dark. Degradation of trans-rubixanthin, trans-lycopene, and trans-β-carotene followed a pseudo-first-order kinetic model for both encapsulating agents. The gelatin matrix provided a greater protective effect over the main carotenoid pigments, as shown by the lower degradation rate constants and the longer half-life values at 21°C. In contrast, the carotenoid pigments showed the same degradation rate in starch, but trans-β-carotene was more stable in gelatin. The kinetic compensation effect obtained according to the calculated thermodynamic parameters suggests that the carotenoids are degraded by the same mechanism.

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