Effect of relative humidity on lipid oxidation in freezedried emulsions

Oxidative stability was studied in a freeze-dried emulsion consisting of linoleic acid (LA), Tween-20, sucrose and maltodextrin in the presence of a catalyst (FeSO 4 /ascorbic acid). Changes in residual LA and conjugated dienes as a function of time were monitored at 0, 32, 43 and 75% relative humidities (RH). Based on GC analyses, LA oxidation was more significant in the surface fraction than the entrapped. The loss of surface oil upon storage may also be due to partial entrapment. However faster oxidation of the surface oil was confirmed by measurement of diene conjugation. Oxidation was more rapid at the lower relative humidities (0 and 32% RH) and decreased with increasing RH. At high moisture, physical modifications in the sample were observed, including reduced porosity, structural collapse, reduction of the surface oil and coalescence of oil droplets triggered by sucrose crystallization. These may be responsible for the decreased oxidation. Sucrose crystallization at the higher humidities inhibited oxidation. In addition, while samples with similar glass transition temperature (Tg) range behaved differently, samples with different glass transition range showed similar oxidative behaviour. Microstructural changes leading to oil entrapment and oil droplet coalescence were found to be significant, in this case.