Spray-dried food powders often contain oil.In the liquid feed,the oily phase is emulsified to drops in the lower μm size range.Powder and rehydrated liquid product properties strongly depend on the oil drop size distribution.We therefore investigate the effect of stresses occurring during atomization on the oil drop size distribution.In this study we used an effervescent atomizer which is a special type of internal mixing pneumatic atomizer with a measurably lower atomization gas consumption compared to external mixing pneumatic atomizers.The design of the mixing chamber,the operating pressure,the ALR(air-to-liquid-ratio by mass)and the feed viscosity are crucial parameters which influence the emerging flow patterns(gas/liquid)in the mixing chamber.This in turn influences the occurring stresses which are responsible for spray forming,but also may force an oil drop break up in the nozzle.A model emulsion consisting of water,different silicon oils,Tween20 and PEG20,000 was atomized at varying parameters.The gas/liquid flow patterns in the mixing chamber were visualized by a high speed camera.The spray drop size and spray pulsation were measured online using a laser diffraction spectroscope(Malvern Spraytec).Before and after the atomization the oil drop size was analyzed offline by a laser diffraction spectroscope(Retsch Horiba LA950).The results for oil drop break up showed a decreasing oil drop size with decreasing viscosity ratio(ratio of viscosity of disperse phase to emulsion viscosity)and increasing ALR.An influence of flow pattern and resulting pulsation of the spray on the oil drop size distribution was found.For slug flow in the mixing chamber,the mean oil drop size increased compared to oil drop sizes generated by annular flow.Effects are of relevance for the production of spray dried dairy powders,e.g.toppings,or oil encapsulation targets.