Stability of alcohol-containing emulsions in relation to neck-plug formation in commercial cream liqueurs

The stability of alcohol-containing dairy emulsions has been investigated through observations of visual and microscopic appearance, normal gravity creaming, droplet-size distribution, viscosity, free fat content and shelf-life at 45 degrees C. Changes in emulsion properties have been assessed in terms of their possible relevance to the phenomenon of neck-plug formation of commercial cream liqueurs after prolonged storage under market conditions. Of the various physical factors investigated (evaporation, stirring, agitation, inadequate or repeated homogenization, temperature cycling, storage at different temperatures), we have found that only the combination of temperature cycling (involving temperatures <0 degrees C) and continuous mechanical agitation leads to changes in microscopic appearance or free fat content in a laboratory-made cream liqueur which could in any way be associated with commercial neck-plugging. Experiments with pH-adjusted cream liqueurs have shown increasing flocculation and reduction in shelf-life as pH is lowered below 6.4, but again no tendency towards neck-plug formation. Indeed, nothing resembling a commercial neck-plug in terms of shear modulus and free fat content could be produced in the laboratory under normal gravity conditions. Such a material could be made, however, by centrifuging a cream liqueur into a non-redispersible cream plug, and subjecting the resulting oil-in-water cohesive cream to temperature cycling (with or without agitation), whereby it is converted into a water-in-oil butter-like plug similar in texture and microscopic structure to the neck-plug. Based on these experimental observations, a plausible mechanism for neck-plug formation is proposed, taking account of likely chemical and compositional factors (pH changes, calcium ion content, emulsifier content) which might contribute towards the instability.