Formation of temperature gradients in large- and small-scale red wine fermentations during cap management

Background and Aims: Red wine fermentations are not a homogenous mixture in terms of both density (grape solids vs liquid) and temperature distribution. Winemakers practise cap management techniques to facilitate the extraction of flavour and colour, but also to achieve a more uniform temperature throughout the fermentation tank. The aim of this work was to provide, for the first time, a detailed definition of the temperature gradients in a two-dimensional cross-section of several fermentations and to determine the impact of typical cap management techniques on these. Methods and Results: Commercial (3450 L) and research (50 L) red wine fermentations were monitored with a handheld thermocouple at defined positions in the tank to yield temperature profiles for a virtual cross-section through each fermentation. Differences of over 12 degrees C were observed between the centre of the cap and the liquid of fermentations at both scales, as were differences of up to 8 degrees C between the centre and side of the cap. Cap management techniques typical for industry did not eliminate temperature gradients. A 1600-L tank fitted with in-place temperature sensors allowed nearly instantaneous determination of temperature cross-sections and revealed that a thorough pumpover was needed to reduce temperature differences to < 5 degrees C. Conclusions: Temperature gradients can be large in red wine fermentations, regardless of scale, varying with the must and fermentation management. Significance of the Study: Large temperature differences and high temperatures through and during fermentation have implications for yeast performance, and colour and flavour compound extraction, modification and stability.