The destemming of grapes: Experiments and discrete element modelling

The removal and separation of grape berries from the stems is an important step in the wine making process. Destemming machines are used to mechanically remove and separate the berries from the stems. The biggest problems experienced with these machines include incomplete removal of the berries from the stems, and broken stems ending up with the berries, which can result in unwanted characters and flavours in the wine. The development of these machines is currently limited to prototype testing. A range of experiments were designed to measure the material properties of the grapes, including the stem stiffness and break strength under tension, the berry stiffness under compression, and the berry detachment force. The Discrete Element Method (DEM) was then used to model a bunch of grapes. Spherical particles were elastically bonded together to model the stems and single spherical particles were used to model the berries. In a model of a simplified destemmer, a single bunch of grapes was hit by a set of rotating beaters. The model and material parameters were based on measurements and adjusted by comparing the results of the simplified destemmer to experimental results. The number of berries removed from a bunch could be accurately predicted as influenced by the rotation speed of the beaters and the mass of the bunch. The model of a commercial destemmer could accurately predict the number of berries removed from the bunches, the spatial distribution of berries at the discharge and the number of unwanted stems mixed with the berries.