The invention relates to a straw comprising a tube (11) and a gas-permeable and liquid-tight stopper (12), said stopper (12) being arranged in the tube (11), close to an end (16). The invention is characterised in that the stopper (12) comprises an indicator component (20) having a first pre-determined colour in the absence of previous contact with said substance (21) and a second pre-determined colour, of a different shade from that of the first colour, once it has been in contact with said substance (21). The assembly comprises the straw (10) and a device (26) for controlling the filling of the straw.
