Oxygen is known to accelerate ascorbic acid degradation Up to 100°C oxygen is still soluble allowing ascorbic acid oxidative degradation. In addition, ascorbic acid is not the only molecule that is oxidisable in a food matrix. In apple purée, polyphenols for example can be supposed to consume oxygen too. Oxygen availability is known to be a limiting factor at low temperature, but up to now for oxidation of ascorbic acid quantification of dissolved oxygen at high temperatures was very difficult. A double walled system was employed that permitted to maintain temperature and measurement of oxygen at precise locations. Oxygen sensor spots specially calibrated for intermediate temperatures were therefore stuck at the inside and measurement was conducted from the outside without disturbing the system. Oxygen was followed at the bottom, near the surface and the headspace. Besides, convection vs. diffusion processes were compared by using a colorant. Oxygen contents in ultrapure water heated up to 80°C were of 0.07 ± 0.00 mmol/l. Oxygen values in the sensor of the headspace were equal to those in the medium which can be linked to water saturation of the headspace. When ascorbic acid was added, oxygen values in the medium decreased initially then slowly rose again. Oxygen content curves in the two locations in the medium were equal. The curves approached asymptotically the level in ultrapure water but did not achieve it completely within eight hours. In apple purée, values dropped down within 15 min to almost anaerobic conditions (0.01 ± 0.00 mmol/l) and remained constant over the measurement time. Besides, darker coloration compared to the rest of the medium was noticed only at the very top layer of apple purée. By putting the blue colorant methylblue at the surface of the heated apple purée convection processes could be excluded to appear in apple purée as the colorant was spread in regular circles indicating diffusion. This was linked to the viscous matrix of apple purée as in the liquid supernatant of centrifuged apple purée convection was observed. To conclude, dissolved oxygen in apple purée at 80°C is quickly consumed and thus oxygen’s diffusion from the headspace and ascorbic acid’s diffusion to the surface limit ascorbic acid’s degradation.