Small particle size of flour limits efficacy of carbon dioxide-high pressure-treatment

In the last two decades the treatment of goods with carbon dioxide (CO2) at pressures of 15 to 30 bar has gained importance in Europe. This is partly due to the acceptability for organic production and short treatment times (e. g. three hours at 20 bar). In a systematic study, whole wheat grains and milled grist particles from coarse to fine (> 710 µm, 500-710 µm, 400-500 µm, 300-400 µm, 200-300 µm, 100-200 µm, < 100 µm) were exposed to pressure treatments in polypropylene sewage tubes of 500 mm length and 75 mm diameter (volume approx. 2.1 l). 50 adult granary weevils in a cage with whole wheat kernels were placed into the centre of the tubes prior to treatment. The treatment was carried out in a large commercial triple high-pressure chamber that was evacuated prior to flooding with CO2. Only the smallest particle size of < 100 µm led to the survival of adult granary weevils while in particles > 100 µm all weevils died. This indicates that fine flours may be compressed from the outside keeping a pocket of air in their centre and thus cannot be pressure-treated with CO2. For pest control, larger amounts of fine flour should rather be sifted or sent through an impact mill.