The residual content of sulphur dioxide was measured in the
mushrooms packed in metabisulphite/citric acid 0.6/0.06 g/m2
packaging. The method developed by Monier-Williams was
applied, with some modifications as explained below (Montes et al.,
2012). Twenty grams of mushrooms were triturated crushed with
20 mL of water and homogenised using an Ultra-Turrax T18 from
IKA (Staufen, Germany). The resulting slurry was poured into a
double neck round-bottom flask connected to a condenser. The exit
of the condenser was connected to another flask containing 20 mL
of 0.01 M sodium hydroxide aqueous solution. An air flow input
was connected to the second neck of the round-bottom flask,
bubbling the mixture and carrying the vapours released to the
trapping alkaline solution. The extraction of sulphur dioxide started
with the addition of 20 mL of 25% (w/w) aqueous hydrochloric acid
to the mushrooms slurry. The extraction was carried out at 85 C
using a silicone oil bath for 30 min. Also, a magnetic stick bar was
used to homogenise the solution.