( ( As ( ( a matter ) ( of fact ) ) ) ( , ( ( low ( density PE ) ) ( ( ( is ( ( ( four times ) ( more permeable ) ) ( to ( carbon ( dioxide gas ) ) ) ) ) ( than ( it ( ( ( is ( to ( oxygen gas ) ) ) , ) ( allowing ( ( the bags ) ( to ( ( vent ( excess ( carbon ( dioxide gas ) ) ) ) ( as ( fermentation begins ) ) ) ) ) ) ) ) ) ) . ) ) ) )
(ROOT (S (PP (IN As) (NP (NP (DT a) (NN matter)) (PP (IN of) (NP (NN fact))))) (, ,) (NP (JJ low) (NN density) (NNP PE)) (VP (VBZ is) (ADJP (ADJP (NP (CD four) (NNS times)) (RBR more) (JJ permeable)) (PP (TO to) (NP (NN carbon) (NN dioxide) (NN gas)))) (SBAR (IN than) (S (NP (PRP it)) (VP (VBZ is) (PP (TO to) (NP (NN oxygen) (NN gas))) (, ,) (S (VP (VBG allowing) (S (NP (DT the) (NNS bags)) (VP (TO to) (VP (VB vent) (NP (JJ excess) (NN carbon) (NN dioxide) (NN gas)) (SBAR (IN as) (S (NP (NN fermentation)) (VP (VBZ begins))))))))))))) (. .)))
As a matter of fact, low density PE is four times more permeable to carbon dioxide gas than it is to oxygen gas, allowing the bags to vent excess carbon dioxide gas as fermentation begins.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))