( ( A ( process variable ) ) ( ( is ( ( a factor ) ( in ( ( an experiment ) ( that ( ( is not ) ( ( part ( of ( the ( mixture but ) ) ) ) ( whose ( ( ( ( levels , ) ( when varied ) ) , ) ( could ( affect ( ( the ( blending properties ) ) ( of ( the ( mixture ingredients ) ) ) ) ) ) ) ) ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN process) (NN variable)) (VP (VBZ is) (NP (NP (DT a) (NN factor)) (PP (IN in) (NP (NP (DT an) (NN experiment)) (SBAR (WHNP (WDT that)) (S (VP (VBZ is) (RB not) (NP (NP (NN part)) (PP (IN of) (NP (DT the) (NN mixture) (IN but))) (SBAR (WP$ whose) (S (NP (NP (NNS levels)) (, ,) (SBAR (WHADVP (WRB when)) (S (VP (VBD varied)))) (, ,)) (VP (MD could) (VP (VB affect) (NP (NP (DT the) (NN blending) (NNS properties)) (PP (IN of) (NP (DT the) (NN mixture) (NNS ingredients)))))))))))))))) (. .)))
A process variable is a factor in an experiment that is not part of the mixture but whose levels, when varied, could affect  the blending properties of the mixture ingredients.
(ROOT (S (NP (JJ Dependent) (NN variable)) (VP (VBZ is) (NP (NP (DT the) (NN term)) (PP (IN for) (NP (NP (DT the) (VBN affected) (NN factor)) (PP (IN in) (NP (DT an) (NN experiment))))))) (. .)))