Multiple terms: term1 term2
red apples
returns results with all terms like:
Fructose levels in red and green apples

Precise match in quotes: "term1 term2"
"red apples"
returns results matching exactly like:
Anthocyanin biosynthesis in red apples

Exclude a term with -: term1 -term2
apples -red
returns results containing apples but not red:
Malic acid in green apples