The Brit lost 30 seconds to Quintana on the climb on a day of rare vulnerability after an almost flawless race so far.
The 2013 Tour winner has been dogged by allegations of doping throughout his time in the yellow jersey in this year's race and Team Sky have even released the rider's power data in an attempt to abate the questioning.
But it is the actions of so called 'fans' which has caused Froome the greatest upset in recent stages, having had a cup of urine thrown over him on stage 14, while he and his teammates have suffered verbal abuse from the side of the road.
Asked about the spitting incident after the stage finish, Froome said: "I didn’t see it but a few journalists have told me about it. It’s appalling.
"Primarily we are human beings then sportsmen, so people need to remember that. You can’t come to a bike race to spit at people, or punch them, or throw urine at them. That’s not acceptable."
Froome will take a 2:38 lead over Quintana into Saturday's stage 20 showdown up Alpe d'Huez, with the Colombian's Movistar teammate sitting in third place 5:25 down.
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