Shadow of the Vampire

In 1921, German director F. W. Murnau takes his cast and crew on-location in Czechoslovakia to shoot Nosferatu, an unauthorized version of Bram Stoker's novel Dracula. Murnau keeps his team in the dark about their schedule and the actor playing the vampire Count Orlok. It is left to the film's other main actor, Gustav von Wangenheim, to explain that the lead is an obscure German theater performer named Max Schreck, who is a character actor. To involve himself fully in his role, Schreck will only appear amongst the cast and crew in makeup, and will never break character.
After filming scenes in a studio with leading actress Greta Schroeder, who is displeased about leaving Berlin, Murnau's team travels to the remote inn where they will be staying and shooting further scenes. The landlady becomes distressed at Murnau removing crucifixes around the inn, and the cameraman, Wolfgang Muller, falls into a strange, hypnotic state. Gustav discovers a bottle of blood amongst the team's food supplies, and Murnau delivers a caged ferret in the middle of the night.
One night, Murnau rushes his team up to an old Slovak castle for the first scene with the vampire. Schreck appears for the first time, and his appearance and behavior impress and disturb them. The film's producer, Albin Grau, suspects that Schreck is not a German theater actor, and is confused when Murnau tells him that he found Schreck in the castle. Soon after the completion of the scene, Wolf is found collapsed in a dark tunnel. Upon returning to the inn, the landlady appears frightened by his pale, weak appearance, and mutters "Nosferatu" while clutching at a rosary.
Whilst filming a dinner scene between Gustav and Count Orlok, Murnau startles Gustav, making him cut his finger. Schreck reacts wildly at the sight of the blood, and tries drinking from Gustav's wound. The generator powering the lights fails and when the lights return, Schreck has pinned Wolf to the floor, apparently draining his blood. Albin orders filming ended for the night, and the crew rushes from the castle, leaving Schreck behind. Schreck examines the camera equipment, fascinated by footage of a sunrise.
Schreck is in fact an actual vampire, and Murnau has struck a deal in order to create the most realistic vampire film possible. Murnau has promised him Greta as a reward, but Schreck remains a difficult star. With Wolf near death, Murnau is forced to bring in another cinematographer, Fritz Arno Wagner. During Murnau's absence, Schreck approaches Albin and the screenwriter, Henrik Galeen. They invite him to join them, and question Schreck, believing he is still in character. Schreck points out Dracula's loneliness, and the sadness of Dracula trying to remember how to perform the everyday activities. When they ask how he became a vampire, Schreck says it was a woman. A bat flies by and Schreck catches it, sucking its blood. The others are impressed by what they assume is talented acting.
The production moves to Heligoland to film the final scenes, and Murnau admits Schreck's true nature to Albin and Fritz. The two realise they are trapped, leaving no choice but to complete the film and give Greta to the vampire if they wish to survive. Greta becomes hysterical after noticing Schreck casts no reflection. Murnau, Albin and Fritz drug her, and film the scene as Schreck feeds on Greta, killing her, but the laudanum in her blood puts Schreck to sleep. At dawn, the remaining three attempt to open a door and let in sunlight to destroy Schreck. Schreck previously cut the chain, having learned of their trickery. Schreck kills Fritz and Albin while Murnau continues filming. Henrik and the crew arrive in time to lift the door and flood the set with sunlight, destroying Schreck while Murnau films his death.

Question: What does Gustav find among the crew's food supplies?
Answer: bottle of blood