"I've never touched a dead body," says guide Tom Le Mesurier, "but I imagine this is what it feels like." He passes around the small orange fruit and we all recoil at its cold, clammy texture. In the mouth, it's fibrous and juicy with a strange saliva-sapping aftertaste. "Bit like a disappointing pear isn't it?" he says, smiling.
It transpires this bizarre corpse-like fruit is a cashew and inside its protruding brown seed is a cashew nut. Extracting it, however, is no easy matter. The casing contains a poisonous sap which can only be vapourised by roasting it at extremely high temperatures. No wonder cashews are so expensive.
Le Mesurier is keen to show visitors that there's more to Rio than bronzed bodies, beaches and bars. A passionate cook, he started this food tour after moving from England in 2010. His rationale is simple: "I want people to try new things in places they wouldn't normally go."
Our culinary adventure begins in a street market in Botafogo where we sample fruits I've barely heard of let alone tasted. There's the fig-like sapoti, which resembles a large mottled potato, an oddly cheese-flavoured jackfruit and the alien-looking innards of a cocoa pod (delicious, by the way).
We also try the Brazilian equivalent of a cheese toasty – a kind of bouncy pancake made with fried tapioca flour – and drink a sweet shot of fresh sugarcane juice.
From Botafogo, we take a cab to the bohemian suburb of Lapa. First stop is Nova Capela, one of Rio's oldest restaurants, where we sample crunchy salted cod croquettes and creamy cinnamon-dusted Portuguese custard tarts.
Armed with potent caipirinhas mixed by Fabio at Boteco Belmonte ("he makes the best ones in Rio"), we stroll through graffitied streets lined with historic pastel-coloured mansions. "Lapa has a dodgy reputation," says Le Mesurier, "but we've done more than 600 tours here and never had a problem."
After a quick look at Chilean artist Jorge Selaron's famous colourful tiled staircase, we pause for a palate-cleansing beer before taking the metro to what Le Mesurier calls "our most adventurous stop".
Tacaca is a fiercely salty soup containing a mildly anaesthetic leaf from the Amazon called jambu. I take a tentative sip and feel a soft tingling numbness in my tongue. "This dish always divides the group," says Le Mesurier, "but it's something you'll only find in Brazil."
Sadly, I have to leave for a flight before the tour's finale – a hearty lunch of traditional north-eastern Brazilian cuisine including deep-fried pastries, mashed cassava and coconut shrimp stew.
However, there's time for two last drink specialities. The first is the juice from the Amazonian cupuacu fruit, which resembles an enormous dinosaur egg and tastes like a tsunami of tropical sweetness. The second is a cachaca spirit infused with our old friend the jambu leaf. "You'll get some pretty intense tingles," warns Le Mesurier. "And probably a saliva rush as if you're about to be sick."
What he doesn't mention is the subsequent explosion of saltiness. "Oh yeah, it also puts three of your four tastebuds to sleep."
Next to me a young English girl holds the mysterious brown liquid up to the light and says what we're all thinking: "That is the weirdest thing I've ever tasted."
Rob McFarland was a guest of Chimu Adventures and the Adventure Travel Trade Association.
Eat Rio tours run for five to six hours and cost $R350 (about $130), which includes all food, drink and transport. See eatrio.net
Latin America specialist Chimu Adventures can create a tailor-made Brazil itinerary including flights, accommodation, transfers and tours. See chimuadventures.com