enzymes
plant proteases include papain (from the papaya), actinidin (from the
kiwi fruit), and ficin (from the fig). These proteases may induce a
prickly sensation in the mouth when consumed.

Pineapple / Bromelain - http://en.wikipedia.org/wiki/Bromelain
papaya / papain - http://en.wikipedia.org/wiki/Papain
kiwi / actinidin - http://en.wikipedia.org/wiki/Actinidin

fig / ficin - http://en.wikipedia.org/wiki/Ficin
Ficain (or ficin) is an enzyme which is derived from figs latex.
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cayenne pepper and capsaicin
works great to help keep chickens healthy while keeping dogs away from
areas.