Is there a known isotope that generates anti-matter? I heard that even a banana generates a minute quantity of antimatter. Does any know radioactive nuclear reaction produce antimatter along with alpha, beta and gamma radiation? 
 A: Bananas are notorious for having high levels of potassium, though a quick Google will find plenty of other potassium rich foods. Anyhow, one of the common radioactive isotopes of potassium is potassium-40. This mostly decays by emission of an electron and anti-neutrino but rarely it can decay by emission of a positron i.e. anti-matter. So it is true that bananas emit minute quantities of anti-matter.
But ...
Bananas don't emit a net amount of antimatter because the electron is always paired with an anti-neutrino and the positron is always paired with a (not anti) neutrino. The number of particles emitted is always equal to the number of anti-particles emitted. This always true and is the result of conservation of lepton number.
A: This is the chart of nuclides, i.e. the isotopes of all atoms.

The green color shows that most nuclei have isotopes decaying with beta+ decay, i.e. positrons. If you go to the link there is interactive information.
From this it is seen that everyday items which will always have a tiny percentage of the long lived isotopes will be decaying into positrons, the antiparticle of an electron. 
Nuclides that have too many protons will decay with beta+

alpha, beta and gamma radiation

The "beta" in your question is both a beta+ and beta- , and yes the statement is correct. 
The beta+ decay frees a positron into the environment of the nuclide.
$^\mathtt{A}_\mathtt{Z}X \rightarrow ^\mathtt{\hphantom{Z-}A}_\mathtt{Z-1}X' + e^+ + ν_e$
This decay is used in positron emission tomography, because the positron meeting an electron in the environment will annihilate into two photons  and give medical information:

Positron emission tomography (PET)1 is a nuclear medicine, functional imaging technique that is used to observe metabolic processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. 

