The baryon non-conservation in the quantum theory is caused by the [chiral anomaly](http://en.wikipedia.org/wiki/Chiral_anomaly), which leads to the baryon current $j_B^\mu = \sum_i \bar q_i \gamma^\mu q_i$ being not conserved as
$$ \partial_\mu j_B^\mu \propto F\wedge F$$
where $F$ is the field strength tensor of the electroweak field. Now, $\int \partial_\mu j_B^\mu \propto \int F\wedge F$ is a topological invariant, associated to the [instanton](https://en.wikipedia.org/wiki/Instanton) (see also) sector we're in. The instanton is essentially the classical solution - the vacuum - we're basing our quantum field theoretic perturbative expansion around (see also [this](https://physics.stackexchange.com/a/127880/50583) and [this](https://physics.stackexchange.com/a/224608/50583) post of mine). This means instantons are invisible from the perturbative viewpoint - you have to fix one to do perturbation theory. 

Furthermore, if one expands the partition function as a Taylor series around vanishing coupling constant, which essentially defines the perturbative approach, the instanton contributions vanish because they are suppressed with a factor $\mathrm{e}^{-1/g}$ whose Taylor expansion at vanishing coupling is identically zero, and hence doesn't appear in the perturbation series (cf. [nLab](http://ncatlab.org/nlab/show/non-perturbative+effect)).

Graphically, the anomaly and hence the non-conservation is associated to the triangle diagrams
[![enter image description here][1]][1]

see [*"Lectures on Anomalies"*](http://arxiv.org/abs/0802.0634) by Bilal.


  [1]: https://i.sstatic.net/roscb.png