AVSpeechSynthesizer utterance to audio file

I'm using AVSpeechSynthesizer in my app and I'm looking to save the spoken text to an audio file or AVAsset. I went through Apple's docs and didn't see anything but figured I'd post a question to make sure. Below is my current code.

AVSpeechUtterance * utterance = [AVSpeechUtterance speechUtteranceWithString:textView.text];
float rate = [speedSlider value]/1.5;
utterance.rate =  rate;
[speechSynthesizer speakUtterance:utterance];