1. Yeast Extract or Autolyzed Yeast Extract from by product of beer making:The only two categories of food products which use the yeast extract or autolyzed yeast extract are Cheese Crackers and Cheese products because of its harsh and bitter taste. The type of yeast is Brewer Yeast.
2. Yeast Extract or Autolyzed Yeast Extract from Baker's Yeast:
Saccharomyces cerevisiae yeast or Baker's Yeast(Halal) is used in manufacture of yeast extract or autolyzed yeast or extract in food products other than cheese crackers or cheeses such as soups, fruits, and other foods etc. So if you see Yeast Extract or Autolyzed Yeast or extract in food products other than cheese crackers and chees, it means the Yeast Extract or Autolyzed Yeast or Extract is from Baker's yeast and it is Halal.
3. Torula Yeast:
According to manufacturer currently the Torula Yeast is manufactured from sugar source such as Molasses or corn by many manufacturer. They stop manufacturing Torula yeast from alcohol or waste paper.
Yeast cells are pasteurized to inactive them. Mostly from Baker's yeast which is a Halal yeast.