There was a time when animal nutritionists knew everything about yeast. In fact, the only available yeast was dried brewer’s yeast. I do recall such a time, when things were simple!
Today, yeast is no longer yeast. It would be naive to assume brewer's yeast is the only strain available — tantamount to saying there is only one species of plants on earth. Modern technology has brought so many yeast strains into mainstream production that animal nutritionists no longer know everything about yeast(s).
Take, for example, a conversation I had a couple weeks ago with a yeast marketer. They have two different types of yeast, differing by about 10 percent points in crude protein — and even more in price. One is to be used at less than 100 kg per metric ton feed, whereas the other (the more expensive) at only 1-2 kg per metric ton feed. Marketing, you will be excused to wonder, but perhaps not because each yeast product comes from a different production process.
It is interesting how yeast, yeast products, yeast blends, yeast extracts and yeast fractions have become mainstream. In fact, I think it is time someone writes a book (or at least an article) about yeast. There is so much information out there but we nutritionists, including me, know so little about yeast. For example, I am still waiting for an article promised to me on how yeast culture works. I am genuinely interested in the subject.
From a marketing perspective, yeast producers like to keep as much information as possible close to their chest, rightfully fearful of competition. But they should also understand that nutritionists often refuse to use ingredients they are not fully familiar with — and to be honest, there are many questions that we have regarding yeast that remain without a concrete answer. This might serve short-term, but in the long run it will hurt the acceptance of yeast as a universal source of protein and other nutrients.