Coolship:
- Picks up a variety of microbes. The key to a good long-term sour is the Brett strain(s).
- Beer is transferred into barrels. Each barrel has different flora, creating different flavors ("character") and acidity in each.
- The barrels are blended to balance flavors/acidity and to increase complexity.
- Wort composition is optimized to support the microbes.
- The attenuation is somewhat predictable.
Contamination:
- Smaller number of microbes. The beer may or may not become sour. The beer may or may not develop any additional flavor, whether beneficial or detrimental.
- Probably not any Brett, much less good Brett. Off-flavors and off-textures may persist without Brett to smooth out the other microbes' byproducts.
- Not usually properly aged or blended.
- Wort composition is usually not suited for wild fermentation and may clash with any flavors that develop, or have too much alcohol potential.
- Unpredictable attenuation.