Ever wondered why professional brewers obsess over cell counts? Let's dive into the technical world of yeast pitching rates, exploring the biochemistry behind fermentation dynamics while keeping things as easy to digest as a well-attenuated ale.
The Biochemistry of Pitching Rates
The pitching rate is defined as the number of viable(healthy) yeast cells introduced per millilitre of wort. Pitching is a critical factor in fermentation kinetics. It directly influences:
- Lag Phase Duration: The period before active fermentation begins
- Flavour Compound Production: Including esters, higher alcohols, and off-flavours
- Attenuation Rate: The speed and extent of sugar consumption
- Flocculation Behaviour: Impacting beer clarity and yeast harvesting
Underpitching Consequences:
- Extended Lag Phase: Increases risk of bacterial contamination
- Elevated Ester Production: Due to increased yeast growth phase
- Higher Fusel Alcohol Formation: Resulting from amino acid catabolism
- Increased VDK (Vicinal Diketone) Production: Leading to diacetyl off-flavours
For more on Off-flavours see Beer Flavour Profiles: A Guide to Eliminating Off-Flavours
Overpitching Effects:
- Reduced Ester Formation: Limiting desirable fruity notes
- Accelerated Nutrient Depletion: Potentially leading to stuck fermentations
- Increased Autolysis Risk: Resulting in meaty, rubber-like off-flavours
- Altered Flocculation: Potentially affecting beer clarity and yeast cropping
Quantifying the Ideal Pitch: Cell Count Calculations
The standard pitching rate formula:
Cells Required = (Pitching Rate) × (Wort Volume in mL) × (°Plato)
Where:
- Typical Pitching Rate for Ales: 0.75 million cells/mL/°Plato
- Typical Pitching Rate for Lagers: 1.5 million cells/mL/°Plato
This formula accounts for both wort volume and gravity, crucial factors in yeast stress and fermentation kinetics.
But a general rule is for a 23L batch: 2 packs of yeast for ales and lagers with an OG of less than 1.055. Above 1.055 2-3 packs for ale and 3-4 packs for lagers.
Viability and Vitality: Beyond Simple Cell Counts
Yeast health is more than just numbers. Consider:
- Viability: The percentage of living cells in a population
- Vitality: The metabolic activity and fermentation capacity of living cells
Viability declines over time, following a roughly linear pattern:
Viability (%) = 100 - (0.7 × Age in Days)
This can mean a 21% drop in viability per month for liquid yeast!
Advanced Pitching Considerations
- Oxygen Levels: Higher pitching rates require increased dissolved oxygen (8-10 ppm for high-gravity beers)
- Nutrient Availability: Ensure adequate FAN (Free Amino Nitrogen) levels, especially for high-gravity worts
- Temperature Control: Proper pitching temperature is crucial for cell membrane fluidity and nutrient uptake
Exceptions to the Rule: Intentional Underpitching
Some styles benefit from stress-induced ester production:
- Traditional British Ales: Underpitch by 20-30% for enhanced fruity esters
- NEIPAs: Reduce pitching rate by up to 50% to complement hop aromatics
Key Takeaways for Technical Brewing Excellence
- Consider investing in cell counting technology (hemocytometer or flow cytometry) for precise inoculation
- Implement oxygen measurement and control systems for consistent dissolved oxygen levels
- Monitor FAN levels in the wort, especially for high-gravity brews
- Establish a yeast management program to track generational changes in performance
- Consider investing in propagation equipment for ultimate control over yeast health
Remember, while these technical aspects are crucial, brewing remains both a science and an art. By mastering the intricacies of yeast pitching, you're not just fermenting wort - you're orchestrating a complex biochemical symphony. So, arm yourself with knowledge, trust your instruments, and may your fermentations always be precisely pitched!
MYO Drinks
Helping you to make your own Great Batch from Scratch.