The short answer
When yield drops after a lot change, three causes account for most cases, in order of likelihood. First, something else in the process changed at the same time (a new seed vial, a new drum of sugar, a modified sterilisation program) and the lot change simply coincided. Second, the new lot is within specification but at the other end of the range, for example amino nitrogen moving from the upper to the lower limit, which sensitive strains notice. Third, the lot is genuinely faulty. Troubleshoot in that order: process first, COA second, and only then a side-by-side peptone trial.
Step 1: confirm the peptone is the only change
Lay out the records for the week before and after the change: seed age and inoculum size, lot numbers of sugar and other raw materials, sterilisation temperature and time, pH meter calibration, incubator or fermenter temperature logs. In practice more than half of "it dropped when we changed lots" turns out to be something else that changed on the same day. If there are two or more variables, do not conclude yet.
Step 2: compare the two COAs side by side
Every line on a COA has a specification range, and two lots can both be in range without being alike. Look at these:
- Total nitrogen (TN) and amino nitrogen (AN). TN reflects protein content; AN reflects the degree of hydrolysis. A higher AN/TN ratio means more small peptides and free amino acids: cells consume them quickly, the lag phase shortens, but nitrogen may run out late in the run. A lower ratio does the opposite. For lactic acid bacteria, Bacillus and other strains sensitive to the form of nitrogen, a difference of one or two percentage points in the ratio is visible.
- Ash and sodium chloride. They affect osmotic pressure and mineral supply, which matters for salt-sensitive strains.
- pH and clarity of the solution. A pH shift changes the starting pH of the prepared medium; poorer clarity usually means more large molecules.
- Loss on drying. A lot with more moisture delivers less peptone per 10 g weighed.
How to read these numbers: Total nitrogen, amino nitrogen and the COA.
Step 3: run a trial where only the peptone differs
Shake flasks are enough; no fermenter needed. Same strain, same seed, same day, same lots of every other raw material, with only the peptone split between old and new lot, three replicates each. Measure cell count (OD or plate count) and the target product. Only if the difference between lots exceeds the difference between replicates is the peptone confirmed. If the replicates already scatter as much as the lots, the issue is process stability, not the raw material.
Keeping a retained sample of the old lot is what makes this possible. Keep 100 g of every incoming lot sealed and refrigerated until six months after the lot is used up.
Why lots of the same peptone differ
Peptones are hydrolysates of natural proteins. The raw material itself (soybean, yeast, casein) varies season to season, and the degree of hydrolysis moves within its specification range. A specification range is a guarantee that every lot in the range performs; it is not a guarantee that every lot is identical. So:
- For processes with tight stability requirements, trialling a new lot before production is standard practice.
- You can ask the supplier for a narrower band, for example only lots with AN/TN within a defined window.
- Buying one large lot before a production campaign and using it throughout is the simplest way to remove the variable.
Once the peptone is confirmed
Send both COAs and the trial data to the supplier and ask them to review the production record of that lot, and to replace it if warranted. STBIO ships a COA with every lot, cultural response data is available on request, and samples of a new lot can be trialled before purchase. Customers who need a defined specification window should say so when requesting a quotation; see sample request.
FAQ
Everything on the COA is in specification and the supplier says the lot is fine, but my yield is down. What now? In specification does not mean identical. Take the trial data to the supplier and ask for the full analytical values of this lot and the previous one, not just the ranges. The parameter sitting at the other end of the range usually turns up.
Can I compensate by dosing more? If the difference is in nitrogen content, adjusting the dose in proportion to total nitrogen is a workable temporary fix. If the difference is in degree of hydrolysis, more peptone does not help; change lot or adjust the formulation.
How often should incoming peptone be checked? Every lot. At minimum compare the COA and measure pH and clarity; for critical processes add a shake-flask growth curve. It is far cheaper than tracing a problem backwards.
Where do I find the COA for a specific lot? For STBIO products, enter the Ref. and lot number on the COA lookup page.