In the standard placement described in the Figure 11.2 discussion, which pair correctly lists the two timing options for adding the precipitating agent?

Question

BlackTom AI · Accepted Answer

Precipitating agents are typically introduced at bulk-processing points where they can aggregate impurities or target proteins efficiently. Two standard timing options are: adding the precipitant prior to harvest in the bioreactor (treating whole broth) or adding it after the capture chromatography step (treating the product-containing eluate).

Relevant knowledge points and concepts:
- Precipitation is a unit operation used to remove impurities or fractionate proteins by inducing aggregation; its effectiveness depends on the matrix (cell-containing broth vs. clarified/partially purified pool).
- Upstream addition (in the bioreactor) treats the crude broth and can simplify downstream clarification but must tolerate cells and process conditions.
- Downstream addition (post-capture chromatography) treats a more purified pool, offering greater selectivity and lower contaminant load.

Step-by-step reasoning:
1. List plausible timing points in a standard downstream train: before harvest (in bioreactor), after cells are removed (post-clarification), after capture chromatography, and after viral filtration.
2. Evaluate suitability for precipitation:
   - Prior to harvest: feasible and commonly used when precipitant is compatible with cells and can be added to the bioreactor to aggregate impurities (option A part).
   - After capture chromatography: feasible because the eluate is concentrated and cleaner, letting precipitation target remaining impurities or concentrate product (option D part).
3. Consider other listed timings:
   - After cells are removed (clarified pool) is a possible timing, but the standard placement discussion emphasized whole-broth (prior to harvest) versus post-capture; the specific pair expected contrasts upstream (bioreactor) with a downstream chromatography point.
   - After viral filtration is typically too late: viral filtration is a final polishing/sterile barrier step, so adding a precipitant afterward would compromise viral clearance and is not standard.

Why the correct answer is correct:
- The standard placement described contrasts an upstream whole-broth addition (prior to harvest in the bioreactor) with a downstream, post-capture chromatography addition. These two points represent the typical strategic choices for precipitation placement.

Why other options are incorrect:
- “Prior to harvest; after cells are removed” mixes two upstream/clarification points rather than the canonical upstream vs. post-capture contrast.
- Options that include “after viral filtration” are incorrect because viral filtration is a terminal polishing step and not a standard site for precipitation.

Conclusion: The correct pair is “Prior to harvest in the bioreactor; after capture chromatography,” which matches the provided answer.

In the standard placement described in the Figure 11.2 discussion, which pair correctly lists the two timing options for adding the precipitating agent?Single choice

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