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A pulper reject sample can explain more than a recovery figure
A recovery figure can look precise while saying very little about what left the machine. Pair the product with the reject, describe the feed, and compare both streams on one declared basis.
Start with the material that actually entered
A recovery figure is a ratio, and every ratio depends on the boundary chosen for its numerator and denominator. In a pulping trial, that boundary begins with the feed. Washed whole fruit, destoned flesh, peeled fruit, crushed material and a prepared pulp are not interchangeable inputs. They carry different amounts of skin, seed, stone, fibre and free liquid, and they present a different load to the rotor and screen.
The FAO guide to small- and medium-scale fruit juice processing places pulping and screening within a longer sequence that can include washing, sorting, inspection, peeling, deseeding, crushing and heat treatment. The guide also notes that fruit type changes the preparation route: soft seeded fruit, firm fruit with inedible skin, and fruit with large stones are handled differently. That is a useful warning against treating one recovery percentage as a property of a machine alone.
Write down the fruit or blend, variety where known, maturity range, preparation step, measured temperature, feed rate and the screen installed for the run. If the feed description changes between two trials, their recovery figures are not directly comparable, even when the same pulper is used.
Keep the product and reject samples paired
A product sample taken from one moment and a reject sample taken from another can tell contradictory stories. Collect both over the same stable interval, with the same settings and the same feed lot. Label each container with time, run identifier, screen, rotor setting and sampling point. If the run was interrupted, record the interruption rather than averaging it away.
The sampling plan should identify three boundaries: the prepared feed, the through-screen product and the retained solids. Where a machine has two stages, sample each stage separately as well as the final combined product. That prevents a good final sample from hiding a first-stage problem, and it shows whether the second stage is doing a distinct job or merely reprocessing a stream that was already acceptable.
Retain enough material for a repeat inspection and for any agreed laboratory method. A small grab sample may be adequate for a visual check but not for a particle, texture or composition test. Agree the sample size and the chain of custody before the trial, especially when the result will be used to compare suppliers or approve a purchase.
Inspect seed integrity before calling the reject clean
For seeded fruit, the reject is not simply waste. It is evidence of how the machine treated hard bodies. Count or classify intact, cracked and fragmented seeds using an agreed method, and inspect the product stream for fragments that may have passed through the screen. A reject that looks dry can still contain broken seed, and a product that looks smooth can still carry fine hard particles.
The FAO tropical-fruit chapter describes a paddle pulper with brushes and a screen for mango, with a discharge throat large enough to eject the intact seed. That example is specific to the fruit, preparation and equipment arrangement described by the source. It does not establish that every pulper with a similar screen will preserve every seed, or that a fine screen is safe for an unspecified stone load.
The practical question is not whether a reject sample is perfectly intact. It is whether the agreed product specification tolerates the observed fragments, and whether the machine arrangement and upstream preparation are appropriate for the fruit. If stone or seed damage is unacceptable, the trial should test that requirement directly rather than infer it from a recovery percentage.
Read fibre, skin and useful pulp in the reject
Reject composition matters as much as reject mass. Skin, fibre, seed and stone are different materials with different product consequences. A high reject mass may be expected when the feed contains a large inedible fraction, while a low reject mass may simply mean that more fibre or skin passed into the product. Neither number is self-interpreting.
Inspect the reject for useful pulp as well. A dry-looking sample can contain flesh that was carried out with the solids, and that loss may be unacceptable even when the product stream meets its texture target. The FAO guide notes that a pulper or finisher can separate juice from particulate matter, and that screen choice and machine arrangement affect the result. It does not provide a universal acceptable loss for every fruit.
Use a consistent assessment method across runs: weigh the sample, describe visible flesh, wash or separate where the method allows, and record the observations alongside the product result. The point is to compare like with like, not to convert a visual impression into a precise recovery claim.
Compare results on a declared mass basis
Weigh the prepared feed, any added liquid or other additions, the collected product, the reject and any other measured outputs over the same interval. Record the running time. Then calculate the mass closure and report the unaccounted mass. A closure close to the collection boundary is a measurement check; it is not proof that the product meets its quality brief.
Product mass divided by prepared feed is often called recovery, but it is not fruit yield when liquid has been added or when material inventory inside the machine has changed. A high product mass can reflect added water, retained product from a previous run or an unrepresentative collection window. State the calculation and its inputs so that another person can reproduce it.
Measured rates describe the interval that was actually run. They do not become a machine rating or a daily production guarantee. If the trial is intended to support a quotation, keep the material, screen, settings and collection boundary attached to the result, and ask the supplier to state the same basis in the proposal.
Turn the samples into a quotation question
A useful enquiry does not begin with a single recovery target. It begins with the feed, the product specification and the evidence that will be accepted. Ask what will be sampled, who will take the samples, which method will be used for seed integrity and residual pulp, and what acceptance criteria will be agreed before the run.
Request a trial on the intended fruit and preparation, with the proposed screen and stage arrangement. Ask for the product and reject results to be reported together, on a declared mass basis, with photographs or retained samples where practical. If a supplier offers only a recovery number, ask for its feed basis, reject inspection and mass closure.
That approach does not make a trial infallible. It does make the result reviewable. A paired sample, a described feed and an honest mass balance are more useful than a polished percentage that cannot be traced back to the machine, the material and the collection interval.
What to keep with each trial sample
- Run identifier, date, start and end time, and the stable collection interval.
- Fruit or blend, preparation, maturity range, measured temperature and feed rate.
- Machine arrangement, stage, screen aperture and condition, rotor setting and any adjustments.
- Product and reject mass, added liquid, other outputs and material retained in the machine.
- Seed or stone integrity observations, residual pulp assessment and the agreed product quality result.
Sources and scope
These references support the processing principles and method discussed above. They do not establish a material-specific result for any machine, and the article does not convert their examples into a fruitpulper.com performance claim.
- FAO — Principles and practices of small- and medium-scale fruit juice processing, Chapter 6: general juice manufacture principles
Unit operations, feed preparation and the use of pulpers and screens.
- FAO — Principles and practices of small- and medium-scale fruit juice processing, Chapter 15: tropical fruits
Fruit-specific preparation examples, including mango and passion fruit.
Put the note to work
Plan a trial around both outlets
Record prepared feed, through-screen product and retained solids together, then bring the samples to an equipment discussion.