Pouch Filling: How Puree Viscosity & Particle Size Affect Line Performance
Pouches are efficient, convenient, and scalable—but pouch lines demand tight control of puree behavior. In baby food manufacturing, the same puree that tastes perfect in a cup can be a nightmare on a high-speed pouch filler. Two variables drive most line performance issues: viscosity (how thick the puree is) and particle size (how smooth or coarse the puree is). If viscosity is too high, the filler struggles to hit target weights at speed, pumps cavitate, and nozzles drip or string. If particle size is too large or poorly controlled, nozzles clog, fill weights drift, and seals fail due to product contamination at the seal area. Even “small” differences matter because pouch filling runs fast and every second of downtime is expensive. This guide explains how viscosity and particle size translate into real operational outcomes, what to specify for reliable pouch runs, and how to align ingredient procurement with line performance.
For baby food base fruit puree requirements, see Topic 061. For vegetable + fruit blends that often challenge viscosity and particle specs, see Topic 062. For aseptic vs frozen inputs (thaw variability impacts viscosity), see Topic 063.
Why pouch fillers are sensitive to viscosity and particle size
Pouch filling is fundamentally a flow-control problem. The filler must deliver accurate volume or weight into a flexible package at high speed. Anything that makes flow inconsistent forces the filler to slow down, overfill, or reject packages. Viscosity determines how easily the puree can be pumped, metered, and cut cleanly during dosing. Particle size determines whether the product flows smoothly through nozzles and valves without clogging or “bridging.” Together, these variables control: fill accuracy, fill speed, nozzle cleanliness, seal integrity, and downtime frequency.
Viscosity basics: what it means operationally
Viscosity is often treated as a lab number, but on pouch lines it shows up as: pump load, fill consistency, drips, stringing, and variability in shot cutoff. Thick products may require higher pump pressure and can increase mechanical stress on equipment. Very thick products can also trap air, which creates foaming and fill-weight variation. On the other side, products that are too thin may splash, create inconsistent headspace, or migrate into the seal area. The goal is not “as thick as possible” or “as thin as possible”—it is “consistent and predictable at fill temperature.”
Temperature matters: viscosity is not constant
Purees change viscosity with temperature. A blend that is perfect at 20°C can behave differently at 35–45°C after preheating or thermal staging. Plants must align viscosity specifications to the actual fill temperature range. This is a common reason why bench trials succeed but production struggles: the plant fills warmer (or colder) than the bench assumed. If you are making a change (new supplier, new fruit ratio, new process), measure viscosity at the real operating temperature.
Shear and mixing: how processing changes puree flow
Mixing energy matters. Some purees are shear-thinning: they flow more easily under agitation and then thicken again at rest. High-shear mixing can reduce apparent viscosity, break down structure, or change particle distribution. Low-shear blending may leave localized thick zones that create inconsistent fills. This is why a “viscosity spec” should be paired with an SOP: mixing time, speed, and sequence, plus hold-time limits before filling.
Particle size: why “smooth” needs a real spec
Particle size issues are responsible for a large fraction of pouch downtime. Even if a product looks smooth in a cup, small particles can accumulate at valves or nozzle tips. Larger particles can bridge at restrictions and cause intermittent clogging. In addition, particles can contaminate seal zones, leading to leakers and increased rejects. “Smooth” should be defined as measurable limits: sieve specs, maximum particle size, or standardized screen controls at production.
This matters even more for vegetable blends (carrot, sweet potato) and fibrous fruits, which naturally carry more insoluble material than apple or pear. See Topic 062.
Deaeration and foaming: hidden drivers of fill-weight variability
Air in puree causes inconsistent fill weights because air compresses and expands. It also increases the chance of oxidation and can create a “spongy” texture perception. Foaming can be introduced by aggressive mixing, recirculation loops, or transfer pump cavitation. In pouch lines, foaming can create messy nozzles and higher reject rates. Many plants use gentle mixing, controlled pump selection, and (when appropriate) deaeration steps to stabilize filling behavior.
Solids and °Brix: how base ingredients shift viscosity
Solids drive viscosity. If your apple puree (or fruit carrier base) has higher solids, it can push viscosity upward across the blend. If your vegetable component is fibrous or concentrated, it can increase thickness and change flow. In baby food portfolios, small solids drift can cause large operational impact because pouch fillers magnify variability. That’s why procurement specs around solids/°Brix matter for line stability.
For how to specify °Brix/solids, acid, and pH, see Topic 095.
Aseptic vs frozen: thaw history affects viscosity and particle distribution
Frozen purees can behave differently depending on thaw rate and re-homogenization. Ice crystal formation and thaw separation can create localized thick zones or watery zones. If the plant does not re-mix consistently, pouch filling behavior can drift across a run. Aseptic purees generally reduce this variability because they do not require thaw staging, which is why many baby food plants prefer aseptic for high-speed pouch operations.
For the full risk and logistics comparison, see Topic 063.
QA checkpoints that protect line performance
Pouch lines run best when the plant uses a small set of practical QA checkpoints tied to performance: viscosity at fill temperature, particle size/sieve compliance, homogeneity after mixing, and foam/air checks. These are operational metrics—not just lab data. The goal is to identify deviations early, before they become downtime. If you are onboarding a new puree supplier or changing fruit ratios, run a structured line trial with these checkpoints.
Micro and hold-time risk: infant products require discipline
Even if a puree is aseptic on receipt, pouch operations can introduce risk through open handling and long hold times. For baby food, plants typically define strict hold-time rules and sanitation SOPs. This also intersects with viscosity: thick products are harder to clean out of lines and can increase sanitation burden. Align the product design with realistic cleaning and sanitation capabilities.
For micro specs and buyer expectations, see Topic 094. For baby food documentation expectations, see Topic 067.
Procurement specs: how to buy for pouch performance
Pouch-friendly puree procurement often includes: viscosity range (defined at a specific temperature), particle size or sieve specs, solids/°Brix range, pH and acidity (for finished product control), and documentation (COA, micro, traceability). The goal is to reduce variability so the line can run at full speed with fewer adjustments.
For COA interpretation, see Topic 093. For packaging formats and handling, see Topic 096. For traceability, see Topic 099. For a spec sheet template you can adapt to pouch operations, see Topic 100.
Next steps
If you share your pouch filler type, target throughput, nozzle configuration, typical fill temperature, product viscosity challenges, and the fruit/vegetable blend profile, PFVN can recommend puree formats and specification targets that reduce downtime and stabilize fill accuracy. Use Request a Quote or visit Contact. You can also browse Products and Bulk Juice Concentrates.
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