GOS Powder Sachet Compression Moisture Pickup and Fill Weight Control

Galacto-oligosaccharide (GOS) powder is produced through enzymatic transgalactosylation of lactose followed by membrane purification, concentration, spray drying or freeze drying, and blending to a defined oligosaccharide profile. The dried powder is an amorphous carbohydrate matrix with a particle size distribution typically spanning 20 µm to 250 µm, a bulk density that can range from 0.45 g/cm³ to 0.70 g/cm³ depending on spray-dryer atomization settings and post-drying conditioning, and a pronounced hygroscopicity that shifts bulk handling properties within minutes of exposure to ambient air. Fill weight control on high-speed sachet lines is therefore not a fixed volumetric setting but a dynamic system governed by bulk density, flow function, hopper residence time, moisture sorption from the environment, and the compression history of the powder inside the formed sachet. Moisture pickup can occur during pneumatic conveying, hopper holding, auger or cup dosing, seal jaw closing, and post-packaging storage; each stage introduces either mass change or rheological change that affects the delivered net weight and the uniformity of the anhydrous GOS content. Production-scale failure modes observed on vertical form-fill-seal equipment include cohesive arching above the auger inlet, powder build-up on seal jaws, film static attraction of fine particles, and checkweigher reject chute obstruction caused by sticky fines accumulating after the seal area cools.

What Drives Fill Weight Variability in a Compression-Sensitive Amorphous Powder?

Fill weight variability in GOS powder sachets is primarily governed by the interplay between the dosing mechanism and the powder’s shear stress response under consolidation. In an auger filler, the delivered mass per dose equals the fill volume displaced per auger revolution multiplied by the bulk density of the powder entering the auger flights; if the powder develops a cohesive arch above the auger inlet, the material feed becomes intermittent and under-dosing occurs even when the auger rotation count remains constant. Vacuum volumetric cup fillers introduce a separate failure mode: if the vacuum is set too high, the powder compacts in the cup and delivers a higher mass per volume; if the powder is cohesive from moisture, the vacuum draw may fail to pick up a consistent cup volume, producing under-dosing. On a high-speed line running at 120 sachets/min, the standard deviation of fill weight can increase as hopper level fluctuates because the consolidation state of the powder bed changes with bed depth, altering the bulk density at the auger intake. A loss-in-weight feeder equipped with a 20 mm auger and 2.0 mm flight pitch can reduce level-related variation, but only when the hopper agitator speed is tuned to prevent clump retention and shear-induced moisture redistribution. The fill weight target is specified as a net weight with a legal tolerance band of ±5% for many prepackaged food supplements under EU 76/211/EEC or NIST Handbook 133; however, internal control limits of ±2.0% are common for GOS formulations because moisture uptake and bulk density drift consume a portion of the legal tolerance before the product reaches the consumer. Bulk density measurements per USP <616> Method I should be performed on powder sampled from the dosing chamber rather than from a drum, because the tapped density and Hausner ratio change with relative humidity; a Hausner ratio above 1.25 generally indicates that external vibration, flow aid addition, or automated weight correction is required. Flow function coefficient values obtained by shear cell testing per ASTM D6773 at 3 kPa normal stress provide a more direct indication of hopper-to-auger discharge behaviour than bulk density alone, and the control limit should be derived from batch history rather than from a single universal powder classification.

Moisture sorption in GOS powder is controlled by the difference between the powder’s water activity and the relative humidity of the surrounding headspace, and by the water vapour transmission rate of the sachet film after sealing. At 25 °C and 60% RH, unprotected amorphous GOS can sorb water within minutes, increasing surface cohesion and reducing the flow function coefficient; at 75% RH, surface water uptake is usually sufficient to cause visible caking and particle fusion in the hopper. The critical water activity for stickiness and caking is grade-specific because galacto-oligosaccharide powders differ in degree of polymerization, residual glucose and lactose content, and drying history; published data for this specific configuration is limited, so the caking threshold must be established by controlled moisture sorption isotherm studies using a dynamic vapour sorption analyser or saturated salt slurries. Water activity measured by a dew point chilled mirror at 25 °C per USP <922> is more predictive of caking and flow loss than total moisture content, because total moisture includes bound water that does not plasticize the amorphous matrix. Karl Fischer titration per USP <921> Method Ia and loss on drying per USP <731> at 105 °C should be used together; a divergence between the two results can indicate residual organic volatiles from spray drying or degradation of lactose-derived impurities. The moisture sorption isotherm for GOS powder typically shows a steep uptake above 60% RH, and the glass transition temperature decreases as water content increases because water acts as a plasticizer for the amorphous carbohydrate matrix; when the powder temperature exceeds the glass transition, the material can undergo viscous flow and form interparticle necks, causing caking and dosing instability even if the water content remains below a specification limit.

When Seal Jaw Compression Interacts with Hygroscopic Powder

Seal jaw compression during vertical form-fill-seal packaging does not change the net mass of the powder but can create a densified zone near the seal area, displace air from the sachet, and force fine particles into the seal region if the product has become sticky from moisture uptake. Pneumatically or hydraulically actuated seal jaws closing with a force of 400 N to 800 N and a dwell time of 0.1 s to 0.3 s compact the powder bed; if the powder is above its glass transition temperature due to moisture plasticization, this compression can cause particle sintering at the seal interface and powder build-up on the jaw faces. The resulting product-in-seal defect reduces seal strength and can lead to leakers, while product loss from the sachet cavity to the jaw surface alters the net fill weight of affected sachets. If the jaw cooling water is set below the dew point, condensation forms on the jaw face and transfers water to the powder during compression; the cooling water temperature should be maintained at least 5 °C above the filling suite dew point. Compression also creates a transient headspace pressure increase that can rupture the film or open the seal if the fill volume is too high; therefore, the fill volume and headspace must be designed so that the sachet is not fully compacted before the seal dwell is complete. The sachet fill volume should be limited so that the compaction ratio does not exceed approximately 1.35 times the settled bulk density; a higher compaction ratio may densify the powder beyond the point where it can be re-dispersed and may increase seal contamination. On high-speed lines, seal jaw temperature, pressure, dwell time, and jaw flatness interact with powder moisture content; a processing window of ±5 °C around the seal jaw set point is often required because jaw temperature below the set point may fail to melt the film adequately, while jaw temperature above the set point can heat the powder at the seal area and accelerate moisture-induced stickiness. The relationship between sachet fill weight and seal jaw compression is most visible when the fill weight standard deviation is plotted against sachet height after sealing; densified and under-filled sachets show a characteristic shift in the height-to-weight ratio that can be used as an early warning of moisture-related caking.

Moisture Sorption Isotherms, Glass Transition, and Caking Induction

The sorption isotherm of an amorphous galacto-oligosaccharide powder can be divided into a low-humidity region dominated by monolayer adsorption, an intermediate region where water clusters form and plasticization becomes significant, and a high-humidity region where the powder can absorb enough water to dissolve soluble oligomers and form sticky bridges. The rate and extent of moisture pickup depend on particle size, porosity, surface area, the presence of glassy lactose residues, and the degree of crystallinity retained after spray drying; spray-dried GOS powder is usually fully or partially amorphous while freeze-dried powder may have a higher surface area and faster initial sorption. Dynamic vapour sorption testing over a range from 10% RH to 80% RH at 25 °C can identify the critical relative humidity at which mass gain accelerates, and this value should be used to set the maximum allowable hopper room humidity. If the powder is exposed to humidity above the critical threshold, the surface water activity rises, the glass transition temperature falls below the process temperature, and interparticle bridge formation leads to caking; this caking is not reversible by simple re-milling because the amorphous structure has undergone viscous flow and may require re-drying and re-cooling to restore an acceptable flow function. The hopper and dosing chamber should be maintained at 40% RH or lower for GOS powder, with excursions above 60% RH triggering pre-drying or dry air purging of the hopper headspace. The processing window for post-drying cooling is frequently narrow—within ±5 °C of the filling room temperature—because powder that is too warm may stick to contact surfaces and powder that is too cold may develop condensation when transferred into a warmer room. The addition of a hydrophobic flow aid such as fumed silica within a narrow window of 0.3 wt% to 0.8 wt% can reduce moisture-induced cohesion and improve fill weight uniformity, but above 1.0 wt% the same flow aid may lower bulk density, generate dust, and alter dissolution behaviour in a way that is unacceptable for the final sachet application.

Dry Air Purging at the Hopper Is a First Defence, Not a Complete Solution

Dry air purging of the feed hopper and dosing chamber reduces moisture pickup during line residence, but it does not correct water activity or caking that has already developed in the powder before it reaches the filling machine. Plant compressed air must be dried to a dew point of −40 °C or lower for contact with hygroscopic GOS powder, and the purge flow rate should be balanced against the hopper exhaust so that fine particles are not entrained and removed from the process. Compressed air quality should meet ISO 8573-1:2010 class 2.2.2 or better, with particular attention to the pressure dew point and the absence of oil aerosol that could contaminate the sachet contents. A purge system with a dew point of +3 °C may still permit moisture transfer to the powder, especially when the hopper is opened for replenishment or when the room dew point is high. Hopper residence time itself is a critical process parameter: the longer the powder remains in a partially emptied hopper, the greater the opportunity for moisture pickup, particle settling, and cohesive arch formation. Production-scale observations indicate that fill weight drift often begins after a line stoppage of 15 min or more, because the static powder bed consolidates and the surface of the powder in the hopper gains moisture from the headspace; restarting the line without purging the first few doses can produce a cluster of over- or under-weight sachets. The hopper should therefore be operated with a defined maximum residence time, and the auger or cup dosing chamber should be purged continuously with dry air when the line is idle. The use of nitrogen purge is warranted only when the powder is oxygen-sensitive or when the film structure is prone to oxidative odour transfer; nitrogen is not a substitute for low-humidity air in controlling moisture pickup. In addition, the hopper and dosing chamber contact surfaces should be fabricated from 316L stainless steel with a surface finish of Ra < 0.8 µm to reduce particle adhesion, and all gaskets and seals should be checked for water traps after wet cleaning.

Fill weight control on a GOS sachet line is closed-loop only when the checkweigher data are used to adjust the dosing mechanism rather than only to reject nonconforming sachets. A checkweigher with 100% inspection at 120 sachets/min must have a sampling rate and filter setting that separates real weight trends from mechanical vibration and film flap noise; the weigh cell should be installed on a rigid, isolated frame and the transfer belts should be cleaned frequently to prevent fine powder accumulation. The checkweigher should meet the accuracy requirements of OIML R 51, and its repeatability standard deviation should be less than one-quarter of the internal tolerance band so that the measurement system does not dominate the observed fill weight variation. The short-term standard deviation of fill weight is usually corrected by adjusting the auger rotation speed or cup fill volume, while the long-term drift is corrected by monitoring the moving average against the target and feeding back a bias to the dosing system. The control algorithm should be configured so that a sudden increase in standard deviation does not trigger immediate over-correction of the mean, because moisture-induced caking often appears first as an increase in variation rather than a shift in the mean. Near-infrared or microwave moisture sensors can provide real-time hopper moisture indication, but calibration transfer across particle size and lactose residues is non-trivial; release decisions should still rely on USP <731> and USP <921> methods. Reject limits should be set at ±2.0% of target for internal control, with the legal tolerance of ±5% retained as a secondary boundary; sachets outside the internal limit should be quarantined and inspected for moisture content, water activity, and seal defects. The fill weight target itself should be expressed on the basis of anhydrous GOS content per sachet rather than simple gross mass, because moisture ingress during storage increases gross mass but reduces the mass fraction of GOS. A moisture gain allowance of 0.5–2.0% should be included in the label claim calculation only when the barrier film evaluation confirms the actual shelf-life moisture ingress under the intended distribution environment. Statistical process control charts for fill weight, moisture content, water activity, and seal strength should be maintained as a single integrated database so that shifts in one parameter can be traced to upstream changes in the other.

Cleaning and changeover procedures exert a direct influence on fill weight control because residual water on auger flights, cup surfaces, or hopper walls causes the next batch of GOS powder to stick, clump, and dose unpredictably. After wet cleaning, the disassembled contact parts should be dried with desiccant-dried air at 40 °C to 60 °C and verified by wipe tests or dew point measurement before reassembly; a single water droplet in a stainless steel auger bore can create a nucleation site for powder build-up and can increase fill weight variability for the first 10–20 sachets after restart. Product changeover from a non-hygroscopic powder to GOS powder should include a dry purging step with low-humidity air to remove residual fines and moisture from dead zones; the use of water alone for cleaning can leave thin films that are not visible but are sufficient to alter powder flow. GOS powder should not be dry blended with amine-containing excipients or amino acid premixes unless compatibility studies have been completed, because Maillard browning and hygroscopicity interactions may alter the glass transition and flow properties of the mixture; published data for this specific configuration is limited, so a binary mixture sorption study should be performed before any such formulation is introduced to the sachet line. The same caution applies to the addition of hygroscopic sweeteners or fruit powders, which can lower the critical relative humidity of the blend and narrow the processing window.

Engineered Barrier Films and Sachet Water Vapour Transmission Rate Selection

The sachet film structure is the primary barrier against moisture pickup after sealing, and its selection must balance water vapour transmission rate, seal strength, puncture resistance, and the mechanical demands of the form-fill-seal process. For hygroscopic GOS powder, a high-barrier laminate containing aluminium foil with a water vapour transmission rate of 0.05 g/m²/day to 0.5 g/m²/day at 38 °C and 90% RH is usually specified, but the actual sachet performance depends on the integrity of the foil layer and the seal area. The water vapour transmission rate of the film should be measured per ASTM F1249 using a modulated infrared sensor, and the seal strength should be measured per ASTM F88/F88M-21 at 300 mm/min to ensure that seals withstand internal pressure and distribution compression. For a pillow sachet with external dimensions of 60 mm × 80 mm, the surface area available for transmission is approximately 96 cm²; at a film WVTR of 0.5 g/m²/day, the calculated moisture ingress is small per day but significant over a 24-month shelf life if the external humidity is high. The driving force for moisture ingress is the difference between the external water vapour partial pressure and the sachet headspace water vapour partial pressure, not simply the relative humidity difference; therefore, the moisture load is higher in tropical distribution environments and the film must be specified for the worst-case distribution condition. In addition to the WVTR of the film, the seal geometry and the presence of product in the seal area affect the effective moisture barrier; product-in-seal defects can create capillary pathways that bypass the barrier layer and allow localised moisture ingress even when the film itself meets specification. Seal jaws should be inspected for build-up after every production run, and the seal width should be verified against the film supplier’s minimum seal width requirement. The interaction between the film’s inner sealant layer and GOS powder under compression should also be considered: a low-melt sealant may soften at seal jaw temperatures and accept powder particles, while a high-melt sealant may require jaw temperatures that are high enough to heat the powder at the seal interface. The selected film structure must therefore be validated with the actual product, fill weight, headspace, and seal jaw settings rather than by film supplier data alone.

The operational boundary for GOS powder sachet filling is defined by incoming water activity, hopper room dew point, seal jaw temperature, and the barrier film’s water vapour transmission rate. Incoming powder with a water activity above 0.35 should be re-dried or rejected; a room relative humidity above 60% requires pre-drying and dry air purging; and a seal jaw temperature excursion of more than ±5 °C from the validated set point should trigger a line stop because the combined effects of heat and moisture cause powder stickiness that degrades fill weight control. The filling suite should be maintained at 21 °C to 25 °C and 40% RH to 50% RH, with continuous recording of dew point and humidity so that transient spikes during shift changes and door openings are not concealed by daily averages. The maximum hopper residence time should be defined during process qualification by measuring fill weight standard deviation and water activity at scheduled stoppages of 10 min, 20 min, and 30 min, because the onset of variability is often abrupt once the powder bed consolidates and the surface moisture content exceeds the critical limit. The use of a barrier film with a low WVTR does not eliminate the need to control upstream moisture pickup; it only reduces the shelf-life moisture ingress after sealing. Similarly, checkweigher feedback does not correct the underlying flow property change that occurs when GOS powder cakes in the hopper. The entire line must be managed as a single mass-and-moisture balance, with fill weight, moisture content, water activity, seal strength, and film WVTR recorded in the same batch record and reconciled against the anhydrous GOS label claim.