Set Yogurt Syneresis and Starter Acidification after Prepasteurization GOS Dosing

Galacto-oligosaccharide syrups are introduced into the standardized milk base before pasteurization in set yogurt manufacture when the formulation requires co-pasteurization of the prebiotic with the protein phase. The base is prepared from raw milk standardized to 3.0–4.0% w/w milk fat and 3.4–4.0% w/w protein, with added GOS syrup at 1.5–4.5% w/w. Commercial GOS syrups according to supplier technical bulletins contain 55–60% w/w GOS on dry matter, with residual lactose, glucose, and galactose at 20–35% w/w combined and water at 25–30% w/w. In a set yogurt line, the standardized milk is preheated to 55–60 °C, homogenized in two stages at 150–200 bar first stage and 30–50 bar second stage, then pasteurized at 90–95 °C for 5–10 min, cooled to 42 °C, and dosed with a thermophilic starter culture before cup filling and fermentation. The dosing point before pasteurization subjects GOS to thermal load and to heat-induced interactions with milk proteins, and the subsequent acidification by Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus occurs in the presence of heated GOS and its reaction products. Published full-scale data for this specific configuration is limited because most controlled studies dose GOS after pasteurization or use whey protein model systems.

Does Pre-Pasteurization GOS Dosing Shift Starter Acidification Kinetics?

Acidification curves for GOS-fortified milk bases are generated with continuous pH recording using CINAC or iCinac systems at 42.0 °C ± 0.5 °C, with pH probes calibrated according to ISO 26323:2009. Starter culture addition rate is typically 1.0–2.0% w/w of a frozen or freeze-dried thermophilic culture. Fermentation room air temperature must be maintained at 42 °C ± 1 °C, because cup center temperature above 45 °C can reduce Streptococcus thermophilus viability and stall acidification before pH 4.40. The time required to reach pH 4.60, the maximum acidification rate expressed as dpH/dt, and the time to reach maximum acidification rate are the primary kinetic indicators. When GOS syrup is dosed before pasteurization, several process variables intersect. The increase in total solids from approximately 12.0% w/w in a non-fortified base to 15–17% w/w with GOS syrup raises osmotic pressure and reduces water activity, which can extend the lag phase and reduce the maximum acidification rate. The residual lactose in GOS syrup may provide additional fermentable substrate, while glucose and galactose are readily metabolized by S. thermophilus. The net effect cannot be extrapolated from post-pasteurization GOS dosing because heating alters the low-molecular-weight sugar profile and produces Maillard-derived organic acids and melanoidins. Titratable acidity measured according to ISO 11869:2012 should be tracked, but the buffering capacity of GOS and residual protein-bound minerals can cause deviations between pH and titratable acidity. Production-scale batches with GOS dosage above 3.0% w/w have shown increased time-to-pH 4.60 by 10–25 min in some installations, although published reproducibility data for this observation is limited and must be confirmed with site-specific culture rotation.

In continuous processing lines, the dosing point for GOS syrup is often placed in the raw milk balance tank after fat and protein standardization. The syrup must be dispersed before the plate heat exchanger to avoid localized high osmolarity zones that can destabilize casein micelles and increase fouling. A high-shear mixer or venturi injection system operating at 3,000–3,500 rpm for 10–15 min is used in production-scale batches, with recirculation through the balance tank until the GOS concentration measured by refractometry or HPAEC-PAD is homogeneous within ±0.1% w/w. Undissolved GOS syrup can settle in the balance tank or dead legs and create batch-to-batch variability in acidification and syneresis. The syrup pH is typically 3.5–5.0, which may reduce the milk base pH before pasteurization by 0.05–0.15 pH units at 3.0% w/w addition; this pre-acidification shift must be considered when setting the starter culture inoculum and when interpreting the initial pH of the pasteurized base. The dosing point should also be upstream of any vitamin or mineral premix addition to avoid localized precipitation of calcium phosphate, which can be promoted by GOS and by heating. If GOS syrup is cold from storage at 4 °C and added directly before pasteurization, the preheating load must compensate for the syrup enthalpy; in-line dosing of a cold syrup at 10–15 °C can reduce the blend temperature by 1–3 °C, depending on addition rate.

Syneresis Quantification and Whey Separation Limits in Set Gels

Syneresis is assessed by volume or mass of whey released after 24 h at 4 °C without disturbance, and by forced centrifugation of intact set yogurt cups at 1,200 × g for 10 min at 4 °C. The forced syneresis method uses a fixed cup weight of 50 g and a centrifuge fitted with a swing-bucket rotor; results are expressed as the percentage of released whey to initial sample mass. No ISO method exists for yogurt syneresis; the centrifugal method is an internal quality-control procedure that must be validated against sensory and packaging drip observations. A control limit of ≤2.0% w/w released whey after 24 h is common in set yogurt specifications, but this is a commercial target rather than a Codex requirement. Pre-pasteurization GOS dosing introduces two opposing effects on whey separation. The high water-binding capacity of GOS and residual lactose may reduce free water in the gel pores and lower spontaneous syneresis. Conversely, if GOS or its heat-degradation products interfere with denatured whey protein attachment to casein micelles during pasteurization, the gel may form a coarser protein network with larger pores and increased whey expulsion. Protein denaturation indices such as residual native β-lactoglobulin concentration after heating should be measured by HPLC or SDS-PAGE; in a non-fortified set yogurt base heated at 90–95 °C for 5–10 min, residual native β-lactoglobulin is typically below 5% of the original content, but published data for GOS-fortified bases is limited. The firmness of the set gel can be measured by penetration force using a texture analyzer with a 10 mm diameter cylinder probe at 20 mm/min crosshead speed and 10 mm penetration depth after 24 h at 4 °C; values are used for comparative process validation only.

When Pre-Pasteurization GOS Dosing Is Combined with HTST and Two-Stage Homogenization

Because GOS syrup is added before the plate heat exchanger, the product-side pressure drop and fouling tendency can differ from an unfortified base. The higher total solids and viscosity may raise pressure drop across the regeneration, heating, and holding sections by 10–30%, depending on the syrup composition and base flow rate. The hold time and temperature must be verified with a holding tube calculation using the fastest-flowing particle and the measured flow rate; the nominal holding time of 5 min at 95 °C is not equivalent to the minimum safe time if product viscosity changes. The two-stage homogenization step is preferably placed after preheating to 55–65 °C and before final heating. First-stage pressure of 150–200 bar and second-stage pressure of 30–50 bar reduce fat globule median diameter to 0.8–1.2 µm in unfortified bases, but GOS syrup can increase the viscosity and alter droplet breakup efficiency. Process validation for GOS-fortified set yogurt should include particle size analysis by laser diffraction according to ISO 13320:2020. If the fat globule size exceeds 1.5 µm after homogenization, free fat may appear in the set gel and increase syneresis. Pre-pasteurization GOS dosing may also increase heat-induced lactulose formation because the added reducing sugars are present during the thermal load. Lactulose concentration in pasteurized milk is typically 10–50 mg/L at 95 °C for 5 min; in GOS-fortified bases no universal correction factor exists and published data for lactulose in this specific configuration is limited. Product viscosity at 20 °C may be measured by Brookfield rotational viscometry with a T-bar spindle at 5 rpm; however, set yogurt is not stirred after fermentation, so viscosity is less informative than penetration force.

Cleaning of the pasteurizer and dosing lines after GOS-fortified product requires verification of rinse-water sugar removal. GOS syrups are sticky when dried on plate surfaces, and residual sugar can caramelize during alkali cleaning if the pre-rinse temperature is above 60 °C. A cold-water pre-rinse at 20–30 °C until the rinse water refractometer reading is ≤1.0 °Brix is therefore inserted before the caustic circulation. The subsequent caustic wash uses 1.5–2.0% w/w NaOH at 75–80 °C for 20–30 min, and the acid wash uses 0.8–1.2% w/w nitric acid at 65–70 °C for 15–20 min. Verification of cleaning efficacy by ATP bioluminescence is performed before production; the post-rinse ATP reading must return to the plant-established background value. The same balance tank and recirculation loop must be checked for residual GOS in dead legs and sampling ports because residual GOS can support adulterant microbial growth in the pasteurized milk if post-pasteurization contamination occurs. Published data on GOS-specific fouling rates in plate heat exchangers is limited, but production-scale observations indicate that the regeneration section shows higher deposit formation when the GOS addition rate exceeds 3.5% w/w and the product is held at 95 °C for more than 5 min.

Thermal Damage Indicators and Furosine Accumulation

Because GOS syrups contain reducing sugars, co-pasteurization can generate furosine, carboxymethyllysine, and lactulose. Furosine is measured by ion-pair reversed-phase HPLC according to ISO 18329:2004; in pasteurized milk not fortified with GOS, furosine values are typically below 10 mg/L, whereas in GOS-fortified bases the value depends on the reducing sugar content of the syrup and the thermal load. No universal acceptance limit for GOS-fortified set yogurt exists in Codex STAN 243-2003; process validation should compare the furosine value of the fortified base against the same base without GOS using identical pasteurization conditions. An increase of furosine by more than 5 mg/L relative to the unfortified control indicates that the syrup is contributing reactive reducing sugars and that the thermal load or holding time may need to be reduced. The starter culture may be inhibited by Maillard products if their concentration becomes high, but published data for the specific inhibition threshold in thermophilic yogurt starters is limited. Residual native β-lactoglobulin content, lactulose concentration, and furosine should be reported together when establishing the upper limit for prepasteurization GOS addition. The operational boundary is site-specific because the plate heat exchanger configuration, regeneration efficiency, and final cooling time all influence the integrated thermal load. A production facility using high regeneration and a holding time of 5 min at 95 °C may achieve acceptable acidification at 4.5% w/w GOS, while a facility with longer cooling residence times may exceed the furosine control limit at 3.0% w/w. No single universal maximum can be stated without measuring the integrated F0 and the reducing sugar profile of the syrup.

Process capability for GOS-fortified set yogurt is established by running three consecutive production lots with the same syrup lot, the same starter culture rotation, and the same pasteurizer hold time. The batch record must include the GOS addition rate expressed as percentage of final product, the pH of the raw base after GOS addition, the pasteurization temperature and hold time, the homogenization pressures, the time to pH 4.60, the final pH after cooling, the titratable acidity per ISO 11869:2012, and the syneresis value after 24 h at 4 °C. A batch with a final pH above 4.60 or a spontaneous syneresis above 2.5% w/w should be diverted for process review and not released as set yogurt without corrective action. The starter culture rotation should be maintained, because repeated use of a single culture may lead to phage-mediated acidification failure that is not caused by GOS but can be misattributed to the prebiotic. The operational boundary includes pre-drying or conditioning of powdered GOS if used, because high-moisture powder can clump in the dosing line; syrup or powder feeding equipment must be integrated into the plant allergen and cleaning matrix. No conclusion is drawn from any single pilot batch, because the interaction between prepasteurization GOS dosing and set yogurt syneresis is subject to mineral equilibrium, heat load, homogenization efficiency, and starter culture strain selection.