Cold Flow Stability and Boiling Point Depression in Sugarless Confectionery

In sorbitol-free hard candy manufacture, continuous vacuum cooking of isomalt and maltitol syrup is carried out in scraped-surface heat exchangers at absolute pressures of 0.15–0.35 bar, where the equilibrium boiling point of the aqueous phase is suppressed below 70–85 °C. The final product temperature at the discharge nozzle is typically maintained between 168 °C and 175 °C for isomalt-dominant formulations because residual moisture must fall below 2.0 wt%; a lower discharge temperature produces a soft, hygroscopic glass that cold-flows under the force of twist-wrapping and case stacking. Cold flow in this context is not a bulk melting event but a creep process in the glassy state, and it becomes measurable when the storage temperature approaches Tg − 15 K. Differential scanning calorimetry performed in accordance with ISO 11357-2:2020 on vacuum-dried isomalt glass samples yields a glass transition midpoint from 58 °C to 66 °C, depending on residual moisture and cooling rate. The corresponding creep compliance measured on a controlled-stress rheometer fitted with sandblasted 20 mm parallel plates at 40 °C and 50% RH increases by an order of magnitude when moisture exceeds 1.8 wt%. Production-scale deposits of isomalt-based hard candy have shown that slabs stored at 35 °C and 75% RH develop visible surface dimpling within 48 h unless sealed packaging with a water vapor transmission rate below 1.0 g/m²/day at 38 °C and 90% RH is used. Maltitol-based glass with a lower glass transition of 40–48 °C cold-flows at a lower ambient temperature, and its formulation therefore requires a final moisture below 1.2 wt% and the inclusion of 0.3–0.8 wt% hydrogenated starch hydrolysate to increase the average molecular weight of the glass. The same thermodynamic constraint explains why boiling point control must be interpreted through formulation-specific refractometric curves rather than a single sucrose-based cook chart.

Does Higher Average Molecular Weight in Polyol Syrups Depress the Boiling Point at Fixed Refractometric Solids?

In sugar-confectionery practice, the boiling point of a cooking syrup is interpreted as an indirect measurement of dissolved solids and final moisture. The relationship is not determined solely by sugar content; it follows the colligative and thermodynamic water activity of the solution, and therefore the number-average molecular weight of the dissolved species controls the boiling point at a given refractometric dry solids. Sucrose-based control syrup with dissolved sucrose and 42 DE glucose syrup exhibits a number-average molecular weight near 275–310 g/mol. A maltitol syrup or hydrogenated starch hydrolysate with a typical oligomer distribution of sorbitol, maltitol, maltotritol, and higher polyols has a number-average molecular weight of 420–520 g/mol for high-maltose HSH grades. At the same apparent dry solids of 85 wt%, the molality of the HSH solution is lower than that of the sucrose/glucose syrup; the result is a lower boiling point at atmospheric pressure, commonly by 2–5 °C, and a lower vapor pressure reduction per unit mass. This is the boiling point depression encountered when a sugarless cook profile is transferred directly from a sucrose formula without correcting for average molecular weight. In contrast, when sorbitol or xylitol crystals are dissolved, the reduction in molecular weight to 182 g/mol or 152 g/mol raises molality at the same solids and produces a boiling point elevation relative to sucrose of approximately 3–7 °C. The practical consequence is that cooking to a fixed discharge temperature will yield different residual moisture for different polyol blends. Vacuum pans equipped with 4–20 mA pressure transmitters and calibrated Pt100 RTDs with an accuracy of ±0.2 °C are required to distinguish these small but formulation-critical differences. The relevant conversion can be expressed through Dühring’s rule: a plot of solution boiling temperature versus solvent boiling temperature at constant composition is linear, and calibration lines for each polyol blend must be generated at plant elevation because barometric pressure changes shift the apparent final solids by up to 1.5 wt% for a fixed temperature. Refractometric dry solids determined at 20 °C should be used to correct the boiling point target. Published data for specific ternary polyol blends is limited, so commissioning runs require in-plant boiling point curves generated under actual vacuum conditions. If the target final moisture is 1.2 wt% for an isomalt/maltitol blend, the discharge temperature may need to be adjusted upward by 3–6 °C when replacing 20 wt% of the maltitol syrup with sorbitol to maintain equivalent water activity.

Chewy sugarless confectionery based on maltitol syrup, hydrogenated starch hydrolysate, and gelling agents such as gelatin or gum arabic presents a different cold flow failure mode because the matrix is intentionally above its glass transition and behaves as a highly viscous melt at ambient temperature. In these products, cold flow is controlled by the plateau modulus of the gel network and the water activity of the humectant phase, rather than by the glassy-state creep of hard candy. Maltitol syrups with 75–80 wt% dry solids, mixed with 0.5–1.5 wt% high-acyl gellan or 5–10 wt% gelatin 250 Bloom, are cooked in a batch vacuum caramel cooker to a final moisture of 6–10 wt%. The final water activity, measured at 25 °C by ISO 18787:2017, is held between 0.45 and 0.60; below this range, the product develops hard, glassy texture; above it, the piece loses shape under its own weight within 24–72 h. Cold flow in soft sugar-free candies is frequently observed at the interface between the product and a starch-free tray or paper cup after storage at 30 °C and 65% RH, where surface moisture uptake softens the outer 0.5–1.0 mm layer. Production lines address the problem by applying a thin film of gellan gum or a coating of high-melting vegetable fat with a slip melting point of 34–38 °C, but fat barriers alter mouthfeel and require cocoa butter-compatible triglyceride blends. The deformation kinetics can be characterized by creep-recovery measurements on a 25 mm parallel-plate rheometer in accordance with ISO 6721-10:2015, with an applied shear stress of 100 Pa for 300 s; the recovered strain after 600 s provides an operational limit of less than 15% permanent deformation. Twin-screw continuous mixers with an L/D ratio of 40:1 and jacket zones maintained at 92–105 °C are used to disperse the gelling agent without excessive shear that would degrade molecular weight and reduce network strength. A batch-to-batch variation of ±2 °C in cook temperature shifts moisture by ±0.5 wt%, which can move the product from stable shape retention to cold flow failure.

Glass-Rubber Transition Boundaries in Deposited Isomalt and Maltitol Systems

The cold flow stability of sugarless glass is governed by the gap between the storage temperature and the plasticized glass transition. In production, this gap is narrowed by residual moisture, low molecular weight polyol fractions, and compressive stress in bulk packaging. Table 1 summarizes the relevant published transitions and moisture tolerance limits for the major glass-forming sugarless systems. The values are compiled from industrial technical bulletins and peer-reviewed thermal analysis studies; they should be treated as indicative rather than universal because the measured glass transition depends on heating rate, moisture equilibration time, and thermal history. A hard candy matrix formulated with 60 wt% isomalt and 40 wt% maltitol syrup solids can exhibit a glass transition that is 8–12 °C lower than pure isomalt because the maltitol component lowers the average glass transition. Consequently, the addition of maltitol syrup to isomalt increases flexibility and reduces brittleness but also reduces cold flow resistance. Conversely, replacing 10 wt% of maltitol syrup with crystalline isomalt raises the glass transition and permits bulk storage in warm climates without twist-wrap deformation. At a storage temperature of 30 °C, a glass transition of 38 °C is insufficient to prevent creep in a compression-loaded case, whereas a glass transition of 52 °C provides a margin of 22 K and generally limits deformation to below 2% after 6 months under 2 kPa stack pressure.

For high-speed depositing of sugarless hard candy into Teflon-coated dies at 72–80 °C die temperature, release requires that the glass transition of the cooling surface layer remain below the die release temperature. Sticking and cold flow are simultaneous risks: if the die is too cold, the surface quenches below the glass transition and cracks; if it is too hot, flow continues after forming and the piece loses dimensional accuracy. The transition from rubbery to glassy state on cooling occurs over a narrow window of 8–12 °C, so die temperature control using electrically heated water-jacketed depositing heads with ±0.5 °C uniformity is required for production speeds above 1,200 pieces/min. Sugarless hard candy with residual moisture below 1.0 wt% can still cold-flow if the cooling tunnel residence time is shorter than the time required for the center of the piece to cool below Tg − 5 K. The thermal diffusivity of an isomalt glass is approximately 1.0–1.2 × 10⁻⁷ m²/s; for a 2.0 g piece with a thickness of 5.0 mm, center cooling from 170 °C to 60 °C requires 3–6 min under forced-air cooling at 12–15 °C and 2.0–3.0 m/s air velocity. Flow-wrapping in that condition transfers compressive stress to the still-warm piece and produces flattened or domed ends. Equipment suppliers therefore specify a minimum cooling tunnel length of 8–12 m for a 1,200 piece/min line when the product thickness exceeds 4.0 mm. At the opposite end, sugarless chewy candies are rope-formed on a pair of batch rollers operating at 65–75 °C; if the mass temperature drops below 55 °C, the gel network fractures, and if it remains above 80 °C, the rope sizer cannot maintain diameter because the mass cold-flows under its own weight. The processing window of ±10 °C is narrower than for sucrose/glucose syrups, which tolerate a wider plastic range. These constraints are documented in equipment commissioning reports and technical service bulletins from depositor and forming-line manufacturers; published data for non-standard polyol combinations is limited, and start-up trials are normally required.