Optimizing cryopreservation protocols of Saccharomyces eubayanus using heat transfer modeling

Efficient cryopreservation is essential for maintaining the viability of Saccharomyces eubayanus, a cryotolerant wild yeast of industrial importance as the cold-adapted parent of Saccharomyces pastorianus. This study integrated post-thaw viability and vitality assessments of cryopreserved S. eubayanus CRUB 1568ᵀ with experimental measurements of transient heat transfer and numerical simulation of the freezing stage. Two protocols were evaluated: (A) direct freezing of cryovials in cryoboxes at - 80 °C, governed by convection, and (B) freezing inside a CoolCell® device (Corning Inc., Corning, NY, USA), where heat transfer occurs by conduction through the insulated plastic material. A mathematical model was developed to numerically solve the transient heat transfer equation with phase change using the finite element method. Experimental temperature-time data validated the simulations, allowing estimation of overall heat transfer coefficients (UA = 18.04 W m^-2 K^-1; UB = 4.76 W m^-2 K^-1) and characteristic freezing times (t𝚌 = 10.9 min; 27.6 min, respectively). Calculated Biot numbers confirmed uniform temperature distribution within cryovials. PROTOCOL A achieved optimal cooling rates (5-7 °C min^-1) and yielded higher post-thaw viability (71.7 ± 3.5%) compared with PROTOCOL B (51.2 ± 3.6%) after 1 year at - 80 °C. The integration of modeling and experimental data demonstrates that the overall heat transfer coefficient is a key engineering parameter influencing cryopreservation performance. Direct freezing of cryovials in cryoboxes represents a simpler, faster, and lower-cost approach that ensures uniform cooling and higher cell survival, providing a valuable basis for standardizing yeast cryogenic storage in industrial and biotechnological applications. KEY POINTS: Finite element modeling assessed heat transfer during yeast cryopreservation. Direct freezing in cryoboxes achieved higher viability than CoolCell®. Overall heat transfer coefficient (U) is key for cryogenic performance analysis.