Optimizing cryopreservation protocols of Saccharomyces eubayanus using heat transfer modeling

Abstract

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.