Convective heat transfer coefficients of open and closed Cryotop® systems under different warming conditions

The warming of cryopreserved samples supported by small volume devices is governed by heat transfer phenomena which are mathematically described by the solution of the transient heat conduction partial differential equations; the convective heat transfer coefficient (h) is an important parameter involved in the boundary condition which is related to the fluid dynamic behavior at the interface device-warming fluid (water, sucrose solution or air). Unfortunately, h values for small volume devices (i.e. Cryotop^®) have not been experimentally determined. Moreover, heat transfer coefficients during warming of Cryotop^® cannot be obtained through classical dimensionless correlations expressed in terms of Nusselt vs. Reynolds and Prandtl numbers that are available for regular geometries and single materials. It is the purpose of present work to determine the convective heat transfer coefficients (h) by numerically solving the heat transfer equation applying the finite element method. Numerical simulations allowed to predict time-temperature histories and warming rates under different protocols in Cryotop^® system which were compared with literature warming rates reported for this device. The h values were calculated considering the heterogeneous structure of the domain (microdrop, plastic-support) and the irregular three-dimensional geometry. The warming conditions analyzed were: a) open system in contact with air and sucrose solution at 23 °C) and b) closed system in contact with air and water at 23 °C. The h values of the Cryotop^® open system immersed in sucrose solution (23 °C), that according to literature achieved a survival in the order of 80%, are in the range of 1800-2200 W/m²K. The h values obtained in this work for warming conditions are critical parameters for cryobiologists when studying heat transfer rate in this small volume device.

Experimental determination of surface heat transfer coefficient in a dry ice-ethanol cooling bath using a numerical approach

BACKGROUND: Dry ice-ethanol bath (-78 degree C) have been widely used in low temperature biological research to attain rapid cooling of samples below freezing temperature. The prediction of cooling rates of biological samples immersed in dry ice-ethanol bath is of practical interest in cryopreservation. The cooling rate can be obtained using mathematical models representing the heat conduction equation in transient state. Additionally, at the solid cryogenic-fluid interface, the knowledge of the surface heat transfer coefficient (h) is necessary for the convective boundary condition in order to correctly establish the mathematical problem.

OBJECTIVE

The study was to apply numerical modeling to obtain the surface heat transfer coefficient of a dry ice-ethanol bath.

MATERIALS AND METHODS

A numerical finite element solution of heat conduction equation was used to obtain surface heat transfer coefficients from measured temperatures at the center of polytetrafluoroethylene and polymethylmetacrylate cylinders immersed in a dry ice-ethanol cooling bath. The numerical model considered the temperature dependence of thermophysical properties of plastic materials used.

RESULTS

A negative linear relationship is observed between cylinder diameter and heat transfer coefficient in the liquid bath, the calculated h values were 308, 135 and 62.5 W/(m²K) for PMMA 1.3, PTFE 2.59 and 3.14 cm in diameter, respectively.

CONCLUSION

The calculated heat transfer coefficients were consistent among several replicates; h in dry ice-ethanol showed an inverse relationship with cylinder diameter.

Methylcellulose Coatings Applied to Reduce Oil Uptake in Fried Products

Reducing fat content of fried foods by application of coatings is an alternative solution to comply with both health concerns and consumer preferences. The objective of the present work was to analyse the effect of coating formulation on the quantity and composition of oil-uptake by potato strips and dough discs, and the quality of frying oil after several frying batches. The effect of coating composition showed that the most efficient formulations were 1% methylcellulose (MC) and 0.5% sorbitol for fried potatoes and 1% MC and 0.75% sorbitol for dough discs. The most effective coating formulations reduced oil uptake by 35-40%, depending on the product. The increase in water content was 6.3% and 25.7% for potato strips and dough discs, respectively. Although coatings were effective barriers to reduce oil uptake in fried products, they did not modify oil composition as detected by HPLC analysis. Oil stability of frying oils, evaluated by total polar compounds and acidity, was within the allowed limits established by the Argentine Food Code (acidity <0.6mg KOH/g oil).

DETERMINATION OF HEAT TRANSFER COEFFICIENTS FOR FRENCH PLASTIC SEMEN STRAW SUSPENDED IN STATIC NITROGEN VAPOR OVER LIQUID NITROGEN

BACKGROUND

The use of mathematical models describing heat transfer during the freezing process is useful for the improvement of cryopreservation protocols. A widespread practice for cryopreservation of spermatozoa of domestic animal species consists of suspending plastic straws in nitrogen vapor before plunging into liquid nitrogen. Knowledge of surface heat transfer coefficient (h) is mandatory for computational modelling; however, h values for nitrogen vapor are not available.

OBJECTIVE

In the present study, surface heat transfer coefficients for plastic French straws immersed in nitrogen vapor over liquid nitrogen was determined; vertical and horizontal positions were considered.

MATERIALS AND METHODS

Heat transfer coefficients were determined from the measurement of time-temperature curves and from numerical solution of heat transfer partial differential equation under transient conditions using finite elements. The h values experimentally obtained for horizontal and vertically placed straws were compared to those calculated using correlations based on the Nusselt number for natural convection.

RESULTS

For horizontal straws the average obtained value was h=12.5 ± 1.2 W m(2) K and in the case of vertical straws h=16 ± 2.48 W m(2) K. The numerical simulation validated against experimental measurements, combined with accurate h values provides a reliable tool for the prediction of freezing curves of semen-filled straws immersed in nitrogen vapor.

CONCLUSION

The present study contributes to the understanding of the cryopreservation techniques for sperm freezing based on engineering concepts, improving the cooling protocols and the manipulation of the straws.

How important are internal temperature gradients in french straws during freezing of bovine sperm in nitrogen vapor?

The subject of present work was to predict internal temperature gradients developed during freezing of bovine sperm diluted in extender, packaged in 0.5 ml French plastic straws and suspended in static liquid nitrogen vapor at -100 degree C. For this purpose, a mathematical heat transfer model previously developed to predict freezing times (phase change was considered) of semen/extender packaged in straw was extended to predict internal temperature gradients during the cooling/freezing process. Results showed maximum temperature differences between the centre and the periphery of semen/extender "liquid" column was 1.5 degree C for an external heat transfer coefficient, h = 15 W per (m(2) K), and only 0.5 degree C for h = 5 W per (m(2) K). It is concluded that if a thermocouple wire were inserted in a 0.5 ml plastic straw to monitor the freezing process in nitrogen vapor, its radial position would have little importance since expected internal gradients may be safely neglected. This finding facilitates the interpretation of freezing rates in 0.5 ml plastic straws immersed in nitrogen vapor over liquid nitrogen, a widely used method for cryopreservation of bovine spermatozoa.

42 COMPARISON OF COOLING RATES IN OOCYTE VITRIFICATION SYSTEMS USING A NUMERICAL SIMULATION

Interest in oocyte cryopreservation has increased due to the application of assisted reproductive technologies and the need for the establishment of ova/gene banks worldwide. In order to maintain cell viability, biological functions must be halted, inducing a suspended animation state by cooling it into a solid phase. Compared to cryopreservation of male gametes, oocytes represent a greater challenge due to their low surface area:volume. Vitrification, the solidification into an amorphous, glassy state while maintaining absence of intra- and extracellular ice crystals, requires high concentrations of cryoprotectants and extremely rapid cooling rates. Several vitrification devices such as open pulled straws (OPS), ultra fine pipette tips, nylon loops and polyethylene films have been introduced to manipulate minimal volumes and achieve high cooling rates. However, experimental comparison of cooling rates presents difficulties mainly because of the reduced size of these systems. To circumvent this limitation, a numerical simulation of cooling rates of various vitrification systems immersed in liquid nitrogen was conducted, solving the non-stationary heat transfer partial differential equation using the finite element method. Three external heat transfer coefficients (h = 200, 1000 and 2000 W m–2 K) were considered. The Cryotip® and OPS were approached as 2 concentric finite cylinders; differential equations representing heat transfer in cylindrical coordinates were described considering radial and axial coordinates and were numerically solved as a 1-dimensional heat conduction problem in an infinite cylinder. The Cryoloop® was approximated as a 1-dimensional heat flow system in Cartesian coordinates and Cryotop® was numerically described as an irregular bi-dimensional axial-symmetric problem. All differential equations were numerically solved using the finite element method in COMSOL Multiphysics 3.4. The domain was discretized in triangular (Cryotip®, OPS and Cryotop®) and linear elements (Cryoloop®) in order to obtain accurate numerical approximations. In each case, the warmest point of the system was identified to determine the time-temperature curve that allows the evaluation of the slowest cooling rate (worst condition). Results indicate the nylon loop (Cryoloop®) is the most efficient heat transfer system analysed, with a predicted cooling rate of 180 000°C min–1 for an external heat transfer coefficient h = 1000 W m–2 K when cooling from 20 to –130°C; in contrast, the pipette tips (Miniflex® showed the lowest performance with a cooling rate of 6164°C min–1 at same value of external heat transfer coefficient. Predicted cooling rates of OPS and Cryotop® (polyethylene film) were 40 909 and 37 500°C min–1, respectively for the same heat transfer coefficient. It can be concluded that in oocyte cryopreservation systems, in which experimental comparison of cooling rates presents difficulties due to the reduced size of the vitrification devices, the numerical simulations and the analysis of the predicted thermal histories could contribute to determine the performance of the different techniques.

Assessment of external heat transfer coefficient during oocyte vitrification in liquid and slush nitrogen using numerical simulations to determine cooling rates

In oocyte vitrification, plunging directly into liquid nitrogen favor film boiling and strong nitrogen vaporization. A survey of literature values of heat transfer coefficients (h) for film boiling of small metal objects with different geometries plunged in liquid nitrogen revealed values between 125 to 1000 W per per square m per K. These h values were used in a numerical simulation of cooling rates of two oocyte vitrification devices (open-pulled straw and Cryotop), plunged in liquid and slush nitrogen conditions. Heat conduction equation with convective boundary condition was considered a linear mathematical problem and was solved using the finite element method applying the variational formulation. COMSOL Multiphysics was used to simulate the cooling process of the systems. Predicted cooling rates for OPS and Cryotop when cooled at -196 degree C (liquid nitrogen) or -207 degree C (average for slush nitrogen) for heat transfer coefficients estimated to be representative of film boiling, indicated lowering the cooling temperature produces only a maximum 10 percent increase in cooling rates; confirming the main benefit of plunging in slush over liquid nitrogen does not arise from their temperature difference. Numerical simulations also demonstrated that a hypothetical four-fold increase in the cooling rate of vitrification devices when plunging in slush nitrogen would be explained by an increase in heat transfer coefficient. This improvement in heat transfer (i.e., high cooling rates) in slush nitrogen is attributed to less or null film boiling when a sample is placed in slush (mixture of liquid and solid nitrogen) because it first melts the solid nitrogen before causing the liquid to boil and form a film.

Numerical simulation of cooling rates in vitrification systems used for oocyte cryopreservation

Oocyte cryopreservation is of key importance in the preservation and propagation of germplasm. Interest in oocyte cryopreservation has increased in recent years due to the application of assisted reproductive technologies in farm animals such as in vitro fertilization, nuclear transfer and the need for the establishment of ova/gene banks worldwide. However, the cryopreservation of the female gamete has been met with limited success mainly due to its small surface-area:volume ratio. In the past decade, several vitrification devices such as open pulled straws (OPS), fine and ultra fine pipette tips, nylon loops and polyethylene films have been introduced in order to manipulate minimal volumes and achieve high cooling rates. However, experimental comparison of cooling rates presents difficulties mainly because of the reduced size of these systems. To circumvent this limitation, a numerical simulation of cooling rates of various vitrification systems immersed in liquid nitrogen was conducted solving the non-stationary heat transfer partial differential equation using finite element method. Results indicate the nylon loop (Cryoloop®) is the most efficient heat transfer system analyzed, with a predicted cooling rate of 180,000°C/min for an external heat transfer coefficient h= 1000 W/m(2)K when cooling from 20 to -130°C; in contrast, the open pulled straw method (OPS) showed the lowest performance with a cooling rate of 5521°C/min considering the same value of external heat transfer coefficient. Predicted cooling rates of Miniflex® and Cryotop® (polyethylene film system) were 6164 and 37,500°C/min, respectively, for the same heat transfer coefficient.

Theoretical prediction of the effect of heat transfer parameters on cooling rates of liquid-filled plastic straws used for cryopreservation of spermatozoa

Heat transfer plays a key role in cryopreservation of liquid semen in plastic straws. The effect of several parameters on the cooling rate of a liquid-filled polypropylene straw when plunged into liquid nitrogen was investigated using a theoretical model. The geometry of the straw containing the liquid was assimilated as two concentric finite cylinders of different materials: the fluid and the straw; the unsteady-state heat conduction equation for concentric cylinders was numerically solved. Parameters studied include external (convection) heat transfer coefficient (h), the thermal properties of straw manufacturing material and wall thickness. It was concluded that the single most important parameter affecting the cooling rate of a liquid column contained in a straw is the external heat transfer coefficient in LN2. Consequently, in order to attain maximum cooling rates, conditions have to be designed to obtain the highest possible heat transfer coefficient when the plastic straw is plunged in liquid nitrogen.

Inhibitory effect of a surfactant on pure cultures of a filamentous and a floc forming micro-organism

Activated sludge is the most widely used biological process for wastewater treatment. Inorganic and organic compounds are removed by a biotic community in the aeration basin. Problems of these systems are loss of settleability and poor sludge compaction due to excessive growth of filamentous micro-organisms. The filamentous bulking can be controlled by the addition of chemical agents. Strong oxidants, such as chlorine, are utilized to eliminate filamentous bacteria; however, these substances also tend to attack floc-forming bacteria and to cause process breakdown. Besides, chlorine may become hazardous owing to the formation of chemical products as chloramines. Surfactant addition constitutes an interesting alternative for the control of filamentous bulking. In this work the effect of a surfactant Triton X-100 (octylphenol ethoxylate), on the respiratory activity (RA) of pure cultures of a filamentous (Sphaerotilus natans) and a floc-former microorganism (Acinetobacter anitratus) was evaluated. In the concentration range tested (60-220 mg l(-1)), the surfactant was observed to exhibit high RA specific inhibition of the filamentous micro-organism with no significant effect on the floc-forming bacteria. Light microscopy observations showed that the surfactant induced cell lysis, leaving only empty sheaths in the case of filamentous micro-organisms. A kinetic equation to predict the microbial RA fraction of a S. natans pure culture as a function of surfactant concentration and contact time was proposed. The effect of Triton X-100 on the inactivation of pure cultures of both micro-organisms was compared to that of chlorine. Triton X-100 results were adequate to eliminate filamentous bacteria emerging as an alternative for filamentous bulking treatment.

Effect of aluminum sulfate and cationic polyelectrolytes on the destabilization of emulsified wastes

Emulsified oil in wastewater constitutes a severe problem in the different treatment stages. Aluminum salts have been traditionally used as coagulants in wastewater treatments. Polyelectrolytes are used to coagulate and flocculate colloidal systems. The performance of aluminum sulfate in comparison to polyelectrolytes (chitosan and polyacrylamide) as conditioning chemicals for an emulsion waste was tested, and the predominant mechanisms acting in each case were analyzed. Turbidimetry, jar test, colloidal titration and microscopy were used to test emulsion destabilization. Both charge neutralization and bridge formation were identified and confirmed as mechanisms of interaction of polyelectrolytes with waste constitutents. Charge neutralization would be more important for chitosan than for polyacrylamide treatment. A coincidence between the doses necessary to reach zero colloidal charge and minimum turbidity was observed for polyelectrolytes. The time necessary to produce system clarification was larger for aluminum sulfate than for polyelectrolytes; this time was shortened for higher aluminum sulfate concentration. The pH showed a marked effect on aluminum sulfate performance with the optimum at pH 6; polyelectrolyte action was practically not affected by pH. Polyelectrolyte addition produced the minimum turbidity for the same doses that zero colloidal charge; at higher doses, emulsion was restabilized and became turbid again. However, aluminum sulfate treatment did not produce emulsion restabilization.

Modeling the aerobic growth and decline of Staphylococcus aureus as affected by pH and potassium sorbate concentration

The effects of pH (5.0, 5.2, 5.4, 5.6, and 5,8) and concentration of potassium sorbate (10.0 and 16.6 mM) at two water activity values (0.90 and 0.92) on the aerobic growth and decline of Staphylococcus aureus ATCC 6538P, 196-E, and FDA-C243 were studied using brain-heart infusion broth. The inoculum was approximately 4 to 5 log CFU/ml, and the incubation temperature was 30 degrees C. Samples were periodically enumerated on tryptic soy agar. The Gompertz model was used to obtain microbial growth parameters, specific growth rate was obtained as a derived parameter, and the inhibition index was calculated. A linear model was fitted in cases of bacteriostatic or bactericidal action of the treatment. The ATCC 6538P strain showed the highest resistance in the range of tested conditions. Microbial behavior was modeled considering the main controlling factors, and a response surface methodology was used to determine the effects of undissociated acid concentration and pH. These results can be used to establish treatment conditions for microorganism growth or inhibition.

Mathematical modelling of microbial growth in packaged refrigerated beef stored at different temperatures

Gompertz and logistic models were fitted to experimental counts of microorganisms growing in beef stored at 0, 4, 7, 9 and 10 degrees C. Samples were packaged in polyethylene (high gaseous permeability) and in EVA/SARAN/EVA (low gaseous permeability) films, being EVA ethyl vinyl acetate and SARAN polyvinyl and polyvinylidene chloride copolymer. Lag phase duration (LPD) and specific growth rate (mu) were obtained as derived parameters for lactic acid bacteria, Enterobacteriaceae, Pseudomonas sp. and psychrotrophic microorganisms. The reciprocal of LPD was fitted to an Arrhenius type equation; LPD of lactic acid bacteria showed a marked dependence on temperature, with activation energy values (ELPD) of 222.2 and 216.9 kJ/mol for polyethylene and ESE respectively. The effect of initial microbial population at different storage temperatures on adaptation period was analyzed. As the initial microbial population increased, adaptation period decreased for all studied microorganisms and for both packaging films. The effect of temperature on specific growth rate was better interpreted by the Arrhenius model than by the linear or the square root equations. Psychrotrophic microorganisms in beef showed the highest activation energy values for specific growth rate (E mu) in both packaging films, being E mu 85.50 and 103.10 KJ/mol for polyethylene and ESE film respectively. In both films, Enterobacteriaceae showed the lowest E mu values, being 15.33 and 59.89 kJ/mol in ESE and polyethylene respectively. The final number of microorganisms (maximum population density) did not show significant changes with storage temperature.