The effect of the electric field on lag-phase, ethanol and β-galactosidase production of a recombinant S. cerevisiae growing on lactose

The production of ethanol and β-galactosidase from cheese whey and other sub-products from industries can be a good way of minimizing environmental problems and producing valuable products from low-cost raw materials. A recombinant S. cerevisiae NCYC869-A3/pVK1.1 flocculent strain expressing the lacA gene (coding for β-galactosidase) of Aspergillus niger under ADHI promotor and terminator was used in the experiments, once this strains has higher ethanol and β-galactosidase productivities. Batch culture experiments were performed in SSlactose medium with 50 g/L lactose. A 2 dm3 bioreactor with agitation, temperature and pH measurement and control, was used. The experiments were conducted under aerobic and anaerobic conditions. The temperature was maintained at 30 ◦C and the pH at 4.0. Moreover, electrodes were placed inside the bioreactor and experiments were conducted at three different electric fields, ranging from 0.5 to 2.0 V.cmˉ¹ to determine the effect of the electric field in the fermentation profile. For all the experiments the biomass, protein, β-galactosidase activity, lactose, glucose, galactose and ethanol were measured. Finally, the lag phase and specific growth rate were calculated. Significant changes in lag phase and biomass yields were found when using 2.0V.cmˉ¹. The results show that ohmic heating enhances early stages of the fermentation, indicating that ohmically heated fermentations may be extremely useful in food industry.Fundação para a Ciência e a Tecnologia (FCT

Aseptic processing of a strawberry pulp in a continuous ohmic heater: numerical simulations and model validation

Continuous food-sterilization processes often involve the flow of liquids or solid-liquid mixtures, frequently highly viscous and with non-Newtonian behaviour, in pipes and reactors. Heat treatment of such flows is complex and it is necessary to guarantee that each part of the material is adequately processed, preventing overcooking as much as possible. The use of ohmic heating technology as an alternative heating method has regained interest for complex fluids or multiphase foods

CFD simulations of RTD of a strawberry pulp in a continuous ohmic heater

A pilot ohmic heater is to be tested for the continuous aseptic processing of strawberry pulps and jams. The hydrodynamics and the fluid residence time distribution (RTD) have been experimentally investigated for Newtonian and non-Newtonian fluids (water and an industrial strawberry pulp, respectively), for several inlet flow rates. The results were obtained using Computational Fluid Dynamics (CFD) with a user-defined function (UDF) description of the fluid phase (pulp). For all of the conditions tested the fluid phase is described using the laminar flow model. The results show that the RTD is affected by the inlet flow rate but not so significantly by the process temperature. Some shortcuts and dead zones were detected in the ohmic heater specially for Newtonian fluids. The heater behaves like a piston flow with longitudinal mixing

Numerical simulations of the hydrodynamic behaviour of a continuous ohmic heater operating with Newtonian and Non-Newtonian fluids

A pilot ohmic heater was tested for the continuous aseptic processing of strawberry pulp. The fluid mechanics and the fluid residence time distribution (RTD), in the whole volume of the vessel and exclusively in the heating zone (between the electrodes), have been numerically simulated using Computational Fluid Dynamics (Fluent™) for a Newtonian (water) and a non-Newtonian fluid (strawberry pulp). These simulations were performed at different mass flow rates (from 0.5 to 2 kg/min.) and different inlet temperatures (from 40 to 90 ºC). RTDs were determined defining a unit scalar at the inlet and monitoring its value at the outlet. These results allowed the construction of F-diagrams and the calculation of the hold back (H) and segregation (S). The simulations showed that for 0.5 kg/min water induces the formation of more shortcuts (10 % higher H) then pulp. However, for 2 kg/min both fluids display a similar behaviour (less than 3 % difference between H values). Furthermore, RTD is affected by the inlet mass flow rate but not so significantly by process temperature. The values of S calculated for water show that the reactor approaches plug flow behaviour. The RTD determinations in the heating zone allowed the establishment of an equation for the prediction of the outlet temperature of the fluid, allowing setting criteria for the aseptic processing of fluids thus guaranteeing the microbiological safety of the products

Simulações numéricas do processamento asséptico de polpas de morango num aquecedor óhmico contínuo

Neste trabalho efectuaram-se simulações numéricas dos padrões de fluxo num aquecedor óhmico contínuo, em estado estacionário, com recurso ao programa Fluent™. Para tal, criou-se uma grelha hexagonal (não estruturada) com base na geometria do aquecedor óhmico. A precisão dos resultados simulados foi optimisada através da diminuição do espaçamento entre as células junto às paredes. O número total de células na grelha foi minimizado por aplicação de uma condição de simetria axial (378 348 células). Para um fluido newtoniano (água) e um outro não newtoniano (polpa de morango) simularam-se diversas condições de escoamento, correspondendo a valores do número de Reynolds (Re) inferiores a 2000 (regime de escoamento laminar). Para a simulação com polpa de morango foi necessário determinar experimentalmente a dependência da viscosidade com a temperatura, cujo efeito foi expresso num modelo reológico do tipo “plástico de Bingham”. Após obter os padrões de fluxo no interior do aquecedor, calculou-se a distribuição dos tempos de residência (DTR) do fluido em todo o seu volume. Recorrendo à DTR exclusivamente na zona de aquecimento (entre os eléctrodos) foi possível prever o perfil térmico do fluido tratado no final da etapa de aquecimento. Consequentemente, obteve-se uma equação de projecto que relaciona a temperatura final com as variáveis operacionais (caudal de fluido, voltagem aplicada e tipo de fluido), conseguindo-se optimizar as condições de operação do aquecedor óhmicode acordo com a segurança microbiológica dos produtos processados e na minimização dos gastos energéticos. No final do estudo concluiu-se que a aplicação deste aquecedor às polpas de morango está limitada a caudais máximos de 2 kg/min e voltagens mínimas de 100 V

The effect of electric field on important food-processing enzymes : comparison of inactivation kinetics under conventional and ohmic heating

This work deals with the determination of the inactivation kinetics of several enzymes, most of them used as time-temperature integrators in the food industry. The tested enzymes were polyphenoloxidase, lipoxygenase, pectinase, alkaline phosphatase, and β-galactosidase, and the inactivation assays were performed under conventional and ohmic heating conditions. The thermal history of the samples (conventional and ohmically processed) was made equal to determine if there was an additional inactivation caused by the presence of an electric field, thus eliminating temperature as a variable. All the enzymes followed 1st-order inactivation kinetics for both conventional and ohmic heating treatments. The presence of an electric field does not cause an enhanced inactivation to alkaline phosphatase, pectinase, and β-galactosidase. However, lipoxygenase and polyphenoloxidase kinetics were significantly affected by the electric field, reducing the time needed for inactivation. The results of the present work can be used industrially to determine processing effectiveness when ohmic heating technology is applied