Electroporation of stomata: effect on drying kinetics and aroma compounds of Genovese basil leaves

Abstract

The aroma components of herbs and spices are very sensitive to heat, complicating their drying process. Until now, exporters of dried herbs face the challenge of low total phenolic contents and efforts are constantly being made to improve the drying efficiency of herbs and conserve their aroma. This thesis looks at the influence of extracellular trehalose impregnated by vacuum impregnation (VI) and electroporation achieved with reversible pulsed electric field (PEF) treatment affecting stomata opening on the convective air drying kinetics, rehydration properties, colour changes and aroma preservation of basil leaves. In describing the drying kinetics of treated basil samples, attention is given to the determination of the drying rate and the effective moisture diffusivity.
The results indicate that reversible electroporation of opened stomata resulted in faster drying rate (from 0.025 to 0.00039 min-1) compared to samples with opened stomata not electroporated (from 0.0202 to 0.0021 min-1) throughout the drying period. Vacuum impregnation with trehalose increased the initial drying rate of the control from 0.012 to 0.016 min-but towards the end of drying, the drying rate of the control and vacuum infused control were slightly the same (0.0025 and 0.0024 min-1) respectively. Similar trends were also observed for effective moisture diffusivity where samples with stomatal electroporation resulted in increased effective moisture diffusivity (1.857*10-9 m2 s-1) compared to samples where the opened stomata were not electroporated (1.309*10-9 m2 s-1). As a result of the increased drying efficiency, irreversible electroporation of the leaf tissue resulted in 47% reduction, reversible electroporation of opened stomata resulted in 37% reduction whilst leaves samples that were reversible electroporated but opened stomata was not electroporated resulted in 18% reduction in the drying time. The quality parameters analysed on the dried herb indicate that reversible electroporation of opened stomata resulted in less colour change, produced good rehydration properties and resulted in 1.3 to 5.7 fold increase in the major aroma compounds relative to the untreated dried samples.

Popular Abstract

Reversible electroporation of stomata speed up drying and preserve aroma in dried basil leaves

Drying of plant materials induces structural changes which leads to tissue damage and low- quality dried products. In recent years, different pre-drying and drying techniques have been developed in a bid to improve the quality of dried products. By reversible electroporation of stomata prior to drying, the drying rate and the aroma content of dried basil leaves was significantly improved.

There is a constant increase in consumer demand for processed food products that retain most of the characteristics of the raw commodity. Most aromatic herbs are preserved by drying but until now, exporters of dried herbs face the challenge of low aroma content. Drying involves the evaporation of water from a wet surface to a stream of moving air. There is a simultaneous migration of water from the interior of a drying material toward the surface as water is being removed from the surface. Growing plants loss water through specialised structures called stomata located in the leaves. Regulating the size of the stomata pores prior to drying of leaves resulted in faster convective air drying and improved quality parameters of dried basil leaves. The leaves were first incubated in a solution of either fusicoccin to open the stomata or ABA to close the stomata before drying. Leaves with opened stomata dried faster and this effect was imitated by reversible electroporation of the opened stomata and the surrounding epidermal cells.
Pulsed electric fields have been used in the past to improve the drying rate of most materials. But the improved drying rate is often attributed to irreversible membrane permeabilization as an upstream process to enhance mass transfer. But irreversible membrane permeabilization cannot be used as a pre-treatment for the drying of herbs as the aroma contents will leak out. By targeting the stomata, reversible electroporation of opened stomata resulted in faster drying rate and increased the rate at which moisture diffuses from the drying material to the drying surface compared to samples with opened stomata not electroporated. As a result of the increased drying efficiency, reversible electroporation of opened stomata resulted in 37% reduction whilst leaves samples that were reversible electroporated but opened stomata was not electroporated resulted in 18% reduction in the drying time compared to the untreated dried samples. The quality parameters analysed on the dried herb indicate that reversible electroporation of opened stomata resulted in less colour change, produced good rehydration properties and resulted in 1.3 to 5.7 fold increase in the major aroma compounds relative to the untreated dried samples.

• author: Kwao, Stephen
• supervisor: Federico Gómez Galindo
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLTM01 20151
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Basil leaves, Vacuum, Impregnation, Reversible electroporation, Drying kinetics, Aroma compounds, food engineering, livsmedelsteknik
• language: English
• id: 8057290
• date added to LUP: 2015-10-20 16:14:59
• date last changed: 2017-06-16 04:07:50