Aroma characterisation and retention after heat treatment and drying of fruits using extraction and GC-MS analysis

Abstract

This study concerns the identification and characterisation of volatile

components of fruits, and evaluation of the effect of heat treatment and

drying on retention or loss of volatiles of fruits. The investigation included

developing a procedure to extract volatile components from the fruit

matrix, a purification step, separation, identification and quantification.

Initial experiments with Vangueria infausta L. showed that some

components, especially sugars, degrade during heating in the GC analysis,

producing furfural, hydroxyl methyl furfural (HMF) and other volatiles.

These compounds are obtained together with the native aroma components

of the fruit, making analysis difficult.

We developed a procedure using a hydrophobic column that could retain

the hydrophobic aroma components and eliminate sugars that could

disrupt the analyses. The extract was analysed by GC.

The volatile components found in pulp of Vangueria infausta were

primarily hexanoic acid, octanoic acid, ethyl hexanoate, ethyl octanoate,

methyl hexanoate and methyl octanoate. Based on the odour activity

values, it could be concluded that the odour of the fruit is mainly attributed

to ethyl hexanoate and ethyl octanoate (paper I).

Drying is often used to process and preserve food stuff but many food

attributes including aromas which are important for palatability and

consumer interest are affected by the process. Our research showed that

the principal aroma components are well preserved during the initial phase

of drying (down to about a relative water activity of 0.65). However the

aromas are lost after more extensive drying. A possible explanation for

volatilisation is the sugar crystallisation that occurs below a RH of around

0.70. (paper II).

Also we evaluated the effects of drying with or without blanching on

volatiles of mango (Mangifera indica L.). Fresh, blanched and dried

mango samples were analysed (paper III). The fresh sample presented a

very large number of peaks. The blanching was carried out in water and

microwave at 70°C during 10 minutes and at 90°C during 2 minutes.

Blanching increased the levels of aroma components. Both blanching

procedures had no dramatic effect on the impact of the blanching.

Prolonged hot air drying (aw=0.65) reduced most of the aroma. α-pinene

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and 1-butanol were strongly affected due their volatility. Drying had no

great effect on components with high boiling points, which displayed

significant retention even after extensive dehydration. Ethyl butanoate was

high retained despite its high volatility.

To evaluate a possible influence of sugar crystallisation on aroma

retention a further study involved three model matrixes based on

carbohydrates aqueous solution plus oils: I) pectin-sucrose-oils, II) pectinmicrocrystalline

cellulose-sucrose-oils and III) microcrystalline cellulosesucrose-

oils. The oil phase comprised the reference materials of the most

powerful aroma components found in Vangueria infausta (hexanoic acid,

ethyl hexanoate and ethyl octanoate). The model mixture was dried at

80°C, 3 m/s for 60-420 min prior to GC analysis (Paper IV). The aroma

components were preserved in all models throughout the drying process

(until aw ≈0.8). So sugar crystallisation did not induce the loss of volatiles.

However noticeable sugar crystallisation was observed in model II. We

assume that the presence of pectin and microcrystalline cellulose increased

the ability of the matrix to compact, as water activity decreased during

drying. So sugar crystallisation is probably the reason for aroma retention

within the matrix.

The results of the studies in this thesis illustrate what happens to volatiles

during thermal processing of fruits. These results could help design a

better strategy for aroma isolation and characterisation, and explain the

aroma entrapment due to sugar crystallisation during drying of fruits. The

results can also be used to design a strategy for sustainable utilisation of

volatile components of fruits like Vangueria infausta, one of the wild

fruits to be included in a formulation of new industrial food products.

However, more studies are needed in order to learn more about sustainable

utilisation of various wild fruits growing in Mozambique and southern

Africa.

Abstract (Swedish)

Popular Abstract in English

Fruits are a key complement to the diet of many people in the southern

region of Africa, especially in rural areas. maphilwa (Vangueria infausta

L.), maçanica (Ziziphus mauritiana), maçala (Strychnos spinosa),

mapsincha (Salacia kraussi), cajú (Anacardium accidentale L.),

mavungwa (Landolpia kirki) are some of the most commonly found fruits

in sub-Saharan Africa, particularly in Mozambique, Botswana,

Madagascar, South Africa, Zambia and Zimbabwe. They play an

important role in the diet and gastronomy of the rural communities.

Several investigations of nutritional aspects have been carried out, and

showed that the fruit is rich in dietary fibre and sugars and have a high

micronutrient content in the form of minerals and vitamins. Vengueria

infausta L. belongs to the family Rubiaceae. The common names are

African medlar in english or maphilwa in ronga (one of local language

in southern Mozambique). The fruit is usually harvested between

February and April. It is brownish orange when ripe and is spherical in

shape. The fruit is about 2-5 cm in diameter and contains 3-5 seeds. The

fresh fruit is sweet and tastes like medlar (Mespilus sp.), although with

some similarities to green apple and pineapple. The fruit can be

eaten fresh, cooked or dried. It’s used also to prepare juice, jam

puddings and marmalade. We assume that the fruit may be useful and

the taste and aroma profile attractive and appreciated. Information

about identification of aroma on African medlar is limited.

The investigation included developing a procedure to extract volatile

components from the fruit matrix, a purification step, separation,

identification and quantification. The extraction procedure used solvents

(ethanol, diethyl ether and pentane). Initial experiments showed that some

components, especially sugars, are degraded during the heating in the Gas

Chromatography (GC) analysis, producing furfural, hydroxyl methyl

furfural (HMF) and other volatiles. These compounds are obtained

together with the native aroma components of the fruit, making analysis

difficult.

We developed a procedure using a hydrophobic column with a

capability to retain the hydrophobic aroma components and wash out

the hydrophilic components (sugars) using water. The aromas were

released using a mixture of pentane and diethyl ether prior injection

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into the GC. The aroma components were separated on the basis of their

retention times, followed by identification through MS. The identification

was verified using standards. Each peak was quantified, taking into

account the peak areas of components relating to the internal standard.

The main aroma components identified in Vangueria infausta were

hexanoic acid, octanoic acid, ethyl hexanoate, ethyl octanoate, methyl

hexanoate a n d methyl octanoate. The esters (methyl hexanoate and ethyl

octanoate) are the main contributors to the aroma of the fruit (Paper I).

The second aspect of this work was to evaluate the effect of drying upon

aroma components of the fruits. Samples of Vanueria infausta pulp were

convectively dried at 80°C, 3 m/s for up to 480 min. The results showed

that the principal aroma components of pulp are well preserved during the

initial phase of drying down to a relative water activity of about 0.65, but

are lost after more extensive drying. This is due to the volatilisation

induced by sugar crystallisation that is likely to occur below a

relative humidity of around 0.70 during the drying process (Paper II).

The third study of this thesis aimed to evaluate the effect of blanching

and drying on the aroma of mango (Mangifera indica L.). Three samples

of mango (fresh, blanched and dried) were analysed (Paper III). The

blanching was carried out in water at 70°C during 10 min and in a

microwave at 90°C during 2 min. The most relevant aromas Mangifera

indica are 1-butanol, α-pinene, 3-carene, myrcene, limonene, terpinolene,

and ethyl butanoate. The experiments show that the levels of aroma

components are increased when the material is blanched while hot air

drying reduced most of the aroma when the drying is prolonged below

0.65 in aw. Also the study shows that water blanching, microwave

blanching, long period/low temperature or short period/high temperature

had no marked effect on the impact of the blanching.

Another goal of this thesis was to investigate the influence crystallisation

of carbohydrates on retention or loss of aroma. (Paper IV). Three models

were evaluated: I -pectin-sucrose-aromas; II -pectin-microcrystalline

celulose-sucrose-aromas and III -microcrystalline cellulose-sucrosearomas.

The aroma fraction was composed of the main aroma components

identified in Vangueia infausta: hexanoic acid, ethyl hexanoate and ethyl

octanoate. Each model was dried on over at 80°C, 3 m/s during 60-420

min. GC results showed considerable aroma retention in all models at least

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starting when the aw value reached 0.8. Model with pectin and

microcrystalline cellulose rapidly exhibited low aw values and more

consistency. Our assumption is that the crystallisation of sugars could play

a role on aroma retention during the drying of fruits.

The results from these studies show what happens to aroma during heat

treatment of fruits. It is useful to understand the encapsulation of aroma

due to sugar crystallisation during drying. The results can help design a

better strategy for sustainable utilisation of aroma components of fruits,

like the African medlar, one of the wild fruits now included in local

industrial processing of new products. We believe that greater knowledge

on volatiles can be useful in sustainable utilisation of wild fruits grown in

Mozambique and southern Africa.

• author: Ráice, Rui
• supervisor: Björn Bergenståhl
• opponent: Dr. Grauwet, Tara, Catolic University, Loeven, Belgium
• organization: Department of Food Technology, Engineering and Nutrition
• publishing date: 2015
• type: Thesis
• publication status: published
• subject: Chemical Process Engineering
• keywords: Fruits, Vangueria infausta, Mangifera indica, volatile, aroma, blanching, drying, encapsulation, modelling, GC-MS.
• categories: Higher Education
• pages: 158 pages
• defense location: Lecture hall C, Kemicentrum, Getingevägen 60, Lund University, Faculty of Engineering LTH, Lund
• defense date: 2015-11-26 10:00:00
• ISBN: 978-91-7422-415-3
• language: English
• LU publication?: yes
• id: e765a7f5-7dc6-4d0d-aa08-908f6c7c70cf (old id 8083396)
• date added to LUP: 2016-04-04 11:36:05
• date last changed: 2024-01-30 10:28:42