The effect of thermal and freeze drying on the content

The effect of thermal and freeze drying on the content of organic acids and
flavonoids in fruit of European cranberry (Oxycoccus palustris Pers.)
Artur Adamczak, Waldemar Buchwald*, Jan Kozłowski,
Sebastian Mielcarek
The Branch of Medicinal Plants
Institute of Natural Fibres and Medicinal Plants
Libelta 27
61-707 Poznań, Poland
* corresponding author: [email protected]
Summary
The present paper shows the results of a study on the effect of freeze and thermal drying
on the content of organic acids and flavonoids in fruit of European cranberry (Oxycoccus
palustris Pers.). They indicate a significant effect of drying conditions on the content of the
above mentioned active compounds. In lyophilized fruit, distinctly more organic acids but
less flavonoids were found than in fruit dried at a temperature of 35–400C. The largest
relative differences were observed in the case of ascorbic acid. Its content in the fruit dried
at high temperature, compared to the lyophilized material, was lower by a mean value of
5 mg/100 g of dry matter (DM), that is, by 42% on the average. With regard to the other
organic acids (quinic, citric and malic acids), the average difference was 0.9–2.1 g/100 g
DM (14–17%). The mean flavonoid content in the thermally dried fruit was larger by
34 mg/100 g DM (converted into hyperoside) and 24 mg/100 g (converted into quercetin),
that is on average by 19% as compared to freeze-dried samples.
Key words: Oxycoccus palustris, medicinal plants, drying, organic acids, flavonoids
INTRODUCTION
European cranberry fruit contains large amounts of organic acids (including vitamin C) as well as pectins, flavonoids, anthocyanins and other active compounds
[1-6]. Research on American cranberry shows that species of the genus Oxycoccus
(Hill) A. Gray can be used in the treatment of urinary tract infections as well as in
The effect of thermal and freeze drying on the content of organic acids and flavonoids in fruit of European cranberry ...
95
the prevention of heart diseases and ulcer diseases of the digestive system. Cranberry fruit extracts show antibacterial and antifungal activity [7-13].
Above all, the use of European cranberry in the herbal industry requires the
development of optimal methods for drying collected raw material which would
ensure the stabilisation of active compounds [14, 15]. Studies on the effect of
drying conditions on the chemical composition of fruit of the species in question
are few and do not exhaust the issue [6, 16, 17]. There are more data on this topic
with respect to American cranberry [18, 19] and berry fruit of other species of
the Ericaceae family: blueberry [19-21] and bilberry [22]. So far, researchers have
devoted their attention primarily to the determination of the effect of fresh fruit
storage conditions on their chemical composition [23]. These have been studies
relating to highbush and lowbush blueberry [21, 24-33]; much less frequently
– American cranberry [28, 34, 35].
The aim of this study was to determine the effect of freeze and thermal drying
on the content of organic acids and flavonoids in fruit of Oxycoccus palustris.
MATERIAL AND METHODS
European cranberry fruits (23 samples) were collected in the year 2008 on
9 peatlands situated in the regions of Wielkopolska (Greater Poland) and Western Pomerania. Each sample was divided into two parts and subjected to either
lyophilization or drying in a laboratory dryer at a temperature of 35-40C. The
content of organic acids was determined using the HPLC-DAD technique after extraction with water [36]. The total content of flavonoids (flavonol glycosides) was
determined based on the spectrophotometric determination of aglycones using
Christ-Müller’s method, converted into hyperoside and quercetin after extraction
from the material [37].
In statistical analysis, t-test for dependent samples was applied, using Statistica 7.1 software [38]. The Shapiro-Wilk test was used to evaluate the normality
of variable distribution in groups and differences between groups. In order to
normalise the distribution of the analysed variables and their differences, outlier observations were excluded in justified cases. The content of flavonoids and
ascorbic acid was given in mg/100 g, whereas the content of malic, quinic and
citric acids in g/100 g of fruit dry matter (DM). The difference (D) in the content of
the investigated compounds in the lyophilized and thermally dried material was
expressed in percent, calculated it in accordance with the following formula:
D = (a – b / a) x 100%,
in which:
a – the content of a given compound (or a group of compounds) in lyophilized
fruit;
b – the content of a given compound (or a group of compounds) in fruit dried
at high temperature.
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RESULTS
The obtained results show statistically highly significant (at a level of p<0.001)
differences in the content of the investigated active compounds in cranberry fruit
dried under different conditions (fig. 1, 2). In the material subjected to lyophilization, definitely more organic acids were found, but less flavonoids than in the fruit
dried at a temperature of 35–40C. The largest relative differences (expressed in
percent) occurred in the case of ascorbic acid (fig. 3). Its content in the thermally
dried fruit, compared to the freeze-dried material, was lower by a mean value of
5 mg/100 g DM (by 42%, on average). With regard to the other organic acids (quinic, citric and malic acids), the average difference was 0.9–2.1 g/100 g DM which
accounted for 14–17% of the content of the compounds in question in the lyophilized material. The total content of organic acids in the thermally dried fruit was
lower by a mean value of 4.1 g/100 g DM (by 15%, on average). In turn, the mean
flavonoid content in the thermal-dried fruit was higher, compared to the freezedried samples, by 34 mg/100 g DM (converted into hyperoside) and 24 mg/100 g
(converted into quercetin), that is by 19%, on average.
Figure 1. The content of flavonoids converted into hyperoside (A) and quercetin (B) in freeze (FD)
and thermal (TD) dried fruit of European cranberry. T-test for dependent samples: A) t=–4.66390,
p<0.001, n=23; B) t=–4.62697, p<0.001, n=23; DM – dry matter.
The effect of thermal and freeze drying on the content of organic acids and flavonoids in fruit of European cranberry ...
97
Figure 2. The content of ascorbic (A), malic (B), quinic (C) and citric (D) acids in freeze (FD) and
thermal (TD) dried fruit of European cranberry. T-test for dependent samples: A) t=4.564455,
p<0.001, n=21; B) t=5.499619, p<0.001, n=21; C) t=9.234588, p<0.001, n=23; D) t=7.322546,
p<0.001, n=23; DM – dry matter.
Figure 3. The percentage differences in the content of the investigated compounds in the
lyophilized and thermally dried fruit of European cranberry. FLH – flavonoids converted into
hyperoside; FLQ – flavonoids converted into quercetin; QUA – quinic acid; CIA – citric acid; MAA
– malic acid; ASA – ascorbic acid.
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DISCUSSION AND CONCLUSIONS
Papers presenting results of phytochemical studies of European cranberry fruit
are relatively few [39]. Available data on the content of flavonoids and organic
acids in fruit of the species concerned are difficult to compare due to diverse
analysis methods, the application of different formulas and methods of calculating the content of active compounds in the material as well as due to varying
conditions of test material drying and storage [2, 3, 17, 40-42].
Nevertheless, it is interesting to compare the obtained results with earlier analyses related to commercial raw materials and processed products made from Oxycoccus palustris and O. macrocarpos [5]. Such a comparison shows that commercially
available dried fruit of both cranberry species are distinctly inferior, in terms of
their content of quinic, malic and citric acids, to the investigated cranberry fruits.
It was found in the case of application of both freeze drying and thermal drying.
In the lyophilized material, there was also definitely more ascorbic acid (fig. 2).
In the thermally dried fruit of Oxycoccus palustris, twice more flavonoids were also
found (fig. 1) than in the commercial raw material of this species dried under similar conditions [39].
It is recommended that flavonoid materials should be dried in natural conditions
or at a temperature of 120ºC [43]. In the opinion of Elbanowska [44], high drying temperature, in the order of 120–150ºC, stabilises better flavonoid compounds
than a temperature of 40–60ºC. The presented results demonstrate that drying of
European cranberry fruit at a temperature of 35–40ºC guarantees higher flavonoid
content than the lyophilization process (fig. 1). The studies of Mazur and Borowska
[6] show that lyophilization of fruit of this species results in a twofold reduction in
the phenolic content and as much as a sevenfold reduction in the content of anthocyanins, compared to fresh fruit. According to data cited by Abascal and co-authors
[45], freeze drying insufficiently stabilises some groups of pharmacologically active
compounds, such as volatile compounds, phenolics and carotenoids. In the case of
blueberry, lyophilization causes the loss of several flavour components, including
1,8-cineole, which gives to blueberry its characteristic aroma [20].
On the other hand, however, many authors indicate advantages of freeze drying,
in particular in the context of other traditional methods [46]. It is stressed that
lyophilization preserves well the flavour, appearance, colour, texture and biological
activity of products [47]. Detailed investigations confirm this conviction. For example, there are more polyphenols and anthocyanins in lyophilized bilberry fruit than
in material dried at a temperature of 40ºC [22]. Likewise in the case of European
cranberry, more phenolic compounds were found in lyophilized fruit than in those
dried at 24ºC [16, 17]. Lyophilization also preserves more vitamin C, chlorophyll,
riboflavins and niacins than drying at a temperature of 50 and 70ºC [45].
The study presented in this paper shows that more organic acids (in particular
ascorbic acid) may occur in the lyophilizates than in the thermally dried material
(fig. 2), since high temperature accelerates the degradation of vitamin C [46]. Low
The effect of thermal and freeze drying on the content of organic acids and flavonoids in fruit of European cranberry ...
99
temperature may also cause a significant reduction in ascorbic acid content, however, together with a decrease in pH value, the resistance of the compound in
question to oxidation increases [31]. The stability of vitamin C in fruit is attributed
to a high content of acids [48], but also to the effect of phenolic compounds with
antioxidative activity [49] and intracellular compartmentation of ascorbate and
phenolics [26].
To sum up, the obtained results show that drying conditions have a significant
effect on the content of active substances in European cranberry fruit, with this
effect varying depending on the group of the compounds studied. Lyophilization
stabilises better organic acids in the raw material than thermal drying, whereas it
stabilises flavonoids worse. Therefore, it is not a universal and optimal method for
drying cranberry fruit with respect to all groups of compounds, which is also noted
by Mazur and Borowska [6]. The cited authors point out that a significant reduction
in the content of polyphenols and anthocyanins in lyophilized fruit of the species in
question may be caused by the prolongation of freeze drying due to the occurrence
of thick skin on cranberry fruit, impeding water removal. The economic factor is
not irrelevant, either. Lyophilization is estimated to be 4–8 times more expensive
than other (traditional) drying methods [46, 50]. Given the small number of studies
on the effect of lyophilization on the value of herbal raw materials [45], it seems
justified to continue the undertaken studies on European cranberry, in particular if
they also include other groups of biologically active compounds.
AcknowledgeMENT
This study was financed by the Ministry of Science and Higher Education under
a research project no. N405022 31/1319.
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Wpływ suszenia termicznego i sublimacyjnego na zawartość kwasów
organicznych i flawonoidów w owocach żurawiny błotnej (Oxycoccus
palustris Pers.)
Artur Adamczak, Waldemar Buchwald*, Jan Kozłowski,
Sebastian Mielcarek
Oddział Roślin Zielarskich
Instytut Włókien Naturalnych i Roślin Zielarskich
ul. Libelta 27
61-707 Poznań
*autor, do którego należy kierować korespondencję: [email protected]
Vol. 55 No 3 2009
A. Adamczak, W. Buchwald, J. Kozłowski, S. Mielcarek
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Streszczenie
W niniejszej pracy przedstawiono wyniki badań dotyczących wpływu suszenia sublimacyjnego i termicznego na zawartość kwasów organicznych i flawonoidów w owocach
żurawiny błotnej (Oxycoccus palustris Pers.). Wskazują one na istotny wpływ warunków
suszenia na zawartość wymienionych związków czynnych. W owocach liofilizowanych
stwierdzono wyraźnie więcej kwasów organicznych, natomiast mniej flawonoidów, niż
w owocach suszonych w temperaturze 35-40ºC. Największe różnice względne występowały
w przypadku kwasu askorbowego. Jego zawartość w owocach suszonych w podwyższonej
temperaturze, w stosunku do surowca liofilizowanego, była niższa przeciętnie o 5 mg/100 g
suchej masy (SM), czyli średnio o 42%. Dla pozostałych kwasów organicznych (chinowego, cytrynowego i jabłkowego) przeciętna różnica wynosiła 0,9-2,1 g/100 g SM
(14-17%). Średnia zawartość flawonoidów w owocach suszonych termicznie była większa,
w stosunku do prób liofilizowanych, o 34 mg/100 g SM (w przeliczeniu na hiperozyd)
i 24 mg/100 g (w przeliczeniu na kwercetynę), czyli średnio o 19%.
Słowa kluczowe: Oxycoccus palustris, rośliny lecznicze, suszenie, kwasy organiczne, flawonoidy