Effects of Thermal Treatment on the Phenolic Content and Antioxidant Activity of Some Vegetables
Vegetables are valuable sources of nutrients and they are also being recognised for their antioxidant properties. To assess the benefits and value of phenolic compounds in extracts, it is imperative to evaluate the effects of different processing methods applied to foods on the ultimate quantities/activities of the phenolics. In this study, the effects of thermal treatment on some vegetables is reviewed.
Received: February 03, 2010;
Accepted: March 19, 2010;
Published: July 01, 2010
Phenolic compounds are widely distributed in fruits and vegetables. Epidemiological
studies have indicated that regular consumption of foods rich in phenolic compounds
such as fruits, vegetables, whole grain cereals, red wine and tea, is associated
with reduced risk of cardiovascular diseases, neuro-degenerative diseases and
certain cancers (Amin et al., 2006; Huang
et al., 2007; Hunterand Fletcher, 2002; Nilsson
et al., 2004; Parr and Bolwell, 2000). The
beneficial effects derived from phenolic compounds have been attributed to their
antioxidant activity, protection and regeneration of other dietary antioxidants
(i.e., vitamin E) and chelating of pro-oxidant metal ions. The amount and species
of phenolic compounds vary dramatically among vegetables. Therefore, it is important
to analyse the composition of phenolic compounds in different vegetables before
their health promoting properties can be claimed. According to Ismailet
al. (2004) each type of vegetable has a different antioxidant activity
contributed by different antioxidant components. Some of present studies Muchuweti
et al. (2007a, b) and Chitindingu
et al. (2007) showed that some vegetables are rich sources of phenolic
compounds that might contribute to their antioxidant activities. These components
are affected differently when exposed to different processing techniques.
Vegetable processing such as blanching, canning, sterilising and freezing,
as well as cooking is expected to affect the yield, composition and bioavailability
of antioxidants (Amin et al., 2006; Chu
et al., 2000; Hunter and Fletcher, 2002)
and some nutritional antioxidants such as the heat labile vitamin C. During
vegetable processing, qualitative changes, antioxidant breakdown and their leaching
into surrounding water may influence the antioxidant activity of the vegetables
(Podsedek, 2007). Some antioxidant compounds like ascorbic
acid and carotenoids are very sensitive to heat and storage and are lost during
different vegetable processing steps (Zhang and Hamauzu,
2004). Hence the present review focuses on the effects of processing vegetables
on their phenolic contents and antioxidant activities.
ANALYTICAL METHODS FOR TOTAL PHENOLICS CONTENT AND ANTIOXIDANT ACTIVITY
One of the methods that have been developed for quantification of phenolic
compounds is the Folin Ciocalteu (Folin C.) method, which is based on the reduction
of phosphomolybdic acid in acid by phenols in aqueous alkali. The method is
used to determine the total free phenolic groups. We found this method to be
fast and simple in most of our studies on the phenolic contents of some fruits
(Ndhlala et al., 2007, 2008a,
b). The method does not differentiate between tannins
and many other phenolics that are not tannins. Interfering substances such as
ascorbic acid, tyrosine and glucose are also measured (Singleton
and Jr. Rossi, 1965; Makkar, 1999). Commercial tannic
acid which used to be the common standard for the Folin C method comprises a
mixture of gallotannins. Tannic acid has been replaced by gallic acid in its
use as a standard because of its heterogeneous structure which consists of a
mixture of galloyl esters (Hagerman, 2002).
The radical scavenging assays involve direct measurement of hydrogen atom donation
or electron transfer from the potential antioxidant to free radical molecules
in lipid free systems. Such assays are available as commercial kits and or a
laboratory can prepare their own reagents (Becker et
al., 2004). The assays for detection of antioxidant activity, scavenging
of stable radicals and scavenging of short lived radicals include; the Ferric
Reducing Antioxidant Power (FRAP) assay, Trolox Equivalent Antioxidant Capacity
(TEAC) assay, 1, 1-diphenyl-2-picrylhydrazyl radical (DPPHÿ) assay, the
Oxygen Radical Absorbance Capacity (ORAC) assay and model systems. The evaluation
of antioxidants in model systems is based on measuring changes in the concentration
of compounds being oxidized, on depletion of oxygen or on the formation of oxidation
products (Becker et al., 2004; Kumazawa
et al., 2004).
Vegetables are usually processed by the application of dry or moist heat to improve their organoleptic properties or extend their shelf life. This technique has been known from time immemorial to reduce the quantity of nutrients in foods, especially the heat labile vitamins. Hence, consumer demand for nutritious foods, which are minimally and naturally processed, has led to interest in some non thermal technologies. These non thermal technologies are not commonly used by industries in developing countries. Therefore, in most of the processing unit operations heat is applied. When subjected to heat treatment vegetables are affected differently.
EFFECTS ON TOTAL PHENOLIC CONTENT
In most studies on the effects of heat treatment on the total phenolic content, the results are contradicting. Some researchers reported an increase in the phenolic content whilst others observed a decrease. In some researches an attempt was made to simulate the actual cooking conditions and as a result in some papers the cooking conditions were not explicitly specified. The data generated using the actual cooking conditions is beneficial when included in food composition databases as it will enable the users to evaluate the actual amounts of bioactive compounds consumed.
|| Effects of different heat treatments on the total phenolic
content of some vegetables
Effects of different heat treatments on the total phenolic content of some
vegetables are shown in Table 1.
Lima et al. (2009) observed a dramatic loss
of phenolic content on convectional and organic grown food as a result of thermal
treatment. Significant differences between foods obtained by the two cultivation
procedures were not observed. Organic and minimally processed foods are mostly
preferred because of the nominal changes to the nutrients and non-nutrient components
of foods. Hence, the consumption of organically grown and minimally processed
foods will entail food that is free from toxicants and nutritious.
Cooking asparagus was found to increase total phenols by 23% (Fanasca
et al., 2009). The results also indicate that the effect of cooking
process was significant and more pronounced than the effect of cultivars. The
study was in agreement with that by Lima et al. (2009)
in which the type of food did not have a profound effect on the phenolic content.
Ascorbic acid of asparagus was greatly reduced by the cooking process (Fanasca
et al., 2009). The vitamin contributes to the total phenols as it
is capable of reducing the active reagent used in the analysis of phenols. Hence
processes that affect ascorbic acid will ultimately reduce the total phenolic
Decreasing temperature of processing was also found to preserve 80-100% of
phenolic content in some vegetables (Roy et al.,
2007). Convectional and steam cooking caused significant reduction in total
phenol content of red cabbage and decreasing cooking water and time by half
led to better retention (Podsedek et al., 2008).
The short cooking time used in steam cooking preserves the antioxidant components
of the vegetables than convectional cooking.
Total phenolic content of selected vegetables (peas, carrot, spinach, cabbage,
cauliflower, yellow turnip and white turnip) was found to be generally decreased
by boiling, frying and microwave cooking (Sultana et
al., 2008). Microwave cooking was found to cause more significant changes
than the other methods of processing. Thus an appropriate method might be sought
for the processing of such vegetables to retain their antioxidant components
at maximum level.
Normal cooking temperatures were found to detrimentally affect phenolic content
of spinach, komatsuna, haruna, chingensai, cabbage and
Chinese cabbage (Roy et al., 2007). The same
authors noted that the degree of thermal processing affects not only the content
of phenolic compounds in vegetables but also beneficial biological effects associated
with these compounds. Soaking, boiling and steaming processes caused significant
(p<0.05) decreases in total phenolic content of commonly consumed Cool Season
Food Legumes (CSFLs), including green pea, yellow pea, chickpea and lentil.
However steaming treatments resulted in a greater retention of TPC values in
all tested CSFLs as compared to boiling treatments (Xu
and Chang, 2008).
Turkmen et al. (2005) observed that cooking affected
total phenolics content of some vegetables significantly (p<0.05). After
cooking, total activity increased or remained unchanged depending on the type
of vegetable but not type of cooking. The results of the study contradicts some
studies (Lima et al., 2009; Fanasca
et al., 2009) were the type of vegetable did not have any effect
on the phenolic content.
Blanching up to 15 min was found to cause losses of phenolic content, depending
on the species of spinach (Amin et al., 2006).
Semi-drying of tomatoes was found to lower the phenolic content by 30% but drying
of pepper gave contradicting results (Toor and Savage, 2006).
Vega-Galvez et al. (2009) observed that the total
phenolic content of red pepper content decreased as air-drying temperature decreased.
EFFECTS ON ANTIOXIDANT ACTIVITY
Phenolic compounds and other reductones in vegetables contribute to their antioxidant activities; hence processes that affect the total phenolic content will also affect antioxidant activity. Effects of different heat treatments on the antioxidant activity of some vegetables are shown in Table 2.
Canning is a common industrial process for preserving vegetables and was shown
to cause a more pronounced loss of antioxidant activity on frozen vegetables
than fresh vegetables (Murcia et al., 2009).
The reduction in antioxidant activity as a result of canning might be caused
by the high temperatures used during sterilisation and the degradation of phenolic
compounds as a result of storage.
Faller and Fialho (2009) evaluated the effect of boiling,
microwaving and steaming on fresh conventional and organic retail vegetables
(potato, carrot, onion, broccoli and white cabbage). Organic vegetables showed
higher sensitivity to heat processing than did conventionally grown vegetables.
In contrast significant differences between foods obtained by the two cultivation
procedures were not observed by Lima et al. (2009).
In general, cooking was found to lead to reductions in the antioxidant capacity
for most vegetables, with small differences between the cooking methods applied
(Faller and Fialho, 2009). In his studies, Sultana
et al. (2008) reported that different cooking methods affected the
antioxidant properties of vegetables differently, with microwave treatment exhibiting
more deleterious effects when compared with those of other treatments.
|| Effects of different heat treatments on the antioxidant activities
of some vegetables
acid] ,2TEAC: Trolox equivalent antioxidant capacity, 3DPPH.-1,1-diphenyl-2-picrylhydrazyl,
4ß-CLAMS-ß- carotene linoleic acid model system,
5ORAC: Oxygen radical absorbance capacity
Normal cooking temperatures detrimentally affected the antiradical and anti-proliferative
activities of spinach, komatsuna, haruna, chingensai, cabbage
and Chinese cabbage. However, mild heating of vegetable juices (50°C, 10-30
min) preserved the antioxidant activity and cell proliferation inhibition activities
(Roy et al., 2007). The results showed that foods
should be minimally processed so as to preserve the compounds that contribute
to their antioxidant and cell proliferation inhibition activities. Hence consumer
demand for safe and nutritious food has led to the development of a number of
non thermal food preservation techniques.
Xu and Chang (2008) reported the effects of soaking,
boiling and steaming processes on the antioxidant activity in commonly consumed
Cool Season Food Legumes (CSFLs), including green pea, yellow pea, chickpea
and lentil. As compared to original unprocessed legumes, all processing steps
caused significant (p<0.05) decreases in DPPH free radical scavenging activity
in all tested CSFLs. All soaking and atmospheric boiling treatments caused
significant (p<0.05) decreases in oxygen radical absorbing capacity (ORAC).
However, pressure boiling and pressure steaming caused significant (p<0.05)
increases in ORAC values. Steaming treatments resulted in a greater retention
of DPPH and ORAC values in all tested CSFLs as compared to boiling treatments.
Steam cooking was also reported to increase the antioxidant activity of broccoli
by 230% (Roy et al., 2007). Therefore, steam
cooking should be used instead of boiling the vegetables for prolonged periods.
When cooking, time and temperature combinations should be closely monitored,
as prolonged exposure of vegetables to cooking conditions may lead to deleterious
A study by Turkmen et al. (2005) on pepper, peas
and broccoli gave interesting results, were after cooking total antioxidant
activity increased or remained unchanged depending on the type of vegetable
but not type of cooking. Amin et al. (2006) also
reported that blanching up to 15 min may affect losses of antioxidant activity,
depending on the species of spinach (Amin et al.,
2006). The type of vegetable or species differences might be due to the
easy of extraction of phenolic compounds from different plant matrices. Processes,
such as freezing plant material prior to cooking were found to reduce antioxidant
activities to some greater extent. Processes applied on fresh material reduced
the activity against ABTS●+ to a smaller extent (cooking by
11% and freezing by around 20%), while cooking frozen beans caused a further
decrease by 10-17% (Wolosiak et al., 2009).
In most studies on effects of heat treatment on vegetables, the antioxidant
activity decreased as a result of processing, in contrast Fanasca
et al. (2009) reported an increase in antioxidant activity. In his
study on antioxidant properties of raw and cooked spears of green asparagus
cultivars, the cooking process increased the antioxidant activity by 16%. Podsedek
et al. (2008) reported that steam-cooking is recommended to prevent
the major loss of scavenging activity, because under these conditions, the corresponding
TEAC (Trolox Equivalent Antioxidant Capacity) values were reduced only by 5-20%.
Roy et al. (2007) and Xu
and Chang (2008) also reported an increase in antioxidant activity as a
result of steam cooking.
The effects of different cooking methods (boiling, frying and microwave cooking)
on the antioxidant activity of some selected vegetables (peas, carrot, spinach,
cabbage, cauliflower, yellow turnip and white turnip) were assessed by measuring
the reducing power and percentage inhibition in linoleic acid system (Sultana
et al., 2008). The author reported contradicting results found by
most researchers as there was a significant (p<0.05) increase in reducing
power as a result of frying. However, boiling and microwave cooking did not
affect reducing power. Inhibition of peroxidation was also increased by boiling
and frying, whereas, in contrast it was decreased by microwave cooking.
The results of the present investigations showed that all the thermal treatment methods affected the total content of phenolics and antioxidant properties of the vegetables; however, other treatments such as microwave treatment exhibited more deleterious effects than the other methods. Thus appropriate methods might be sought for the processing of such vegetables to retain their antioxidant components at maximum level. Most losses are due to the leaching of antioxidant compounds from the vegetables into the cooking water during the prolonged exposure to water and heat. Therefore, it is vital to use less water and cooking time and also to consume the water used for boiling so as to obtain the optimum benefits of bioactive compounds present in vegetables.
Amin, I., Y. Norazaidah and K.I.E. Hainida, 2006.
Antioxidant activity and phenolic content of raw and blanched Amaranthus
species. Food Chem., 94: 47-52.CrossRef | Direct Link |
Becker, E.M., L.R. Nissen and L.H. Skibsted, 2004.
Antioxidant evaluation protocols: Food quality or health effects. Eur. Food Res. Technol., 219: 561-571.CrossRef |
Chitindingu, K., A.R. Ndhlala, C. Chapano, M.A. Benhura and M. Muchuweti, 2007.
Phenolic compound content, profiles and antioxidant activities of Amaranthus hybridus
(Pigweed), Brachiaria brizantha
) and Panicum maximum
(Guinee grass). J. Food Biochem., 31: 206-216.CrossRef |
Chu, Y.H., C.L. Chang and H.F. Hsu, 2000.
Flavonoid content of several vegetables and their antioxidant activity. J. Sci. Food Agric., 80: 561-566.Direct Link |
Faller, A.L.K. and E. Fialho, 2009.
The antioxidant capacity and polyphenol content of organic and conventional retail vegetables after domestic cooking. Food Res. Int., 42: 210-215.CrossRef |
Fanasca, S., Y. Rouphael, E. Venneria, E. Azzini, A. Durazzo and G. Maiani, 2009.
Antioxidant properties of raw and cooked spears of green asparagus cultivars. Int. J. Food Sci. Technol., 44: 1017-1023.CrossRef |
Gawlik-Dziki, U., 2008.
Effect of hydrothermal treatment on the antioxidant properties of broccoli (Brassica oleracea
var. Botrytis italica
) florets. Food Chem., 109: 393-401.CrossRef | Direct Link |
Hagerman, A.E., 2002.
Tannin Chemistry. Miami University Oxford, USA
Huang, Z., B. Wang, D.H. Eaves, J.M. Shikany and R.D. Pace, 2007.
Phenolic compound profile of selected vegetables frequently consumed by African Americans in the Southeast United States. Food Chem., 103: 1395-1402.CrossRef |
Hunter, K.J. and J.M. Fletcher, 2002.
The antioxidant activity and composition of fresh, frozen, jarred and canned vegetable. Innovation Food Sci. Emerg. Technol., 3: 399-406.CrossRef | Direct Link |
Ismail, A., Z.M. Marjan and C.W. Foong, 2004.
Total antioxidant activity and phenolic content in selected vegetables. Food Chem., 87: 581-586.CrossRef | Direct Link |
Kumazawa, S., T. Hamasaka and T. Nakayama, 2004.
Antioxidant activity of propolis of various geographic origins. Food Chem., 84: 329-339.CrossRef | Direct Link |
Lima, G.P.P., T.D.V.C. Lopes, M.R.M. Rossetto and F. Vianello, 2009.
Nutritional composition, phenolic compounds, nitrate content in eatable vegetables obtained by conventional and certified organic grown culture subject to thermal treatment. Int. J. Food Sci. Technol., 44: 1118-1124.CrossRef | Direct Link |
Makkar, H.P.S., 1999.
Quantification of tannins in tree foliage: A laboratory manual for FAO/IAEA coordinated research project on the Use of nuclear and related techniques to develop simple tannin assay for predicting and improving the safety and efficiency of feeding ruminants on the tanniniferous tree foliage. Joint FAO/IAEA Division of Nuclear Techniques in Food and Agriculture, Vienna, pp: 1-29.
Muchuweti, M., C. Mupure, A. Ndhlala, T. Murenje and M.A.N. Benhura, 2007.
Screening of antioxidant and radical scavenging activity of Vigna ungiculata
, Bidens pilosa
and Cleome gynandra
. Am. J. Food Technol., 2: 161-168.CrossRef | Direct Link |
Muchuweti, M., E. Kativu, C.H. Mupure, C. Chidewe, A.R. Ndhlala and M.A.N. Benhura, 2007.
Phenolic composition and antioxidant properties of some spices. Am. J. Food Technol., 2: 414-420.CrossRef | Direct Link |
Murcia, M.A., A.M. Jimenez and M. Martinez-Tome, 2009.
Vegetables antioxidant losses during industrial processing and refrigerated storage. Food Res. Int., 42: 1046-1052.CrossRef |
Ndhlala, A.R., M. Muchuweti, C. Mupure, K. Chitindingu, T. Murenje, A. Kasiyamhuru and M.A. Benhura, 2008.
Phenolic content and profiles of selected wild fruits of Zimbabwe: Ximenia caffra
, Artobotrys brachypetalus
and Syzygium cordatum
. Int. J. Food Sci. Technol., 43: 1333-1337.CrossRef |
Ndhlala, A.R., K. Chitindingu, C. Mupure, T. Murenje, F. Ndhlala, M.A. Benhura and M. Muchuweti, 2008.
Antioxidant properties of methanolic extracts from Diospyros mespiliformis
(Jackal Berry), Flacourtia indica
(Batoka plum), Uapaca kirkiana
(Wild Loquat) and Ziziphus mauritiana
(yellow berry) fruits. Int. J. Food Sci. Technol., 43: 284-288.CrossRef |
Ndhlala, A.R., A. Kasiyamhuru, C. Mupure, K. Chitindingu, M.A. Benhura and M. Muchuweti, 2007.
Phenolic composition of Flacourtia indica, Opuntia megacantha
and Sclerocarya birrea
. Food Chem., 103: 82-87.CrossRef |
Nilsson, J., R. Stegmark and B. Akesson, 2004.
Total antioxidant capacity in different pea (Pisum sativum
) varieties after blanching and freezing. Food Chem., 86: 501-507.CrossRef |
Parr, A.J. and G.P. Bolwell, 2000.
Phenols in the plant and in man. The potential for possible nutritional enhancement of the diet by modifying the phenols content or profile. J. Sci. Food Agric., 80: 985-1012.CrossRef | Direct Link |
Podsedek, A., 2007.
Natural antioxidants and antioxidant capacity of brassica vegetables: A review. LWT-Food Sci. Technol., 40: 1-11.CrossRef | Direct Link |
Podsędek, A., D. Sosnowska, M. Redzynia and M. Koziołkiewicz, 2008.
Effect of domestic cooking on the red cabbage hydrophilic antioxidants. Int. J. Food Sci. Technol., 43: 1770-1777.CrossRef |
Roy, M.K., M. Takenaka, S. Isobe and T. Tsushida, 2007.
Antioxidant potential, anti proliferative activities, and phenolic content in water-soluble fractions of some commonly consumed vegetables: Effects of thermal treatment. Food Chem., 103: 106-114.CrossRef |
Singleton, V.L. and J.A. Rossi, 1965.
Colorimetry of total phenolics with phosphomolybdic-phosphotungstic acid reagents. Am. J. Enol. Viticult., 16: 144-158.Direct Link |
Sultana, B., F. Anwar and S. Iqbal, 2008.
Effect of different cooking methods on the antioxidant activity of some vegetables from Pakistan. Int. J. Food Sci. Technol., 43: 560-567.Direct Link |
Turkmen, N., F. Sari and Y.S. Velioglu, 2005.
The effect of cooking methods on total phenolics and antioxidant activity of selected green vegetables. Food Chem., 93: 713-718.CrossRef | Direct Link |
Wolosiak, R., E. Worobiej, M. Piecyk, B. Druzynska, D. Nowak and P.P. Lewicki, 2009.
Activities of amine and phenolic antioxidants and their changes in broad beans (Vicia faba
) after freezing and steam cooking. Int. J. Food Sci. Technol., 45: 29-37.
Vega-Galvez, A., K.D. Scala, K. Rodriguez, R. Lemus-Mondaca, M. Miranda, J. Lopez and M. Perez-Won, 2009.
Effect of air-drying temperature on physico-chemical properties, antioxidant capacity, colour and total phenolic content of red pepper (Capsicum annuum
, L. var. Hungarian). Food Chem., 4: 647-653.CrossRef | Direct Link |
Xu, B. and S.K.C. Chang, 2008.
Effect of soaking, boiling and steaming on total phenolic contentand antioxidant activities of cool season food legumes. Food Chem., 110: 1-13.CrossRef | Direct Link |
Yasmin, A., A. Zeb, A.W. Khalil, G.M. Paracha and A.B. Khattak, 2008.
Effect of processing on anti-nutritional factors of red kidney bean (Phaseolus vulgaris
) grains. Food Bioproc. Technol., 1: 415-419.CrossRef |
Zhang, D. and Y. Hamauzu, 2004.
Phenolics, ascorbic acid, carotenoids and antioxidant activity of broccoli and their changes during conventional and microwave cooking. Food Chem., 88: 503-509.CrossRef | Direct Link |
Roy, M.K., L.R Juneja, S. Isobe and T. Tsushida, 2009.
Steam processed broccoli (Brassica oleracea
) has higher antioxidant activity in chemical and cellular assay systems. Food Chem., 114: 263-269.CrossRef |
Toor, R.K. and G.P. Savage, 2006.
Effect of semi-drying on the antioxidant components of tomatoes. Food Chem., 94: 90-97.CrossRef |