Thermal treatment of food is done in the food industry for preservation purposes. Food preservation prevents deteriorative reactions, extends a foods shelf life and assures its safety. Thermal processing has most of the characteristics of an ideal food preservation method. However, in some foods the high thermotolerance of certain enzymes and microorganisms, mainly bacterial spores, entails the application of extreme heat treatments, which changes the nutritional, phytochemical and organoleptic food properties. Therefore, alternatives to thermal processing as the main means of inactivating pathogenic and spoilage microorganisms are being developed by researchers. The present review focuses on the effect of some non thermal processing technologies on the bioactive compounds and antioxidant activities of some vegetables.
How to cite this article:
B. Chipurura and M. Muchuweti, 2010. Effect of Irradiation and High Pressure Processing Technologies on the Bioactive Compounds and Antioxidant Capacities of Vegetables. Asian Journal of Clinical Nutrition, 2: 190-199.
Over the last few years consumption of processed fruits and vegetables has increased and there is convincing epidemiological evidence that consumption of fruits and vegetables is in general beneficial to health (Duthie et al., 2000; Netzel et al., 2007). Some of our studies showed that fruits and vegetables are rich sources of phytochemical substances (Muchuweti et al., 2005, 2006, 2007; Chitindingu et al., 2007; Nhukarume et al., 2010). The potential health benefits of phytochemical substances have been related to the action of the compounds as antioxidants and inhibitors of lipid peroxidation (Li-Chen et al., 2005). Phytochemical compounds such as phenolics are strong antioxidants against free radicals and reactive oxygen species. Free radicals and reactive oxygen species have been implicated as the major cause of chronic human diseases such as cardiovascular diseases, diabetes and degenerative diseases (Kyung-Hee et al., 2005; Chen and Yen, 2007).
Conventional thermal processing of fruits and vegetables remains the most widely adopted technology for shelf-life extension and preservation of fruits and vegetables. However, consumer demand for nutritious foods, which are minimally and naturally processed, has led to interest in non thermal technologies. The effect of thermal treatment on the total phenolics contents and antioxidant activities of vegetables has been reviewed recently (Chipurura et al., 2010). Convectional cooking of vegetables was in general found to decrease the total phenolics contents and antioxidant activities of vegetables. Interestingly Tiwari et al. (2009) concluded that apart from ozone; high pressure processing, pulsed electric field, ultrasound, irradiation, dense phase carbon dioxide have minimal influence on anthocyanins content of fruit juices. Most studies on the effects of non thermal technologies were done on fruit juices. Hence, the present study focuses on the effects of some of these novel processing technologies on the phenolics contents and antioxidant activities of some vegetables.
ANTIOXIDANTS IN CELLULAR SYSTEMS AND FOODS
Antioxidants are defined as the substance that when present in low concentrations compared to those of an oxidisable substrate significantly delays or prevents oxidation of that substance. For the in vivo situation the concept of antioxidants includes antioxidant enzymes, iron binding and transport proteins and other compounds affecting signal transduction and gene expression (Percival, 1996). In case of foods and beverages, antioxidants are related to the protection of specific oxidation substrates. Antioxidants are divided into two major classes, namely endogenous antioxidants and exogenous antioxidants.
Endogenous antioxidants are mainly enzymes and three groups of enzymes play important roles in protecting cells from oxidant stress (Becker et al., 2004). Firstly, Superoxide Dismutases (SOD) are enzymes that catalyze the conversion of two superoxides to hydrogen peroxide and oxygen. Secondly, catalase, found in peroxisomes in eukaryotic cells, degrades hydrogen peroxide to water and oxygen and hence completes the detoxification reaction started by SOD. Finally, glutathione peroxidase, a group of enzymes which are the most abundant contain selenium and like catalase, degrade hydrogen peroxide. Glutathione peroxidase reduces organic peroxides to alcohols. In addition to the three enzymes, glutathione transferase, ceruloplasmin, hemoxygenase may participate in enzymatic control of oxygen radicals and their products.
Exogenous antioxidants are mainly obtained from the diet. The common exogenous antioxidants are vitamin E, vitamin C/ascorbic acid, carotenoids, glutathione and phenolics compounds. Vitamin E is the major lipid-soluble antioxidant and plays an important role in protecting membranes from oxidative damage. The primary activity vitamin E is to trap peroxy radicals in cellular membranes. Vitamin C is a water-soluble antioxidant that can reduce radicals from a variety of sources. Vitamin C participates in recycling vitamin E radicals. Vitamin C functions as a pro-oxidant under certain circumstances and sometimes produces oxygen by-products of metabolism that can cause damage to cells. To date carotenoids are known to act as antioxidants in vitro. In addition; to quenching of singlet oxygen, carotenoids may react with radical species either by additional reactions or through electron transfer reactions, which results in the formation of the carotenoid radical cation. Glutathione is the most important intracellular defense against damage by reactive oxygen species. The cysteine on the glutathione molecule provides an exposed free sulphydryl group that is very reactive, providing an abundant target for radical attack. Reaction with radicals oxidizes glutathione but the reduced form is regenerated in a redox cycle that involves glutathione reductase and the electron acceptor NADPH membranes (Percival, 1996).
In addition to vitamin E and vitamin C, phenolic compounds can function as antioxidants. The antioxidant properties of some plant extracts have been attributed partially to their phenolic compound contents (Coinu et al., 2007). There are many classes of phenolic compounds, with phenolic acids, flavonoids and tannins as the main dietary phenolics. Phenolic acids include hydroxybenzoic and hydroxycinnamic acids. Flavonoids are a diverse group of secondary plant metabolites that include flavonols, flavanols, flavanones and flavones. The major classes of tannins in the plant kingdom are hydrolysable and condensed tannins.
ANALYTICAL METHODS FOR TOTAL PHENOLICS CONTENT AND ANTIOXIDANT ACTIVITY
One of the common 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 (HTs and PAs). 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 Rossi, 1965; Makkar, 1999). Other methods used for the quantification of phenolic compounds are the butanol-HCl, vanillin-HCl and rhodanine assays.
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 (DPPH•) assay, the Oxygen Radical Absorbance Capacity assay (ORAC) 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). The variety of different methods/assays and experimental techniques also causes problems when comparing the antioxidant status of foods and biosystems. In view of this, an effort to adopt a standardized widely-applicable testing method is a logical consequence and has been the subject of numerous scientific discussions. From the general point of view, these radical scavenging assays should be recommended for standardization.
THERMAL PROCESSING TECHNOLOGIES
In addition to HPP and irradiation, high intensity Pulsed Electric Field (PEF) and ultrasound processing technologies are also being used in the food industry for preservation purposes. Other non thermal technologies with promising applications in the food industry include the use of Dense Phase Carbon Dioxide (DPCD) and ozone.
High intensity Pulsed Electric Field (PEF) processing involves the application of short high power electrical pulses (typically 20-80 kV cm-1) for short time periods (<1 sec) to fluid foods placed between 2 electrodes (Senorans et al., 2003). Typical systems for the treatment of pumpable fluids consist of a PEF generation unit, which is in turn composed of a high voltage generator and a pulse generator, a treatment chamber, a suitable product handling system and a set of monitoring and controlling devices as shown in Fig. 1.
PEF has been demonstrated to be effective against various pathogenic and spoilage microorganisms and enzymes without appreciable loss of flavour, colour and bioactive compounds (Cserhalmi et al., 2006; Elez-Martinez et al., 2006; Hodgins et al., 2002; Yeom et al., 2000).
|Fig. 1:||Schematics of a PEF processing system for pumpable products (Soliva-Fortuny et al., 2009)|
PEF treatment involves short treatment times to inactivate microorganisms at temperatures below those adversely affecting food qualities (Raso and Barbosa-Canovas, 2003). In comparison to the extensive research devoted to the destruction of microorganisms by PEF and the significant information existing about the effects of PEF on enzymes, there are few studies about the impact of PEF on the nutritional quality and health-related compounds of foods (Martin-Belloso et al., 2005).
Soliva-Fortuny et al. (2009) reviewed some studies done on impart of PEF on some bioactive compounds in foods. The bioactive compounds include proteins, peptides, amino acids, phenolic compounds, isoprenoid compounds, chlorophylls, fatty acids, vitamins and flavour compounds. The author noted that the application of these treatments in order to obtain safe and stable products while maintaining their fresh-like bioactive potential looks promising and requires further investigation.
Ultrasound is defined as sound waves with frequencies higher than 16 kHz. These waves can be propagated in a liquid media as alternating compression. If ultrasound has sufficient energy, a phenomenon known as cavitation occurs. Cavitation involves the formation, growth and rapid collapse of microscopic bubbles. Based on theoretical considerations, extremely high temperatures and pressures are momentarily delivered to the liquid media during the collapse of bubbles (Suslick, 1988). Electrical discharge and production of free radicals have also been associated with extreme conditions occurring inside the collapsing bubble (Leighton, 1998). However, the ultimate reason for microbial inactivation via ultrasound is believed to be the mechanical damage caused by cavitation (Raso et al., 1998).
Ultrasound processing was reported to have minimal effects on the degradation of key quality parameters such as colour and ascorbic (Tiwari et al., 2008), enhances the extraction of phenolic and other bioactive compounds from grape must or wine (Cocito et al., 1995). There are few studies on the effect of ultrasound processing on antioxidant capacities of foods. Hence, there is need to harness this technology which has shown potential in the preservation of foods.
Irradiation of food products is achieved by exposing the product to a source of ionizing or non ionizing energy for the purpose of extending the products shelf life. The ionizing radiation source could be high-energy electrons, X-rays (machine generated), or gamma rays (from Cobalt-60 or cesium-137), while the non-ionizing radiation is electromagnetic radiation mainly from ultraviolet rays (UV-A, UV-B and UV-C), visible light, microwaves and infrared (Mahapatra et al., 2005).
Irradiation is not being widely used because of some misconceptions by consumers about its role in causing cancer. Scientific studies have established that 10 kGy and above does not produce any toxicological hazards or nutritional or microbiological problems in food (WHO, 1999). Radiation processing has also been shown to decrease the antinutritional components in some proteinaceous leguminous seeds (Bhat et al., 2007). The use of irradiation combined with other methods especially low heat has been shown to be effective against a number of pathogenic microbes (Pan et al., 2004).
The results on the effects of irradiation on the phytochemical compounds and antioxidant activities are contradicting. Some researchers reported an improvement while others a decrease in antioxidants of irradiated samples. The observed phenomenon might be attributed by the dose applied (usually low and medium doses have insignificant effects on antioxidants), exposure time, raw material used and the solvent systems used in extracting the phenolics compounds. Alothman et al. (2009) reviewed the possible mechanisms for the radiation-induced changes on antioxidants.
Some authors (Fan, 2005; Costa et al., 2006; Song et al., 2007; Lemoine et al., 2007) reported an increase in the antioxidants of the irradiated vegetables. However, Schindler et al. (2005) reported significant reductions of p-hydroxybenzaldehyde, p-coumaric acid, ferulic acid, rutin, naringenin in tomatoes. A study by Costa et al. (2006) gave interesting results, were the phenolics compounds which contributes to the antioxidant activity were reduced significantly while the antioxidant activity of the broccoli florets increased.
To fully appreciate the benefits of irradiation, studies on the effects of the process on the nutritional, microbiological and organoleptic properties of the vegetables should be conducted. In one such study by Song et al. (2007), carrot and kale juices were irradiated and their microbiological, nutritional and sensory properties evaluated. The results showed that irradiation was effective in reducing the microbial load and preserving the nutritional and organolepric properties of the vegetable juices. Applications of the technology on a variety of vegetables juices should also be explored as literature reveals that most of the reported applications of irradiation are limited to fruit juices.
HIGH HYDROSTATIC PRESSURE PROCESSING (HHP)
High Hydrostatic Pressure Processing (HHP) is a non thermal food preservation method for microbial and enzyme inactivation. The method has nominal effects on nutritional and quality parameters when compared to thermal treatments. HHP is derived from material science in which products are treated above 100 MPa. HPP has been extensively reviewed (Rastogi et al., 2007).
|Table 1:||Effect of HPP on bioactive compounds and antioxidant activities of some vegetables|
Literature reveals that HPP preserves the nutritional value of HPP processed food and food products. Although commercial interest in the use of high-pressure technology in food processing has occurred only since the early 1990s, the effects of high pressure in inactivating microorganisms have been known for more than a century. Earliest investigations using HHP involved the application of pressures in milk and meats (Beattie and Lewis, 1924). The findings were promising as the process was found to reduce the microbial loads of the products and subsequently increased the shelf life.
The use of HPP in the food industry can lead to the production of many products with superior nutritional, functional and organoleptic properties. The technology has found some applications in blanching (Eshtiaghi and Knorr, 1993), dehydration and osmotic dehydration (Farr, 1990; Dornenburg and Knorr, 1993; Rastogi et al., 1994, 2005; Rastogi and Niranjan, 1998), rehydration (Zhang et al., 2005), frying (Knorr, 1999). Other application of the technology include solid liquid extraction, pressure shift freezing and pressure assisted thawing, gelation and rheology (Benet et al., 2004; Fernandez-Garcia et al., 2001; Knorr, 1999).
There are few reports concerning the retention of phenolics compounds and antioxidant activities in vegetables, one such report is that by Patras et al. (2008) who reported an insignificant reduction in antioxidant activity of tomato and carrot purée. The authors also reported retention of more than 90% of ascorbic acid after treatments at 600 MPa. Some reports on the effect of HPP on bioactive compounds and antioxidant activities are shown in Table 1.
It would appear from a nutritional point of view that high pressure processing and irradiation are exceptional food processing technologies which have the potential to preserve compounds with pharmacological properties in foods. Therefore, high pressure processed and irradiated foods could be sold at a premium over their thermally processed counterparts as they will have retained their fresh-like properties. However, these methods should be combined with other preservation techniques to ensure that thermotolerant microbes are inactivated.
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