13 Non thermal processing
Thermal processing of food remains the most widely adopted technology for shelf life extension and preservation. However, growing consumer demand for nutritious foods, which are minimally and naturally processed, has resulted in continued interest in non-thermal technologies. Non-thermal technologies encompass all preservation treatments that are effective at ambient or sub lethal temperatures and are generally found to be more energy efficient. The temperature of foods is held below the temperature range normally used in thermal processing, thereby minimising negative effects on bioactive compounds present in food. A number of novel thermal and non-thermal preservation techniques are being developed to satisfy consumer demand with regard to the nutritional and sensory aspects of foods. Ensuring food safety and at the same time meeting such demands, has resulted in increased interest in non-thermal preservation techniques for inactivating microorganisms and enzymes in foods (P. Cullen, 2011; Vega-Mercado, Martin-Belloso, Qin, Chang, Marcela Góngora-Nieto, Barbosa-Canovas, et al., 1997). This chapter summarises potential non- thermal food preservation techniques currently under investigation. Ensuring food safety, while at the same time preserving bioactive compounds, is a challenge due to variations in intrinsic and extrinsic processing parameters of foods. Novel non-thermal preservation techniques considered in this chapter include high pressure, pulsed electric field, ultrasound, irradiation, dense phase carbon dioxide and ozone processing of solid, semi-solid and liquid foods. The effects of non-thermal techniques on the stability of phytochemical compounds are also discussed.
Irradiation treatment generally involves the exposure of food products (raw or processed) to ionising or non-ionising radiation for the purpose of food preservation. The ionising radiation source could be high-energy electrons, X-rays (machine generated) or gamma rays (from Cobalt-60 or cesium-137), while the non-ionising radiation is electromagnetic radiation that does not carry sufficient energy/quanta to ionise atoms or molecules, represented mainly by UV-A (315–400 nm), UV-B (280–315 nm) and UV-C (200–280 nm). Irradiation of food products typically causes minimal modification in the flavour, colour, nutrients, taste and other quality attributes of food (M. Alothman, R. Bhat and A. A. Karim, 2009). However, the levels of modification (in flavour, colour, nutrients, taste etc.) may vary depending on the product, irradiation dose and on the type of radiation source employed (gamma, X-ray, UV, electron beam) (R. Bhat and Sridhar, 2008; R. Bhat, Sridhar and Tomita-Yokotani, 2007). Depending upon the radiation dose, foods may be pasteurised to reduce or eliminate food-borne pathogens. Inactivation of microorganisms by irradiation is primarily due to DNA damage, which destroys the reproductive capabilities and other functions of the cell (DeRuiter and Dwyer, 2002). Tables 13.1 and 13.2 lists reported applications and effects of irradiation on bioactive compounds in selected food products.
Application of gamma radiation has been investigated for a wide range of foods and food products including fruit juices (Alighourchi, Barzegar and Abbasi, 2008; D. Kim, Song, Lim, Yun and Chung, 2007), fresh cut fruit and vegetables (M. Alothman, R. Bhat and A. A. Karim, 2009; Fan and Sokorai, 2011; Jimenez, Alarcon, Trevithick-Sutton, Gandhi and Scaiano, 2011; J.-H. Kim, Sung, Kwon, Srinivasan, Song, Choi, et al., 2009). Irradiation induces negligible or subtle losses of nutrients and sensory qualities in food compared to thermal processing as it does not substantially raise the temperature of food during processing (Wood and Bruhn, 2000). However, Alighourchi, Barzegar and Abbasi (2008) reported a significant reduction in total and individual anthocyanin content in pomegranate juice after irradiation at higher doses (3.5–10 kGy). Similarly, Jimenez et al. (2011) observed inconsistent changes in the oxygen radical absorbance capacity values and total phenolic content of irradiated fresh cut spinach, where a significant decrease in the ascorbic acid content of irradiated spinach during storage at 4°C was found compared to untreated fresh samples. Irradiation effects on anthocyanin pigments depend upon the nature of anthocyanin, for example, diglycosides are reported to be relatively stable to irradiation compared to monoglycosides. Conversely, Ayed, Yu and Lacroix (1999) reported that the anthocyanin content in grape pomace increases with irradiation dose, with an optimum at 6 kGy. Increase in the anthocyanin content can be attributed to the release of bound pigment as a result of cell wall degradation. Some studies suggest that the decrease or increase in bioactive compounds is not dose dependent. For example, Zhu, Cai, Bao and Corke (2010) observed a decrease in phenolic compounds (p-coumaric acid, ferulic acid and sinapinic acid) and anthocyanins (cyanidin-3-glucoside and peonidin-3-glucoside) in black, red and white rice. They observed that at most irradiation doses a significant reduction in total phenolic acid and anthocyanin content of black rice was found. However, they also observed a significant increase in total anthocyanins and phenolic acids in black rice at doses of 6 and 8 kGy.
Application of UV radiation to whole fruit and vegetables and their products such as juice (Guan, Fan and Yan, 2012; Keyser, Muller, Cilliers, Nel, and Gouws, 2008) has been reported for the inactivation of microorganisms. UV radiated food products are reported to have lower levels of some phytochemicals during storage. For example, Guan, Fan and Yan (2012) observed that UV-C doses of 0.45–3.15 kJ m-2 applied to mushrooms resulted in a reduction in the antioxidant activity, total phenolics and ascorbic acid content compared to non-radiated samples during the first seven days of storage at 4°C. UV radiation is also reported to have a negative influence on anthocyanins. Bakowaska et al. (2003) reported a strong negative influence of UV irradiation on the complex of cyanidin-3-glucoside with copigment compared to thermal treatment at 80°C. However, the presence of certain copigments can inhibit the degradation effect of UV on anthocyanins improving the cyanidin-copigment complex (Kucharska, Oszmianski, Kopacz and Lamer-Zarawska, 1998). Literature reveals that most of the applications of irradiation are limited to solid foods and there is scarcity of information regarding treatment of fruit juices. Application of UV radiation on orange, guava and pineapple juice (Keyser, Muller, Cilliers, Nel and Gouws, 2008) has been reported for the inactivation of microorganisms. Alothman, Bhat and Karim (2009) investigated the effect of UV-C treatment on total phenol, flavonoid and vitamin C content of fresh-cut honey pineapple, banana ‘pisang mas’ and guava. On average, the samples received a UV radiation dose of 2.158 J/m2. In their study, total phenol and flavonoid contents of guava and banana increased significantly with treatment time (p < 0.05). In pineapple, the increase in total phenol content was not significant (p > 0.05), but the flavonoid content increased significantly after 10 min of treatment. In contrast UV-C treatment decreased the vitamin C content of all three fruits. A separate study conducted by López-Rubira et al. (2005) demonstrated insignificant changes in anthocyanins and antioxidant activity of pomegranate arils after exposure to UV-C (0.56–13.62 kJ/m2). González-Aguilar et al. (2007) observed significant increase the antioxidant capacity of UV-C irradiated fresh cut mango during storage at 5°C for 15 days even though they observed a significant decrease in β-carotene and ascorbic acid contents.
Table 13.1 Effect of ionising irradiation on bioactive compounds of selected food and food materials. Reproduced from Alothman et al. (2009). Effects of radiation processing on phytochemicals and antioxidants in plant produce.Trends in Food Science and Technology, 20(5), 201–212. With permission from Elsevier.
Table 13.2 Effect of non ionising irradiation on bioactive compounds of selected food and food materials. Modified from Alothmanet al. (2009). Effects of radiation processing on phytochemicals and antioxidants in plant produce.Trends in Food Science and Technology, 20(5), 201–212. With permission from Elsevier.
Costa et al. (2006) observed higher retention rates for chlorophyll content of UV-C irradiated broccoli florets with improved antioxidant activity associated with total phenol and flavonoid content compared to untreated broccoli florets. Similar increases in antioxidants (total phenol and ascorbic acid) because of UV-C treatment of broccoli was observed by Lemoine, Chaves and Martínez (2010). However, they observed either an increase or no significant change in total phenol and flavonoid content during storage. Irradiation of plant tissues with UV has been shown to have positive interactions, indicating an increase in the enzymes responsible for flavonoid biosynthesis. UV irradiation of fruits is also reported to induce anthocyanin biosynthesis. For example, Kataoka and Beppu (2004) observed an increase in the anthocyanin content in peach with an increase in irradiation dose up to 7.3 W m−2. Enhancement of anthocyanin synthesis as a result of UV light is reported due to an increase in phenylalanine ammonia lyase activity which is involved in phenol synthesis in apple (Faragher and Chalmers, 1977). Similar increases in anthocyanin content in cherries (Takos, Jaffé, Jacob, Bogs, Robinson and Walker, 2006) and pears (D. Zhang, Yu, Bai, Qian, Shu, Su,et al., 2011) were reported. UV treatment of grapes during post harvest treatment is reported to produce stilbene-enriched(resveratrol and piceatannol) red wine. Cantos et al.(2003) reported an increase in resveratrol and piceatannol content of wine by 2- and 1.5-fold respectively, when compared to the control wine without affecting other key quality parameters. Similarly, Jagadeeshet al. (2011) observed higher levels of ascorbic acid and total phenolic content for UV treated mature green tomato fruit exposed to UV stored at 13 °C and 95% RH. However, they observed a significant reduction in the lycopene content of the tomatoes.
High pressure (HP) processing is employed as a potential non thermal preservation technique for microbial and enzyme inactivation while minimising effects on nutritional and quality parameters. High hydrostatic pressure (HHP) processing uses water as a medium to transmit pressures from 300 to 700 MPa to foods resulting in a reduction in microbial numbers (Meyer, Cooper, Knorr and Lelieveld, 2000) and enzyme activity (Weemaes, Ludikhuyze, Van den Broeck and Hendrickx, 1998) leading to an extension of product shelf life. However, processing conditions employed for achieving food safety may have negative effects on phytochemical content. Table 13.3 lists reported applications of HHP for various foods along with reported effects on bioactive constituents. HHP processing offers many advantages over conventional techniques and is particularly useful for producing homogeneous products, such as smoothies (Keenan, Roessle, Gormley, Butler and Brunton, 2012). This has been attributed to the instantaneous transmission of isostatic pressure to the product, independent of size, shape and food composition (Patterson, Quinn, Simpson and Gilmour, 1996). It has been shown that food processed in this way maintains its original freshness, flavour and taste, while colour changes are minimal (Dede, Alpas and Bayındırlı, 2007). Despite alternations to the structure of high-molecular-weight molecules such as proteins and carbohydrates, HHP does not typically affect smaller molecules associated with the sensory, nutritional and health promoting properties. These molecules include volatile compounds, pigments and vitamins. Therefore, HPP imparts fresh-like characteristics and preserves the nutritional value of food (Barba, Esteve and Frigola, 2011; De-Ancos, Gonzales and Cano, 2000; Ferrari, Maresca and Ciccarone, 2010; Frank, Koehler and Schuchmann, 2012; Keenan, Brunton, Gormley, Butler, Tiwari and Patras, 2010; Meyer, Cooper, Knorr and Lelieveld, 2000; Oey, Van der Plancken, Van Loey and Hendrickx, 2008; Patras, Brunton, Da Pieve, Butler and Downey, 2009; Patras, Brunton, Da Pieve and Butler, 2009; Plaza, Colina, de Ancos, Sanchez-Moreno and Cano, 2012).
High pressure processed juices have shown better retention of bioactive compounds during storage compared to thermally processed tomato juices (Hsu, Tan and Chi, 2008). Hsu et al. (2008) reported a significant increase of up to 60% for lycopene and 62% for total carotenoid content during high pressure processing (300–500 MPa/25 °C/10 min) compared to fresh and thermally processed (98 °C/15 min) tomato juice. Whereas, during storage at 25 °C for 28 days they observed no significant decrease in total carotenoid content and lycopene content in HP processed tomato juice. However, a decrease of about 18.4 and 12.5% was reported for thermally processed tomato juice.
There are many reports concerning the preservation of phytochemicals in high pressure processed food and food products. For example, Patras et al. (2009) observed a significant increase in total phenol content and a decrease in anthocyanin content of strawberry and blackberry purées at 600 MPa compared to unprocessed purée. In another study, the effects of pressure treatments of 350 MPa on orange juice carotenoids, β-carotene, ά-carotene, zeaxanthin, lutein and β-cryptoxanthin, associated with pro-vitamin A and radical-scavenging capacity values, resulted in significant increases of 20–43% in the carotenoid content of fresh orange juice (De-Ancos, Gonzales and Cano, 2000). Similarly, Plaza et al. (2012) studied the effect of high pressure processing on carotenoid content of persimmon fruit. They observed that a high pressure treatment at 200 MPa for 6 min significantly increased the extractability of carotenoids by up to 86% for astringent persimmon fruits. Anthocyanins content of raspberry (Suthanthangjai, Kajda and Zabetakis, 2005), strawberry (Zabetakis, Koulentianos, Orruno and Boyes, 2000) and blackcurrant (Kouniaki, Kajda, and Zabetakis, 2004) processed at a pressure of 800 MPa for 15 min are reported to be stable compared to unprocessed samples. Improved stability of anthocyanins at higher pressure is mainly due to inactivation of enzymes associated with the degradation of bioactive compounds. Enzymes such as polyphenoloxidase, peroxidase and β-glucosidase have been associated with the degradation of anthocyanins (Fennema and Tannenbaum, 1996). Garcia-Palazon et al. (2004) reported that the stability of strawberry and red raspberry anthocyanins namely pelargonidin-3-glucoside and pelargonidin-3-rutinoside at 800 MPa for 15 min at moderate temperatures 18–22 °C is mainly due to complete inactivation of polyphenoloxidase. However it must be noted that the effects of HHP processing parameters such as pressure, temperature, time and physicochemical properties of food have varying effects on enzymes responsible for the stability of bioactive compounds processed using HHP (Ogawa, Fukuhisa, Kubo and Fukumoto, 1990; Tiwari, O’Donnell and Cullen, 2009).
Table 13.3 Effect of HHP processing of bioactive compounds of some food and food materials
The use of pulsed electric field (PEF) as a novel pasteurisation method is especially suitable for the pasteurisation of fluid foods, in which microorganisms are inactivated by applying short (in general 1–300 µs), high electric field (10–60 kV cm-1) between two electrodes (Fox, Esveld and Boom, 2007). Because PEF processing is controlled at ambient temperature for very short treatment times of microseconds, it provides fresh-like foods which are safe and have extended shelf life (Qin, Pothakamury, Barbosa-Cánovas and Swanson, 1996). PEF has been demonstrated to be effective against various pathogenic and spoilage microorganisms and enzymes without appreciable loss of flavour, colour or bioactive compounds (Cserhalmi, Sass-Kiss, Tóth-Markus and Lechner, 2006; Elez-Martínez and Martín-Belloso, 2007; Elez-Martinez, Soliva-Fortuny and Martin-Belloso, 2009; Plaza, Sanchez-Moreno, De Ancos, Elez-Martinez, Martin-Belloso and Pilar Cano, 2011; Sanchez-Moreno, De Ancos, Plaza, Elez-Martinez and Pilar Cano, 2009) (Table 13.4). Recently (Y. I. Zhang, Gao, Zhang, Shi and Xu (2010) reported that PEF-treated (bipolar pulse 3 µs wide, at an intensity of 32 kV/cm) longan juice retained greater amounts of vitamin C and flavour compounds than thermally treated juice. Elez-Martínez and Martín-Belloso (2007) have reported that pulses applied in bipolar mode, as well as decreasing the field strength, treatment time, pulse frequency and width, led to higher levels of vitamin C retention (p < 0.05) in both orange juice and ‘gazpacho’ soup. Shivashankara, Isobe, Al-Haq, Takenaka and Shiina, (2004) studied the ascorbic acid content in Irwin mango fruits stored at 5 °C after a high electric field pre-treatment, observing that ascorbic acid decreased after 20 days of storage. To this end, all the studies indicate that vitamin C content significantly depends on the PEF treatment time and electric field strength applied during PEF-processing of the juice, so that the lower the treatment time and electric field strength, the greater the vitamin C retention. Various studies have shown the validity of PEF technology for inactivating microorganisms in more complex foods, such as a mixed orange juice and milk beverage (Rivas, Rodrigo, Company, Sampedro and Rodrigo, 2007; Rivas, Sampedro, Rodrigo, Martínez and Rodrigo, 2006; Sampedro, Geveke, Fan and Zhang, 2009; Sampedro, Rivas, Rodrigo, Martínez and Rodrigo, 2006), fruit (orange, kiwi and pineapple) juice soymilk beverage (Morales-de la Peña, Salvia-Trujillo, Rojas-Graü and Martín-Belloso, 2010) and blends of orange and carrot juice (Rivas, Rodrigo, Martinez, Barbosa-Cánovas and Rodrigo, 2006). A pre-treatment of PEF is reported to increase anthocyanin concentration in grape juice (Knorr, 2003). Corrales et al. (2008) demonstrated that PEF treatment enhances (17%) the extraction of anthocyanins compared to conventional methods and is 10% greater than HHP. However, Plaza et al. (2011) observed that PEF (35 kV/cm, 750 µs) treatment of orange juice retained similar level of carotenoids and flavanones to those of untreated juice while an increase in extractability with HP treatment (400 MPa, 40 °C, 1 min) of orange juice was observed. Similarly, Morales-de la Peña et al. (2010) investigated the effect of PEF on vitamin C in an orange, kiwi, pineapple and soymilk based drink immediately after treatment and concluded that levels were not different from the thermally processed juice. However, the beneficial effects of the PEF treatment were noticeable over a storage period of 31 days, as an 800 µs treatment at 35 kV/cm showed significantly greater retention than both a 1400 µs treatment and a thermal treatment. In general, longer exposure PEF treatment times may induce reductions in the product retention of vitamin C due to product heating.
Table 13.4 Effect of high-intensity pulsed electric field treatments on some health-related compounds in food systems. Modified from Soliva-Fortunyet al. (2009). Effects of Pulsed Electric Fields on bioactive compounds in Foods: a review.Trends in Food Science and Technology, 20, 544–556. With permission from Elsevier.
Comparative analysis of high pressure, pulsed electric field and thermally processed orange juice indicates that HP processed orange juice shows higher retention of carotenoid content (45.19%) and vitamin A (30.89%) compared to freshly squeezed orange juice (Figure 13.1a,b). Whereas, PEF and LPT (low pasteurisation temperature) processed orange juices show non-significant changes in carotenoid content and vitamin A compared to freshly squeezed orange juice (Figure 13.1a,b) (Lucia Plaza, Sanchez-Moreno, De Ancos, Elez-Martinez, Martin-Belloso and Pilar Cano, 2011). During refrigerated storage at 4 °C, HP processed orange juice showed higher retention rates for individual carotenoids and vitamin A compared to freshly squeezed, LPT and PEF processed juice (Figure 13.1c,d).
The use of ozone as a disinfecting agent has widespread application in food processing and preservation. Ozone processing of food may provide microbial food safety with several advantages over conventional disinfectant agents such as chlorine, chlorine dioxide, calcium hypochlorite, sodium chlorite, peroxyacetic acid and sodium hypochlorite. Ozone application has been reported at various post-harvest stages of fruits and vegetable processing with objectives of pathogenic and spoilage microorganism inactivation along with destruction of pesticides and other chemical residues. Both aqueous and gaseous ozone is employed for surface decontamination of whole fruits and vegetables via washing or storage in ozone-rich atmospheres (P.J. Cullen, Tiwari, O’Donnell and Muthukumarappan, 2009; P.J. Cullen, Valdramidis, Tiwari, Patil, Bourke and O’Donnell, 2010). In 2001 ozone was approved in the US as a direct additive in food products (Rice, Graham and Lowe, 2002) which triggered the application of ozone for processing of various fruit juices (P.J. Cullen, Valdramidis, Tiwari, Patil, Bourke and O’Donnell, 2010). Microbial studies to date show reductions of spoilage and pathogenic species most commonly associated with food products including fruit and vegetable juices can be achieved. However, ozone processing is reported to have significant effects on the bioactive constituents due to its strong oxidising activity. Greater impact of ozone on bioactive compounds is observed in the case of ozone processed juices compared to whole fruit and vegetables. For example, Tiwari et al. (2008) observed a 50% reduction in ascorbic acid content in orange juice within 2 min, whereas Zhang et al. (2005) reported no significant difference between ascorbic acid contents for ozonated and non-ozonated celery samples. Moreover, increase in ascorbic acid levels in spinach (Luwe, Takahama and Heber, 1993), pumpkin leaves (Ranieri, DUrso, Nali, Lorenzini and Soldatini, 1996) and strawberries (Perez, Sanz, Rios, Olias and Olias, 1999) in response to ozone exposure have also been documented. Decomposition of ascorbic acid in broccoli florets was reported after ozone treatment by Lewis et al. (1996); however, only a slight decrease in vitamin C content was reported in lettuce (Beltran, Selma, Marin and Gil, 2005). Ozone treatments have been reported to have minor effects on anthocyanin contents in strawberries (Perez, Sanz, Rios, Olias and Olias, 1999) and blackberries (Barth, Zhou, Mercier and Payne, 1995). Anthocyanin content in blackberries stored in air and at 0.1 ppm ozone was found to remain stable, however it was shown to fluctuate in 0.3 ppm ozone treated samples during storage (Barth, Zhou, Mercier and Payne, 1995).
Figure 13.1 Individual carotenoid content (µg/ 100 mL) and vitamin A (retinol activity equivalents/100 mL) of fresh orange juices (FS), high-pressure (HP: 400MPa/40 °C/1min), pulsed electric field (PEF: 35 kV.cm-1/750 µs) and low pasteurisation temperature (LPT: 70 °C/30 s) processed juice immediately after processing (a,b) and after storage for 40 days at 4 °C (c,d).
Ozonation of fruit juices rich in anthocyanins such as strawberry and blackberry juice causes a significant reduction in these pigments. A significant reduction of 98.2% in the pelargonidin-3-glucoside content of strawberry juice was reported at an ozone concentration of 7.8%w/w processed for 10 min (Tiwari, O’Donnell, Patras, Brunton and Cullen, 2009a). Reductions of >90% in the cyanidin-3-glucoside content of blackberry juice were reported under similar treatment conditions (Tiwari, O’Donnell, Patras, Brunton and Cullen, 2009a). Studies on ozonation of fresh cut honey pineapple, banana and guava indicated an increase in the total phenol and flavonoid contents of pineapple and banana, while the reverse was reported for guava (Alothman, Kaur, Fazilah, Bhat and Karim, 2010). However, significant decreases in the vitamin C content of fresh cut pineapple, banana and guava were reported. Similarly, Tzortzakis, Borland, Singleton and Barnes (2007) reported an increase in beta-carotene, lutein and lycopene contents of tomatoes stored in an ozone-enriched (1.0 µmol mol−1) atmosphere at 13 °C.
These studies indicate that the effect of ozone on phytochemicals is matrix dependent. Higher degradation is attributed to greater exposure to bioactive constituents in liquid medium compared to whole fruits where penetration of ozone is limited to surfaces. Storage of fruits and vegetables in ozone rich atmosphere is reported to preserve phenolic constituents of grapes during long-term storage and simulated retail display conditions (Artés-Hernández, Aguayo, Artes and Tomás-Barberán, 2007). Artes-Hernandez et al. (2007) observed that the application of ozone during storage increased the total flavan-3-ol content and continuous 0.1 μL L−1 O3 exposure during storage also preserved the total amount of hydroxycinnamates, while both treatments investigated the flavonol content sampled at harvest. Barboni, Cannac and Chiaramonti (2010) compared the effect of ozone rich storage and air storage over a period of seven months on the vitamin C content of kiwi fruit. Gaseous ozone concentration was 4 mg/h in the chamber at a temperature of 0 °C and a humidity of 90–95%. The authors did not observe any significant change in ascorbic acid content of kiwi fruit over a seven month storage period at an ozone concentration of 4 mg/h in the chamber (2 m3) and a storage temperature of 0 °C.
The degradation of phytochemicals including anthocyanins, phenolic compounds and ascorbic acid during ozone treatment could be due to direct reaction with ozone or indirect reactions of secondary oxidators such as •OH, HO2•, •O2− and •O3−. Such secondary oxidators may lead to electrophilic and nucleophilic reactions occurring with aromatic compounds that are substituted with an electron donor (e.g. OH−) having high electron density on the carbon compounds in ortho and para positions. Direct reaction is described by the Criegee mechanism (Criegee, 1975) where ozone molecules undergo 1–3 dipolar cyclo addition with double bonds present, leading to the formation of ozonides (1,2,4-trioxolanes) from alkenes and ozone with aldehyde or ketone oxides as decisive intermediates, all of which have finite lifetimes (Criegee, 1975). This leads to the oxidative disintegration of ozonide and formation of carbonyl compounds, while oxidative work-up leads to carboxylic acids or ketones. Ozone attacks OH radicals, preferentially to the double bonds in organic compounds leading to the formation of unstable ozonide which subsequently disintegrates. The degradation mechanism for anthocyanin based on Criegee in strawberry juice was proposed by Tiwari et al. (2009a). It is also reported that ascorbic acid degradation in the case of whole or fresh cut fruit and vegetables may also be due to the activation of ascorbate oxidase, responsible for the degradation of ascorbic acid (Alothman, Kaur, Fazilah, Bhat and Karim, 2010).
Ultrasound processing has emerged as an alternative non thermal food processing option to conventional thermal approaches for pasteurisation and sterilisation of food products (O’Donnell, Tiwari, Bourke and Cullen, 2010). Power ultrasound has shown promise as an alternative technology to thermal treatment for food processing (Mason, Riera, Vercet and Lopez-Bueza, 2005) and has been identified as a potential technology to meet the US Food and Drug Administration (USFDA) requirement of a five log reduction of E. coli in fruit juices (Tiwari and Mason, 2012). It has been reported to be effective against food-borne pathogens found in a range of juices, including orange juice (Valero, Recrosio, Saura, Munoz, Martí and Lizama, 2007) and guava juice (Cheng, Soh, Liew and Teh, 2007).
Ultrasound processing on its own or in combination with heat and/or pressure is an effective processing tool for microbial inactivation and phytochemical retention. However the literature indicates that it can negatively modify some food properties including flavour, colour or nutritional value. Ultrasound treatment of liquid foods in general has a minimal effect on the bioactive compounds during processing and results in improved stability during storage when compared to thermal treatment. Rawson et al. (2011) investigated the effect of thermosonication on the bioactive compounds of freshly squeezed watermelon juice. They observed a higher retention of ascorbic acid and lycopene at low amplitude levels and temperatures. They also observed a slight increase in lycopene at low amplitude level. Similarly, Tiwari, O’Donnell, Patras and Cullen (2008) reported a slight increase (1–2%) in the pelargonidin-3-glucoside content of sonicated strawberry juice at lower amplitude levels and treatment times, which may be due to the extraction of bound anthocyanins from the suspended pulp. Whereas at higher amplitude levels and treatment times a maximum of 5% anthocyanin degradation was reported. Cheng, Soh, Liew and Teh, (2007) reported a significant increase in the ascorbic acid content of Guava juice during sonication from 110 ± 0.5 (fresh) to 119 ± 0.8 (sonication) and to 125 ± 1.1 (combined sonication and carbonation) mg/100 mL, which could be due to cavitation effects caused by carbonation and sonication, respectively. Cheng et al. (2007) also observed that during carbonation, sample temperature decreased substantially which could have disfavoured ascorbic acid degradation. Similarly, Bhat et al. (2011) observed a significant increase in the bioactive constituents of sonicated kasturi lime (Citrus microcarpa) in a sonication tub at a frequency of 25 kHz. They observed an increase of about 6.7% for ascorbic acid, 27.4% for total phenolics, 42.3% for total flavonoids and 127.4% for total flavanols at 60 min. Low power sonication tends to increase the level of bioactive compounds in sonicated food materials due to enhanced extraction of bound pigments as a result of cell wall disruption. In some cases sonication treatment also enhances the antioxidant activity of treated samples. This is attributed to the addition of sonochemically generated hydroxyl radicals (OH-) to the aromatic ring of the phenolic compounds at the ortho- or para-positions of phenolic compounds (Ashokkumar, Sunartio, Kentish, Mawson, Simons, Vilkhu, et al., 2008).
Figure 13.2 Percentage increase in the level of bioactive compound over control (untreated) due to sonication at 25 kHz.
Source: adapted from Bhat et al. (2012).
Weak ultrasonic irradiation was reported to promote an increase in the amount of phenolic compounds found in red wine (Masuzawa, Ohdaira and Ide, 2000). Literature also indicates ultrasound processing enhances extraction of phenolic and other bioactive compounds from grape must or wine (Cocito, Gaetano and Delfini, 1995). Ultrasound assisted extraction of bioactive compounds and anthocyanins were recently reviewed by Vilkhu, et al. (2008). Zhao et al. (2006) reported a degradation of (all-E)-astaxanthin into unidentified colourless molecule(s) during extraction using sonication with increased power levels and treatment times. Similarly, the degradation of the pelargonidin-3-glucoside content of strawberry juice (Tiwari, O’Donnell, Patras and Cullen, 2008) and the cyanidin-3-glucoside content of blackberry juice (Tiwari, O’Donnell, Muthukumarappan and Cullen, 2009a) was found during sonication. Figure 13.2 shows the effect of sonication on retention of blackberry and strawberry juice anthocyanins during ultrasonic processing. As can be seen from this figure, the degradation of these anthocyanins is minimal, with a retention rate of over 98%. The reported degradation of anthocyanins is mainly due to cavitation, which involves the formation, growth and rapid collapse of microscopic bubbles. The degradation of quality and nutritional parameters results from the extreme physical conditions which occur within the bubbles during cavitational collapse at micro-scale (Suslick, 1988) and several sonochemical reactions occurring simultaneously or in isolation. The chemical effects produced by cavitation generate high local temperatures (up to 5000 K), pressures (up to 500 MPa) and mechanical action between solid and liquid interfaces (Suslick, Hammerton and Cline, 1986). The anthocyanin degradation could also be due to the presence of other compounds such as ascorbic acid and can be related to oxidation reactions, promoted by the interaction of free radicals formed during sonication (Portenlänger and Heusinger, 1992). Hydroxyl radicals produced by cavitation are involved in the degradation of anthocyanins by opening of rings and formation of chalcone mainly due to temperature rises occurring during sonication (Sadilova, Carle and Stintzing, 2007). The interaction of ascorbic acid with anthocyanin pigments results in mutual degradation (Markakis, Livingston and Fellers, 1957). This is also reported for strawberry juice (Tiwari, O’Donnell, Patras, Brunton and Cullen, 2009b).
Ultrasound treatment of fruit juices is reported to have a minimal effect on the ascorbic acid content during processing and results in improved stability during storage when compared to thermal treatment. This positive effect of ultrasound is assumed to be due to the effective removal of occluded oxygen from the juice (Knorr, Zenker, Heinz and Lee, 2004), a critical parameter influencing the stability of ascorbic acid (Solomon, Svanberg and Sahlström, 1995, 2009b). Tiwari et al. (2009b) reported a maximum degradation of 5% in the ascorbic acid content of orange juice when sonicated at the highest acoustic energy density (0.81 W/mL) and treatment time (10 min). During storage at 10 °C sonicated juice was found to have a higher retention of ascorbic acid compared to thermally processed and control samples. However, for sonicated strawberry juice, a higher reduction of ca. 15% was found. Ascorbic acid degradation during sonication may be due to free radical formation (Portenlänger and Heusinger, 1992). Hydroxyl radical formation is found to increase with degassing. Sonication cavities can be filled with water vapour and gases such as O2 and N2 dissolved in the juice (Korn, Machado Primo and Santos de Sousa, 2002). The interactions between free radicals and ascorbic acid may occur at the gas–liquid interface. In summary, ascorbic acid degradation may follow one or both of the following pathways:
Ascorbic acid → thermolysis (inside bubbles) and triggering of Maillard reaction.
Ascorbic acid → reaction with OH– → HC–OH and production of oxidative products on the bubble surface.
Thus, sonication can be related to advanced oxidative processes since both pathways are associated with the production and use of hydroxyl radicals. The cavitation bubble is mainly responsible for the degradation of volatile organic compounds due to the production of hydroxyl radicals and subsequently reacts with organic compounds in the water shell around the bubble (Petrier, Combet and Mason, 2007).
Supercritical or dense phase carbon dioxide processing is a collective term for liquid CO2 and supercritical CO2 or high pressurised carbon dioxide (HPCD). It is a non-thermal alternative to heat pasteurisation for liquid foods and it is attracting much interest in the food industry (Del Pozo-Insfran, Balaban and Talcott, 2006). SCCO2 extraction has been extensively applied in the fruit and vegetable industry for the extraction of different phytochemicals with desired functionalities (antioxidants, anti-depressants, antimicrobial etc.). Recent studies highlighting the presence of health promoting compounds in fruit and vegetables have stimulated the demand for process technologies capable of extracting such compounds in an environment friendly manner. Apart from extraction of bioactive compounds from fruit and vegetables, SCCO2 has unique properties that make it an appealing medium for food preservation. SCCO2 has strong potential as an antimicrobial agent as it is non-toxic and easily removed by simple depressurisation and out gassing. SCCO2 has significant lethal effects on microorganisms in food and inactivates spoilage enzymes with a minimal effect on end product quality (Damar and Balaban, 2006; Kincal, Hill, Balaban, Portier, Sims, Wei, et al., 2006; Liu, Gao, Peng, Yang, Xu and Zhao, 2008). Plaza (2011) observed no significant change in total phenolic content of guava puree processed using dense phase carbon dioxide (30.6 MPa, 8% CO2 and 6.8 min, 35 °C). Ferrentino et al. (2009) investigated the effect of continuous dense phase carbon dioxide (DPCD) on red grapefruit juice. The authors used pressures of 13.8, 24.1 and 34.5 MPa and residence times of 5, 7 and 9 min as variables at constant temperature (40 ºC), and CO2 level (5.7%). A storage study was performed on the fresh juice and DPCD treated at these conditions. The treatment and the storage did not affect the total phenolic content of the juice. Slight differences were detected for the ascorbic acid content and the antioxidant capacity. The experimental results showed that the treatment can maintain the antioxidant content of grape juice. Application of SCCO2 has been reported for various food products including fruit juices such as apple cider (Gasperi, Aprea, Biasioli, Carlin, Endrizzi, Pirretti etal., 2009; Gunes, Blum and Hotchkiss, 2006; Liao, Hu, Liao, Chen and Wu, 2007); orange juice (Kincal et al., 2006); grapefruit juice (Ferrentino, Plaza, Ramirez-Rodrigues, Ferrari and Balaban, 2009); and grape juice (Gunes, Blum and Hotchkiss, 2006). These studies indicated minimal changes in key quality parameters. In a study conducted by Del Pozo-Insfran et al. (2006) no significant changes in total anthocyanin content was reported for DPCD processed muscadine grape juice compared to a 16% loss observed in thermally processed juice. Enhanced anthocyanin stability was also observed in DPCD processed juice during storage for ten weeks at 4 °C. The greater stability of DPCD processed juice could be due to the prevention of oxidation by removal of dissolved oxygen. The exact mechanism for anthocyanin stability is difficult to establish. However, Del Pozo-Insfran et al. (2007) demonstrated that anthocyanin stability is also dependent on the PPO inactivation potential of DPCD treatment and governed by extrinsic control parameters of pressure and CO2 concentration gradient. Parton et al. (2007) tested a continuous SCCO2 system for liquid foods ranging from orange juice to tomato paste. They reported that the low temperatures used during SCCO2 processing often resulted in improved retention of heat sensitive nutritionally important compounds.
Ensuring food safety and at the same time meeting the demand for nutritious foods, has resulted in increased interest in non-thermal preservation techniques. Research to date indicates that non-thermal techniques have the potential to enhance the retention of bioactive compounds without compromising food safety. In most cases storage conditions for the processed product play an important role in stability of bioactive compounds. Selection of appropriate extrinsic storage conditions for the processed product are necessary to retain optimum levels of bioactive compounds in food. A key issue for the industrial adoption of these non-thermal techniques is process optimisation. There is a need for focused studies on the stability of bioactive compounds using combined approaches. Combinations of various natural plant extracts or antimicrobials agents in response to consumer demand for ‘greener’ additives can be explored further to provide improved stability of phytochemicals through copigmentation while at the same time providing synergy for microbial inactivation. Combinations of thermal and non-thermal techniques such as thermosonication, manosonication, manothermosonication and pressure assisted thermal sterilisation have great potential in improving retention of bioactive compounds in food and food products. The impact of product formulation, extrinsic storage parameters and intrinsic product parameters on the efficacy of novel applications of combined non-thermal systems also requires further study. Overall, studies have shown enhanced stability of anthocyanins by novel non-thermal preservation techniques such as HHP, PEF, DPCD, irradiation and ultrasound.
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