Minimal processing can be defined as fresh produce that has been sliced, shredded, diced or peeled before packaging, and differs from other processing methods as heating is not involved. Therefore plant tissues remain viable, albeit in many cases in a wounded state (Barry-Ryan and O’Beirne, 1999). Market research has indicated that consumers widely perceive fresh produce as more nutritious than processed (Richards, 2003), and minimally processed fruits and vegetables are therefore considered more desirable. In many instances this perception is true as several nutrient phytochemicals in plants are destroyed or reduced during processing (Jones et al., 2006). However, it is also recognised that nutritional value and phytochemical content of plant tissue is usually reduced during normal aging and senescence after harvest (Kays and Paull, 2004). Less well understood is the impact of size reduction on the fate of phytochemicals in plant tissue. The aim of this chapter is to review the impact of minimal processing on the phytochemical content of the most commonly consumed fresh-cut product: leafy vegetables in salad mixes. For the purposes of this chapter, phytochemicals for human health can be defined as ‘non-nutrient chemicals found in plants that have biological activity against chronic diseases’ (Kushad et al., 2003). In addition, we will focus on ascorbic acid, as this compound makes a major contribution, along with phenolics, to the antioxidant capacity (as measured in vitro by ORAC) in leafy vegetables.
In plants, phytochemicals serve a wide range of functions including pigmentation (anthocyanins, lycopene), pest and disease defence (glucosinolates, cysteine sulfoxides), and prevention of UV light-induced oxidative stress (flavonols) (Kays and Paull, 2004). Phytochemicals have been linked to many positive health effects in humans including some cancers, coronary heart disease, diabetes, high blood pressure, inflammation, infection, psychotic diseases, ulcers and macular degeneration (Le Marchand, 2002; Nijveldt et al., 2001; Steinmetz and Potter, 1996). Many phytochemical types, such as polyphenolics, carotenoids and organosulphur compounds, are thought to be involved in this protection and they may act synergistically, or have different modes of action (Chen et al., 2007). Over the past 20 years there has been an increased interest in phytochemicals for human health and the potential economic advantages of creating novel food products based on elevated levels of health-promoting phytochemicals. There has also been an increased stimulus to re-examine the post-harvest practices for fruits and vegetables, in order to ascertain whether conditions that result in the preservation of visual and organoleptic parameters also impact on the preservation of phytochemicals. A range of minimally processed products are available in the market. Hence this chapter will focus on the effects of minimal processing procedures on the post-harvest fate of phytochemicals and ascorbic acid contained in common ingredients of salad mixes.
It has been estimated that up to 80% of the minimally processed market in the US is made up of salad mixes (Cook, 2004). A wide variety of leafy vegetables are used in salad mixes (Table 10.1), with a similar wide variation in phytochemical content, ascorbic acid and antioxidant capacity. Commonly consumed lettuce varieties, such as oakleaf and coral, are high in phenolic compounds and are also a source of carotenoids such as lutein and zeaxanthin (Wilson et al., 2004). Red lettuce, for example, can be up to five times higher in antioxidant capacity than similar green varieties (Wilson et al., 2004), due mainly to higher levels of anthocyanins and other phenolics. Spinach, another common ingredient, is relatively high in ascorbic acid (Yadav and Sehgal, 1995). Other ingredients, being from the Brassicaceae, contain glucosinolates. Water cress contains high levels of carotenoids and the glucosinolate nasturtiin (Cruz et al., 2009), rocket contains high quercetin, kaempferol and isorhamnetin content, and the glucosinolate glucoerucin (Jin et al., 2009), while mizuna and mibuna are high in phenolics (Martinez-Sanchez et al., 2008). With such a wide range of different ingredients at hand it is theoretically possible to produce a number of salad mixes with specific health benefits. Apart from salad mixes, ready to use products such as carrot batons, diced onions, soup mixes and fruit salads are now readily available. The demand for these products derives from their convenience as consumers are spending less and less time preparing fruits and vegetables prior to consumption.
Table 10.1 Common salad mix ingredients used in USA and Australian markets
| Common name/s | Botanical name |
| Lettuce – oakleaf | Lactuca sativa |
| Lettuce – coral | Lactuca sativa |
| Lettuce – frisee | see endive |
| Lettuce – Batavia | Lactuca sativa |
| Lettuce – Cos | Lactuca sativa |
| Lettuce – Iceberg | Lactuca sativa |
| Lettuce – Butter | Lactuca sativa |
| Spinach | Spinacia oleracea |
| Chard/Beet | Beta vulgaris |
| Endive/Frisee (chicory) | Chicorium endiva |
| Radicchio | Chicorium intybus |
| Mustard | Brassica juncea or Sinapis alba |
| Pea tendrils | Pisium sativum |
| Mizuna | Brassica rapa v. nipponsicia and japonica |
| Mibuna | Brassica rapa v. nipponsicia and japonica |
| Tatsoi | Brassica rapac v. group Taatsai |
| Pak choi | Brassica rapac v. group Pak choi |
| Rocket – wild | Diplotaxis tenuifolia |
| Rocket – Arugula | Eruca vesicaria v. sativa |
| Deltona | Lactuca sativa |
| Kale | Brassica oleracea v.sabellica |
Antioxidant capacity in lettuce leaves is derived primarily from phenolic compounds and ascorbic acid (Reyes et al., 2007), so induction of the phenylpropanoid pathway that synthesises phenolics by cutting or shredding should increase antioxidant content. This was found to be the case in ‘Iceberg’ leaves, where cutting caused an increase in Phenylalanine Ammonia Lyase (PAL) activity of approximately ten-fold, and a concomitant increase in phenolic content and antioxidant activity (Reyes et al., 2007). Ascorbic acid content declined, however, and the authors hypothesised that the inherent low ascorbic acid content in Iceberg is used up quickly after wounding and is unavailable to deal with the increase in Reactive Oxygen Species (ROS). Phenolics are therefore rapidly synthesised to partially control this wound-induced increase in ROS (Reyes et al., 2007).
This situation is not seen in all cases and cutting lettuce tissues can have a variety of effects. Phenolic compounds increased significantly in the mid-rib tissues of Iceberg lettuce, contributing to browning symptoms (Ke and Saltveit, 1989). Similarly, excised lettuce leaf discs showed an increase in phenolics if they were from the mid-rib region (Tomas-Barberan et al., 1997a). However there was little effect of cutting found on caffeic acid derivatives or flavonols in red or green lettuce tissues, while anthocyanins declined significantly. Similarly, cyanidin glycosides declined after shredding and 48 h storage at 22 °C in the red ‘Lollo Rosso’ and ‘Red Oak’ lettuce leaves (DuPont et al., 2000). However anthocyanins can also increase during minimal processing as the tissue is still alive and able to continue to synthesise these compounds. Cutting red lettuce leaves induced cyanidin glycoside production in the midrib during the first seven days of storage (Ferreres et al., 1997).
Gil et al. (1998) found soluble phenolic compounds doubled in the mid-rib tissues of the red lettuce Lollo Rosso after wounding and storage in air at 5°C which contributed to enhanced browning. Cutting Lamb’s lettuce leaves, however, caused a decrease in both phenolic content and ascorbic acid (Ferrante et al., 2009). Shredding also resulted in significant losses of flavonoids in a range of lettuces, with losses varying from 6% for Lollo Rosso to 94% for green oak after 48 h at 22 °C (DuPont et al., 2000). There was also a significant decline in flavonoid compounds in endives after shredding (DuPont et al., 2000). The content of many phenolic compounds in fruit and vegetables can decline significantly during processing (Tomas-Barberan and Espin, 2001). This is most likely due to phenolic leaching during washing caused by the significant tissue damage shredding entails, compared with minimal cutting.
Some common salad mix ingredients (e.g. rocket, mibuna, mizuna; Table 10.1) are members of the Brassicaceae family and as such, contain glucosinolates. Any processing step that involved cutting, chopping or disruption of cellular integrity caused a loss of total glucosinolates, as this resulted in the mixture of glucosinolates with the enzyme myrosinase (Jones et al., 2006). There is little published information on the effects of cutting on glucosinolate content in leafy Brassicas, but studies in broccoli offer some clues. After chopping and storage of both broccoli and cabbage at room temperature (approximately 20 °C) there were significant reductions in aliphatic glucosinolates (e.g. glucoraphanin) but an increase in some indole glucosinolates, such as glucobrassicin (Verkerk et al., 2001). As leafy Brassicas are washed thoroughly after cutting it is reasonable to assume some reduction in glucosinolate content will occur due to leaching, but the extent will depend on degree of tissue damage. Proper temperature management after cutting should minimise glucosinolate reduction thereafter (Jones et al., 2006).
Ascorbic acid generally declined rapidly after harvest and during processing (Lee and Kader, 2000). Cutting method can also significantly impact on rate of ascorbic acid loss. Manual tearing of Iceberg lettuce leaves resulted in better ascorbic acid retention than machine cutting, while blunt blades used in cutters resulted in greater loss of ascorbic acid than if sharp stainless steel blades were used (Barry-Ryan and O’Beirne, 1999). This is likely due to less cellular damage caused by sharp blades resulting in lower ascorbic acid leakage and enzymatic degradation due to loss of cellular compartmentation.
There are no known reports on the effect of cutting on carotenoids in leafy vegetables. Carotenoids, such as lutein and zeaxanthin, are inherently more stable than phenolics in the post-harvest environment and are not subject to leaching as they are hydrophobic (Jones et al., 2006). It is therefore reasonable to assume content would not be significantly affected by cutting, but more work is required in this area.
Cutting fresh leafy produce induces a wound response in tissues that has a wide range of effects, including increased respiration (Martinez-Sanchez et al., 2008) and ethylene synthesis, and activation of the phenylpropanoid pathway (Saltveit, 2000b) that can result in increased phenolic synthesis, and resultant antioxidant capacity. This wound response is an integral part of healing in plants as it results in elevated production of compounds that are involved in wound repair and defence against pathogens, specifically lignin and suberin (Hawkins and Boudet, 1996). The production of these, and other, compounds results in lignification, which is ubiquitous in all plants (Dyer et al., 1989). Of particular interest to this review is that lignin is synthesised via the phenylpropanoid pathway via initiation of Phenylalanine Ammonia-Lyase (PAL; EC 4.3.1.5; Dyer et al., 1989), which also results in increased phenolic synthesis and antioxidant capacity. Figure 10.1 represents a simplified schematic of the phenylpropanoid pathway, showing how initiation of PAL can result in lignin production (from 4-Coumarate), and phenolic accumulation. The phenolic compounds quercetin, kaempferol, isorhamnetin and anthocyanins are all commonly found in leafy vegetables and contribute significantly to antioxidant capacity (Rochfort et al., 2006). Wounding is also thought to increase phenylalanine synthesis by stimulation of the shikimate pathway, so it would appear the response involves both initiation of PAL and increased production of the amino acid this enzyme acts upon (Figure 10.1; Dyer et al., 1989).
Figure 10.1 Simplified schematic diagram illustrating the relationship between the wound response in plant tissues and the shikimate and phenylpropanoid pathways.
Wounding initiates phenylalanine induction via the shikimate pathway, and enhances PAL activity, which, in turn, results in increased lignin production (from 4-Coumarate) and production of a range of phenolic compounds with antioxidant activity (in bold).
Activation of the phenylpropanoid pathway by wounding in lettuce leaves is primarily via induction of PAL (Dixon and Paiva, 1995; Tomas-Barberan et al., 1997b), leading to an increase in soluble phenolic compounds. Wounded Iceberg lettuce tissue showed a 6–12-fold increase in PAL within 24 h of wounding, while phenolic content rose within 48 h (Saltveit, 2000a). In Iceberg, Romaine and Butterleaf lettuces caffeic acid derivatives were the major phenolics induced by wounding (Tomas-Barberan et al., 1997a). These phenolic compounds are then thought to be readily oxidised by Polyphenol Oxidase (PPO), which leads to visual browning (Ke and Saltveit, 1989), but cutting had no effect on either PPO or peroxidase (POD) activities in a range of lettuce types (Degl’Innocenti et al., 2007), indicating that endogenous activity was sufficient to produce browning once increased PAL activity resulted in enhanced substrate content. In addition wounding caused cellular decompartmentalisation that allowed mixing of phenolic compounds at the cut surfaces with PPO (Tomas-Barberan et al., 1997b).
The wounding mechanism appears to be similar between lettuce types, but differences in susceptibility to browning could be caused by changes in enzymatic activity that lead to increases in phenolics, i.e. PAL activity (Saltveit, 2000a). Although wounding leaf tissue also leads to a transient increase in ethylene, it is not thought this is responsible for the increase in phenolics (Ke and Saltveit, 1989; Tomas-Barberan et al., 1997a).
Reactive oxygen species (ROS) are produced during senescence and wounding and may act as messengers during episodes of stress (Desikan et al., 2001). Hydrogen peroxide, for example, acted as a secondary messenger in tomato leaves after wounding by activating defence genes (Orozco-Cardenas et al., 2001). However, hydrogen peroxide is also known to oxidise phenols, producing browning in lettuce leaves, and this oxidation is catalysed by PPO and POD (Degl’Innocenti et al., 2007). Hence wounding can cause an increased incidence of browning in a range of salad mix ingredients.
There are two key steps to controlling the wound response in leaf tissues: inhibiting the induction of the wound signal; and/or direct inhibition of PAL activity (Saltveit, 2000a). The nature of the wound signalling compound(s) is not known, but ethylene, jasmonic acid, salicylic acid and ascorbic acid are known not to be involved (Saltveit, 2000a). PAL inhibitors, however, are well known and are dealt with in the section on browning inhibition.
As the wound response in plant tissue results in both the production of lignin and phenolic compounds via the phenylpropanoid pathway, fresh cut salad mixes would be expected to be higher in antioxidant capacity than intact leaves. However, this is not certain, as there are no known studies comparing antioxidant capacity in fresh cut and intact lettuce leaves, and the wound response is transitory. Not all lettuce mixes, contain tissues that are wounded other than during harvest. The popular ‘Baby leaf’ mixes are an example of this where all ingredients used are small and immature and therefore do not need size reduction through cutting after harvest. These ingredients behave differently to cut or shredded leaves as a) the phenylproponoid pathway may not be as highly induced, and b) there are few cut surfaces for antioxidant compounds (phenolics, ascorbic acid) to leach from during processing. More work is required to determine the antioxidant capacity of minimally processed mixes compared with intact leaves.
The process of peeling minimally processed ready to eat products also constitutes a unit operation which can induce a wound response. For example, industrially produced ready to use carrot disks are peeled using mechanical abrasion, which induces a wound response and thus phytochemical content. Initially, machine peeling resulted in a greater accumulation of phenolic compounds and increased total antioxidant activity compared to that of hand peeled carrot disks (Kenny and O’Beirne, 2010). However these higher levels were not maintained during storage.
Not all salad mix ingredients exhibit browning symptoms after cutting. While radicchio (also called chicory or escarole) and lettuce showed extensive browning during storage, symptoms were not seen until the end of the storage period in rocket leaves (Degl’Innocenti et al., 2007). Red lettuce varieties also tend to be more resistant to browning symptoms (Degl’Innocenti et al., 2005). This resistance to browning is thought to be related to endogenous ascorbic acid content in the tissue, and is therefore also related to the ‘health status’ of lettuces. A green lettuce susceptible to browning showed a rapid loss in ascorbic acid during 72 h storage at 4 °C, while ascorbic acid increased in a resistant red variety stored under the same conditions (Degl’Innocenti et al., 2005). The authors hypothesised that endogenous ascorbic acid had a protective effect against browning in lettuce leaves, and the rapid loss of ascorbic acid in the green variety removed that protection, while in the resistant red variety ascorbic acid content increased, conferring resistance (Degl’Innocenti et al., 2005). Exogenous ascorbic acid is known to inhibit PPO and browning in plants (Alscher et al., 1997), and protected rocket leaves from browning by inhibiting PPO activity by reducing cytostolic pH (Degl’Innocenti et al., 2007). It is possible that browning resistant leaves in general may be higher in ascorbic acid and, therefore, have a higher inherent antioxidant capacity.
Inhibition of browning in leaf tissue can be achieved by slowing PAL enzyme activity with cold temperatures (<4°C), low O2/high CO2 atmospheres, or the application of exogenous inhibitors (Saltveit, 2000a). Cycloheximide is particularly effective at inhibiting PAL action, but cannot be used commercially (Saltveit, 2000a). Other, less toxic, post-harvest PAL inhibitors have been identified. For example, browning in lettuce tissue was inhibited by CaCl2, acetic acid or 2,4-D, with all three treatments significantly depressing PAL activity (Tomas-Barberan et al., 1997b). Heat shock is an intriguing post-harvest browning inhibition treatment that can be very effective in lettuce tissues (Saltveit, 2000a). A short heat treatment (e.g. 90 second at 45 °C) effectively inhibited browning in Iceberg lettuce as the tissue preferentially synthesised heat shock proteins over PAL (Saltveit, 2000a). The wound signal appeared to dissipate before cells recovered from the heat shock, so PAL activity did not increase. All treatments that result in reduced PAL activity and/or inhibit the subsequent rise in phenolic content will also reduce antioxidant activity in lettuce tissues, leading to an inherent disparity between high antioxidant content and poor visual quality.
Phenolic compounds and antioxidant capacity are generally stable during cool storage (<4 °C) of most leafy vegetables provided proper cool temperature control is adhered to (Jones et al., 2006), but performance during storage is variable depending on degree of cutting, variety and species. Storage of whole lettuce heads for 16 days at 4 °C induced an increase in total phenolics (Zhao et al., 2007), but lettuce flavonol glycosides declined 7–46% during seven days at 1 °C, and the rate of decline was cultivar dependent (DuPont et al., 2000). Total flavonoids in intact spinach leaves did not change during storage at 4 °C for seven days, but increased after three days in cut leaves (Bottino et al., 2009).
Results are also variable when cut leaves are cool stored. For example, 4 °C storage for 72 h maintained antioxidant capacity and phenolic content in fresh-cut radicchio, but both increased transiently in lettuce and rocket leaves (Degl’Innocenti et al., 2008). Similarly, total flavonoid content did not change in cut spinach leaves after three or seven days storage in air at 10 °C (Gil et al., 1999), but antioxidant capacity declined, due to a decrease in ascorbic acid. Total phenolics and antioxidant capacity both increased in cut Iceberg and Romaine lettuce leaves after 48 h storage at 10 °C (Kang and Saltveit, 2002), and a linear correlation was seen between phenolic content and antioxidant capacity as measured by FRAP and DPPH. Cut Lamb’s lettuce stored at 4 °C for eight days also showed an increase in total phenolics, including anthocyanins, while carotenoids declined (Ferrante et al., 2009). In the lettuce variety Lollo Rosso, results were dependent on tissue colour (Ferreres et al., 1997). Storage at 5 °C for 7–14 days after cutting caused an increase in phenolics and anthocyanins in mid-ribs, but no changes were observed in phenolics in green or red tissues, while anthocyanins declined (Ferreres et al., 1997).
Little is known of the behaviour of carotenoids or glucosinolates contained in leafy vegetables during cool storage. Bunea et al. (2008) analysed phenolic and carotenoid compounds after cool storage or freezing of cut spinach and reported that while phenolics declined by approximately 20% during storage at 4 °C or −18 °C, only one day at 4 °C was sufficient to cause a 48% decline in violaxanthin content and a 40% loss of beta-carotene. Storage of shredded kale at 7–9 °C for five days caused a significant decrease in both beta-carotene and lutein (de Azevedo and Rodriguez-Amaya, 2005). It is therefore reasonable to assume that carotenoids in leafy vegetables decline significantly during cool storage. The behaviour of glucosinolates in leafy Brassicas during cool storage is also not clear. Total glucosinolates in cut rocket leaves increased after three days storage at either 4 °C or 15 °C (Kim and Ishii, 2007), but it is not known what effect cooling has on individual glucosinolates. In broccoli florets glucoraphanin content declined by 82% after five days at 20 °C, but by only 31% at 4 °C (Rodrigues and Rosa, 1999). Similarly, Rangkadilok et al. (2002) reported a 50% decrease in glucoraphanin in ‘Marathon’ heads after seven days at 20 oC, but no decrease after seven days at 4 °C. Indole glucosinolates, however, increased in concentration during nine days storage at 10 °C in ‘Marathon’ florets (Hansen et al., 1995), and total glucosinolates did not change significantly, indicating that the rise in indole glucosinolates may have masked any decline in alkenyl forms such as glucoraphanin. It is likely, therefore, that glucosinolates in leafy Brassicas decline during cool storage but the rate of loss is inhibited by temperatures ≤4 °C.
There is a marked tendency for ascorbic acid to decline during post-harvest storage, with lower temperatures acting to alleviate degradation (Lee and Kader, 2000). Ascorbic acid degraded between 2.7 and 2.9 times faster in lettuce leaves held at 8 °C or 15 °C, compared with 0 °C (Moreira et al., 2006). Levels were also better retained in cut rocket leaves stored at 4 °C compared with 15 °C (Kim and Ishii, 2007). Ascorbic acid in spinach leaves declined significantly when stored for 72 h at 4 °C, while total flavonoids did not change (Bottino et al., 2009). Despite the null effect of storage on total flavonoids, antioxidant capacity, as measured by FRAP, declined, reflecting the decline in ascorbic acid.
Once minimally processed salad lines are cut and washed, they are most commonly packed in bags capable of modifying gas atmospheres. Modified Atmosphere Packaging (MAP) occurs when actively respiring produce is placed in sealed plastic bags with differential permeability that results in relatively low O2 (<2%) and high CO2 (>10%) atmospheres (Kays and Paull, 2004). These conditions are thought to generally result in greater antioxidant capacity retention in fruits and vegetables (Kalt, 2005), but this does not appear to be the case in salad mix ingredients. An atmosphere of 2–3% O2 and 12–14% CO2 inhibited the increase in phenolics in lettuce Lollo Rosso mid-rib tissue that was seen when leaves were stored in air, and also inhibited subsequent browning (Gil et al., 1998). A significant decline in phenolics was recorded in MAP-stored green and red lettuce tissues, particularly in red tissue which exhibited increased losses of anthocyanins and total phenolics compared with air storage, indicating that MAP was effective in preventing browning but may have adverse effects on phytochemical retention (Gil et al., 1998). Total phenolic content in spinach leaves did not change when stored in either air or MAP for seven days at 10 °C (Gil et al., 1999). Antioxidant capacity declined during MAP, however, partly due to a marked decrease in ascorbic acid content (Gil et al., 1999). However, storage under low O2/high CO2 atmospheres for eight days caused marked declines in both flavonoids and glucosinolates contained in rocket leaves (Martinez-Sanchez et al., 2006). In contrast, ascorbic acid was retained in rocket leaves stored for eight days in either air or low O2/high CO2 atmospheres (Martinez-Sanchez et al., 2006). Flushing with N2 resulted in better ascorbic acid retention in Iceberg lettuce than low O2/high CO2 atmospheres (Barry-Ryan and O’Beirne, 1999).
Little is known of the effect of MAP on glucosinolate content in leafy Brassicas, but reports on broccoli florets offer some direction. When broccoli heads were stored at 4 °C there was no difference in the glucoraphanin levels between air and MAP after ten days storage (Rangkadilok et al., 2002). At 20 °C, however, broccoli stored in air lost 50% of its glucoraphanin in seven days, while under MAP there was no significant decrease in glucoraphanin over ten days (Rangkadilok et al., 2002). In comparison with the glucosinolate content of freshly harvested broccoli, glucoraphanin content of Marathon broccoli heads stored for seven days at 1 °C under MAP decreased by approximately 48% (Vallejo et al., 2003). A further 17% was lost after three days at 15 oC. If temperatures rise above 4 °C, as they commonly do in the retail environment, then both atmospheres and RH are important factors in maintaining glucosinolate levels in Brassicas. At higher temperatures, CA studies show that O2 levels below 1.5% and CO2 above 6% maintained or improved glucosinolate levels (Hansen et al., 1995; Rangkadilok et al., 2002). We can conclude, therefore, that MAP may be useful in maintaining glucosinolate content after harvest of leafy Brassicas, providing that the atmospheres reached and/or RH achieved were sufficient to have prevented membrane degradation and subsequent mixing of glucosinolates with myrosinase. Based on these studies, we conclude that MAP alone is insufficient to adequately maintain phytochemical and ascorbic acid content in minimally processed salad ingredients, and proper temperature management is of primary importance.
Minimal processing can have a marked effect on both phytochemical and ascorbic acid content in salad mixes. Cutting and shredding commonly induce a wound response in leafy tissues that results in induction of the phenylpropanoid pathway via PAL, and a concomitant increase in phenolic compounds that leads to higher in vitro antioxidant capacity. However, cutting and shredding also lead to a loss of visual quality by enhanced browning and antioxidant loss via leaching during the washing process. Refrigerated storage maintains antioxidant capacity in salad ingredients, providing temperatures are kept at 4 °C or lower. Modified atmosphere packaging reduced antioxidant capacity compared with leaves stored in air. In most commercial salad mixes available today, the use of baby leaves which undergo minimal cutting, refrigeration during marketing and MA packs adequately maintain ascorbic acid content, but may result in lower antioxidant content compared with whole lettuce heads of the same variety and age. More research is required to clarify this.
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* This chapter is a publication funded by Vital Vegetables, a Trans Tasman research project jointly supported by Horticulture Australia Ltd, New Zealand Institute for Crop and Food Research Ltd, the New Zealand Foundation for Research Science and Technology, the Australian Vegetable and Potato Growers Federation Inc, New Zealand Vegetable and Potato Growers Federation Inc and the Victorian Department of Primary Industries.