Changes in nitrate and nitrite concentrations over 24 h for sweet basil and scallions
Highlights
► Basil nitrate and nitrite concentrations varied significantly with light over 24 h. ► Basil and scallions had different responses to light changes over 24 h. ► Nitrate and nitrite concentrations were different in various edible plant parts.
Introduction
Nitrate and nitrite are present in a wide range of foods. Vegetables are known as the major source of nitrate and nitrite intake in the human diet (Amr & Hadidi, 2001). Dennis and Wilson (2003) stated that it is estimated that 75–80% of the total daily intake comes from vegetables. Many people are concerned about the nitrate/nitrite concentration in processed foods, especially processed meats where sodium nitrate/nitrite is added to enhance the flavour and colour, plus extend the shelf life. However, the amount of sodium nitrate/nitrite added to processed meats is much lower (in the UK, the maximum concentration of sodium nitrate in cured meats is 250 ppm) than the natural concentrations in many leafy vegetables (often 10× that amount). Yet, the consumption of more vegetables is widely recommended to get the health benefits from the phytochemicals and fibre in leafy vegetables.
Even though many vegetables, such as the mustard family (also known as brassica vegetables), have functional capabilities to help prevent health problems such as cancer, the consumer also needs to be concerned about the concentration of nitrate/nitrite in those vegetables. Although post-harvest processing, storage or handling can impact nitrate/nitrite content of vegetables (Dennis and Wilson, 2003, Leszczynska et al., 2008), we still need to be concerned about the content at harvest. Many vegetables and culinary herbs are consumed raw or with minimal processing in salads, spices, dips, juices, or mixed in with cooked foods. For example, basil and scallions are preferred to be as fresh as possible in many dishes to obtain the best quality in terms of taste, flavour, colour, aroma and more.
Nitrite is believed to contribute to some forms of cancer (Cassens, 1997). They are also implicated in methaemoglobinaemia (Chan, 2011). Although nitrate is relatively harmless in itself, it is a major precursor of nitrite in the body. It is estimated that around 5% of nitrate ingested is reduced to nitrite by micro-organisms in the saliva (Cassens, 1995). A relationship between nitrate-containing food consumption and the increase of cancer risk has been reported (Mitacek et al., 2008). Nitrate levels can be significant in many leafy vegetables such as lettuce (Lactuca sativa L.), spinach (Spinaciaoleracea L.), celery (Apiumgraveolens L.), and marjoram (Majoranahortensis, Moench) (Samuoliene et al., 2009). Some regulatory agencies are setting maximum levels of nitrate in some vegetables (Santamaria, 2006).
Several factors can potentially influence the level of nitrate and nitrite in various raw vegetables. These factors include type, amount and form of nitrogen fertiliser (Elia et al., 1998, Lips et al., 1990), high levels of organic matter in the soil, growth-depressing temperatures (Habben, 1973), and geographical latitude (due to supplemental light usage). A relationship between light levels and nitrate content in vegetables has been implicated in a number of studies. Lorenz (1978) indicated that nitrate reductase is most active under intense light conditions. The season when the crop is harvested influenced nitrate accumulation due to variation in temperature and light levels with season (Walters, 1991). Amr & Hadidi (2001) harvested a variety of vegetables on different dates that were relatively early, middle and late within their respective harvest seasons (2–3 weeks between successive harvests). They found that date of harvest had a significant effect on nitrite in the leafy vegetables of cabbage, spinach, and lettuce in open field production (spinach had the highest level on the early harvest while cabbage and lettuce had the highest level on the late harvest). The nitrate content of spinach, cabbage, and squash grown in open fields was significantly higher in the earlier harvests than in the last harvest. Dennis and Wilson (2003) state that “when light levels fall, the rate of photosynthesis decreases, and nitrate accumulates in cell fluids and sap. The levels of nitrate in vegetables grown under low light conditions are thus correspondingly higher than those grown under bright light”.
Reduction of nitrate to nitrite in plants is a well-established biochemical process, which is initiated by nitrate reductase (NR) (Beevers and Hagemann, 1969, Campbell, 1999, Hoff et al., 1994). Nitrate reductase is subjected to regulation by several factors. The triggering signal for the NR expression is the presence of nitrate (Crawford, 1995). However, one important factor of NR regulation is irradiance, which is complex and depends on the development stage of the plant (Lilo and Appenroth, 2001, Lilo, 2004, Mohr et al., 1992). According to the “Opinion of the Scientific Panel on Contaminants in the Food chain on a request from the European Commission to perform a scientific risk assessment on nitrate in vegetables. (2008), the consumption of nitrates in vegetables is primarily a concern when high levels of certain vegetables are consumed that have been grown under conditions that result in high concentrations of nitrates. In other words, it is important to grow and harvest the vegetables to minimize nitrate concentration.
Because nitrate reductase responds to light levels, the nitrate content of vegetables would likely change throughout the daily changes in light levels. With this knowledge, one could reduce the nitrate content of raw vegetables by harvesting the vegetables at the time of day when the lowest nitrate content would be expected. To do this, we need to determine the nitrate content of specific vegetables throughout a 24 h light-cycle. The response of nitrate content to changes in the light-cycle may vary with specie of vegetable and even in different parts of the same species. Plant parts vary in their nitrate and nitrite levels. Lorenz (1978) stated that fruits and flowers have low levels, leaves and roots have moderate levels, and petioles and stems have high levels.
The objectives of this study were to:determine if there are variations in nitrate and nitrite concentrations in sweet basil and scallions over a typical 24 h light cycle; compare nitrate and nitrite concentrations between basil and scallions to determine if there are differences in how different vegetables respond to changes in light intensity; determine if there are variations in nitrate and nitrite concentrations in various parts of these plants which may be consumed.
Section snippets
Materials and methods
Several sweet basil (Ocimumbasilicum) plants were raised in an open field in East-Central Texas without the application of fertilisers, pesticides or other chemicals. On October 16, 2011, samples of plant material (leaves with petiole from the upper, younger parts of the plant) were harvested every 3 h for a 24 h period and taken to a laboratory for analysis of the plant material for nitrate and nitrite content. The leaf blades with petiole were blended together for analysis. In addition, during
Results and discussion
All of the nitrate and nitrite concentrations in basil leaves with petioles over a 24 h light cycle are presented in Fig. 1. Also, presented in Fig. 1 are light levels, air temperature and relative humidity over the same 24 h sampling period. There was significant change in nitrate concentrations throughout the 24 h period – from a low of 231 ppm at 9:30 to a high of 2777 ppm at 15:30. An analysis of variance indicated that the nitrate concentrations were significantly different (p < 0.05) between the
Conclusions
For sweet basil, nitrate and nitrite concentrations varied significantly over a 24 h period and appeared to be related to changes in light intensity over that same period. This indicates that future research on nitrate and nitrite concentrations in vegetables may want to consider the 24 h light cycle as a significant factor. It also indicates that the nitrate and nitrite concentrations in raw vegetables can be reduced by timing the harvest for the best time in the daily light cycle. For
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