ABSTRACT
Dry
and fresh spinach (Spinacia oleracea
L.) is considered to be the one of the extremely nutritious vegetables, rich
both in phytochemicals and core nutrients. Nowadays, phytochemicals in plants
are raising interest in consumers for their roles in the maintenance of human
health. Variation in content of bioactive compounds and core nutrients is the
main concern in vegetable production. Factors such as cultural practices
specially fertilization, may affect the nutritional and medicinal properties of
the plants
Therefore,
three parallel trials for NPK to investigate the response of dry and fresh
spinach leaves to nitrogen (N), phosphorus (P) and potassium (K) on chemical
composition were conducted, with treatments arranged as follows: 0, 45, 75,
105, 120 kg·ha-1 N and P and 0, 60, 85, 106, 127, 148 kg·ha-1
K in a randomized complete block design (RCBD) with four replications. The
results demonstrated that, application of nitrogenous, phosphorus, potassium
fertilizers significantly increased the total phenolic content, total
antioxidant activity, total flavonoid content and vitamin C while magnesium,
iron, zinc and selenium did not exhibit significant response to all treatments
applied. The increase in concentrations on total phenolic content, total
antioxidant activity, total flavonoid content and vitamin C was observed,
reaching maximum at 45 kg·ha-1 N, 75 kg·ha-1 P and 85
kg·ha-1 K. The optimum rates
of 45 kg·ha-1 N, 75 kg·ha-1 P, 85 kg·ha−1 K
were then used to formulate the NPK treatment combinations as follows: 0, 30:
30: 40, 45:45:60, 60:60:70, 75:75:90 kg·ha-1,arranged in a RCBD with
three replicates. The results showed that total phenolic content, total
antioxidant activity , total flavonoid content and vitamin C reached maximum in
dry and fresh spinach leaves at N45:P45:K60 kg·ha-1.
Keywords: dry
and fresh spinach, bioactive compounds, minerals, NPK nutrition,
concentrations, variations, phytochemicals, fertilizer, response, quality.
CHAPTER
1
RESEARCH ON DRY AND FRESH SPINACH
1.1 BACKGROUND
Spinach
is a leafy and extremely nutritious vegetable, rich both in core nutrients and
phytochemicals. It is a vegetable that is provided fresh, frozen or canned to
the consumer. Spinach harvested after a shorter growth period than normal is
called dry and fresh spinach, and is marketed fresh to the consumer. This is a
fairly new product that has turned out to be popular in recent years because of
its nutritional value (Hedges and Lister, 2007). Its nutrients comprise a range of vitamins
and minerals, as well as phytochemicals. The major micronutrients in spinach
are vitamins A (from β-carotene), C, K and folate, and the minerals, calcium,
iron and potassium. The phytochemicals of most importance are the carotenoids,
flavonoids, β-carotene, lutein and zeaxanthin and phenolic compounds
(Bergquist, 2006). A number of studies have shown spinach to have strong
antioxidant activity and high levels of antioxidant compounds such as phenolics
and carotenoids (Hedges and Lister, 2007).
Agronomic
practices such as chemical fertilizers have made major contributions to improve
crop yields and food nutrition (Fageria, 2009; Wang et al., 2008). There are many contributing factors in terms of
fertilizer application that can influence the effectiveness of fertilizer such
as the application method, application timing and the rate of application. All
these factors must be investigated before fertilizer application
recommendations to ensure effectiveness and to prevent problems associated with
fertilizers. Problems such as under application and over application of
fertilizers are seen to be challenging. On the other hand, excessive fertilizer
application can have detrimental consequences. It can have adverse
environmental effects on water quality, leaching, and runoff (Heckman, 2007;
Heckman et al., 2003; Manotti et al., 1994; Sims, 1998; Sims et al., 1998). Consequently, it is
imperative to investigate and determine fertilizer application rates that are
beneficial and maximize the yield while minimizing environmental contamination
(Fontes et al., 1997; Heckman et al., 2003). The efficient use of fertilizers
and optimal fertilizer management of nitrogen, phosphorus and potassium are
necessary to minimize production cost and to improve yield and quality
(Fageria, 2009).
Some
studies are available on the identification of bioactive compounds in fruits
and vegetables, pre-harvest and postharvest factors. However, information on the effect of mineral
fertilization on nutritional quality of dry and fresh spinach is scarce. As the
consumption of fresh-cut spinach increases, it is important to investigate the
good management practices, specifically fertilization and its effects on
quality of dry and fresh spinach. The consumption of spinach is increasing
because people are becoming health cautious and spinach is regarded as one of
the healthy vegetables which is rich in nutrients. The purpose of the present
study was to investigate and quantify the variations on different bioactive
compounds, namely, antioxidants, flavonoids, total phenolic content and
carotenoids, as well as the trace elements (magnesium, zinc, iron and
selenium), in dry and fresh spinach with reference to mineral nutrition.
1.2 RESEARCH
PROBLEM
Spinach
is an important agricultural crop, not only because of its economic importance,
but also for the nutritional values of its leaves, mainly due to the fact that
they are an excellent source of nutrients and phytochemicals. Spinach is increasingly becoming important in
health because of its micronutrients and phytochemicals. It has an additional
advantage of being low in calories, which is very important in weight
management. Therefore, it is becoming a food of choice to many people because
of its nutritional importance. Nowadays,
phytochemicals and antioxidants in plants are raising interest in consumers for
their roles in maintaining human health. Phenolics and flavonoids are known for
their health promoting properties due to protective effects against
cardiovascular disease, cancers and other diseases (Kaur and Kapoor, 2001;
Sardas, 2003)
The
intake of these compounds in foods is an important health protecting factor.
They have been recognized as being beneficial for preventing widespread human
diseases, including cancer and cardiovascular diseases, when taken daily in
adequate amounts (Kaur and Kapoor, 2001; Sardas, 2003). It is generally known
that environmental factors and agricultural techniques have an effect on
vegetable and fruit quality. In particular, mineral fertilization influences
antioxidant composition in some fruits and vegetables (Jeppsson, 2000; Kaur and
Kapoor, 2001; Kopsell et al., 2006;
Kemal et al., 2007). Some studies are available on the effect of
postharvest storage and processing on the antioxidant constituents (Flavonoids
and vitamin C) of Fresh-Cut spinach (Bergquist, 2006). However, information on
the effect of mineral fertilization on nutritional quality of spinach is
scarce. Variable content of bioactive compounds and minerals is the main
problem in the production of vegetables, due to many factors especially
fertilization. Consequently, the nutritional and medicinal value of vegetables
may be affected. Producers often apply large quantities of fertilizer in an
attempt to maximize yield and advance growth, which can result in the
accumulation of nitrate in case of excessive nitrogen, reducing the quality of
some vegetables. There are no reports about the content of bioactive compounds
and minerals in dry and fresh spinach grown on the principles of best
management practices in South Africa.
Currently
the demand of dry and fresh spinach exceeds the supply and the applications of
2245Nkg/ha, 22-45Pkg/ha and 63-138Kkg/ha have been reported in California, USA.
These are the application rates from the growers since there is no scientific
validation of these rates. Limited studies are available on the effect of nutrients
on bioactive compounds of dry and fresh spinach and data that provide
recommended rates such as N, P and K on quality have not been well established
under South African conditions.
Therefore,
it was important to investigate the variation in concentrations of bioactive
compounds (antioxidants, flavonoids, carotenoids and total polyphenols) and
trace elements (magnesium, iron, zinc and selenium) in dry and fresh spinach
with reference to mineral nutrition. The main aim of this study was to
investigate the effect of fertilization (N, P and K) dry and fresh spinach on
their bioactive compounds. Additionally, any information on the physicochemical
properties and bioactive compounds of dry and fresh spinach will provide a
knowledge base that may be of some benefit to the production of dry and fresh
spinach in South Africa.
1.3
MOTIVATION FOR MY RESEARCH
Spinach
is one of the world's healthiest vegetables. Rich in vitamins and minerals, it
is also concentrated in health promoting phytonutrients such as carotenoids (betacarotene, lutein, and zeaxanthin) and
flavonoids to provide powerful antioxidant protection (Mehta and Belemkar,
2014). Leafy green vegetables such as
spinach provide more nutrients than any other food, when compared calorie for calorie
(Longnecker et al., 1997). Its nutrients include a range of vitamins and
minerals (micronutrients), which can prevent deficiency diseases and are
essential for normal physiological function, as well as phytochemicals thought
to help prevent chronic health problems such as cancer and heart disease, as
well as other health problems associated
with ageing (Hedges and Lister, 2007).
In
recent years, considerable attention has been directed towards identifying
natural antioxidants, namely those plant-derived that may be used for human
consumption regarding health promotion and disease prevention. It is vital to
determine a method of how we could grow healthy vegetables vegetable so that
human body could have a higher intake of health promoting bioactive compounds.
This fact is very important in these days when people are exposed to lot of
health related challenges. This study addresses such areas where the effect of
mineral fertilization can be related to the concentration of health promoting
phytochemicals.
One
of the advantages of spinach is that it is readily available, just about all
over the world and is easy to prepare. What is more interesting about spinach,
it is easy to find in the market or easy to grow and prepare. With its fine
taste, dry and fresh spinach is a versatile food that can be easily included
into a range of dishes. It can be eaten raw in a salad or it can be cooked
(steamed or boiled) and eaten as a dish on its own or added to soups and other
dishes. Unlike other vegetables, it is harvested after a short period after
planting. Over the years, ready to use fresh-cut spinach has become more
popular as one of the important dietary vegetable (McGill et al., 1966; Burgheimer et
al., 1967; Izumi et al., 1997).
Dry
and fresh spinach was chosen for this study for two main reasons. It is
becoming an increasingly popular product in South Africa and elsewhere in the
world. Spinach is known to be a healthy product and contains relatively high
concentrations of bioactive compounds (USDA, 2005). In addition, dry and fresh
spinach has the advantage of a short culture time and shelf life, making it an
excellent model crop. The objective of the study was to investigate the
response of chemical composition and trace elements in dry and fresh spinach to
fertilization with N, P, and K nutrition.
1.4
AIM OF THE STUDY
The
aim of this study was to investigate and determine the response of total
phenolic content, total antioxidant activity, total carotenoid content, total
flavonoid content and vitamin C as well as minerals to fertilization with
nitrogen (N), Phosphorus (P) and potassium (K) nutrition.
1.5
OBJECTIVE OF THE STUDY
1.5.1
To determine the effect of different rates/levels of application of N, P, and K
fertilizers on chemical composition and minerals in dry and fresh spinach.
1.5.2
To determine the interaction effects of NPK on the bioactive compounds in dry
and fresh spinach.
1.6
HYPOTHESES
1.6.1
Rate of applications of fertilizer N, P and K have no influence on the
bioactive compounds in dry and fresh spinach.
1.6.2
No interaction effects on bioactive compounds in dry and fresh spinach after
NPK application.
1.7
SCOPE AND LIMITATIONS OF THE RESEARCH
Given
the research aim and objectives of the study delineated above, the scope of the
study entailed three aspects – rate of applications of fertilizer N, P and K,
the variation in bioactive compounds in dry and fresh spinach as well the
combined effect of N, P and K on the bioactive compounds.
However,
the study had a set of limitations inherent in the topic investigated. Firstly,
not all varieties of dry and fresh spinach could be investigated for this
research, given the nature of the research methods used and the time-frames
selected. Secondly, the study was conducted in a protected environment and the
effect of this was not taken into consideration in this study. Lastly, only one
method of fertilizer application, side dressing was used in this study.
1.8
ETHICAL CLEARANCE
No
ethical clearance of this study is required (Appendix C).
1.9
ORGANISATION OF THE THESIS
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