Effect of processing on the biochemical contents of Acanthus montanus (Nees) T. Anderson (Acanthaceae) leaves

Abstract The effect of processing on the biochemical contents of Acanthus montanus leaves was investigated. The moisture, crude protein, lipid, fiber, ash, and total carbohydrate contents of the raw vegetable were 59.15, 1.85, 2.32, 3.76, 2.04, and 34.65 g/100 g, respectively. The saponin, alkaloid, tannin, flavonoid, phenol, and anthocyanin contents of the raw vegetable were 5.35, 4.04, 1.10, 3.53, 2.87, and 1.27 g/100 g, respectively, while it contained 2.65 mg/100 g calcium, 1.14 mg/100 g magnesium, 7.66 mg/100 g potassium, 350.75 μg/g vitamin A, 50.87 mg/100 g vitamin C, and 0.25% titratable acidity. There were significant reductions (p < .05) in the protein, lipid, fiber, ash, saponin, alkaloid, tannin, phenol, anthocyanin, calcium, magnesium, potassium, vitamin A, vitamin C, and titratable acidity of the boiled or boiled + sun‐dried A. montanus leaves; significant elevation of the moisture contents but significant reduction of the total carbohydrate contents of the boiled; and significant reduction of the moisture contents of the boiled + sun‐dried vegetable compared with the raw. There were significant increases (p < .05) in the total carbohydrate contents of the boiled + sun‐dried leaves; significant reductions (p < .05) in the moisture, saponin, alkaloid, and vitamins A and C contents of the sun‐dried vegetable; and no significant differences (p > .05) in the lipid, calcium, potassium, and ash, but significant increases (p < .05) in the protein, crude fiber, total carbohydrates, tannins, flavonoids, phenols, anthocyanin, magnesium, and titratable acidity of the sun‐dried vegetable when compared with the raw. Sun drying alone either retained or enhanced the release of some important bioactive compounds in A. montanus leaves. Furthermore, the reduced moisture content of the sun‐dried vegetable together with its increased titratable acidity will make the sun‐dried vegetable uninhabitable for microorganisms, thereby increasing its shelf life.

Despite the nutraceutical relevance of A. montanus leaf, there is paucity of information in literature on the residual constituents of its nutrients/bioactive compounds when cooked/boiled. This is especially of importance as cooking could induce significant changes in the chemical composition and decrease the nutritive quality and chemopreventive compounds in vegetables, so as to decrease their bioavailability when consumed.
One major challenge to ensure food security in most developing countries is making food available throughout the year. Most agricultural products are perishable, and while they are abundant in particular times of the year, they tend to be absent at other times of the year (Habou, Asere, & Alhassan, 2003), underscoring the need for a preservative method that can ensure their availability throughout the year.
One of the methods of stabilizing and extending the durability of vegetables is drying (Chan et al., 2009;Okeke et al., 2008). Drying of vegetables not only increases their shelf lives, but also ensures their availability throughout the year (Habou et al., 2003).
Given the nutraceutical relevance of this vegetable, it has become imperative to explore the preservation method that can increase its shelf life as well as retain its nutrients/bioactive compounds, for which information is lacking in literature.
Consistent with this, the present study was therefore setup to investigate the effect of boiling, drying, or their combination on the nutrient and bioactive constituents of A. montanus leaves.

| MATERIALS AND METHODS
Fresh samples of A. montanus were obtained from Umuahia, Abia State, Nigeria. The nonleafy parts were removed, while the leafy parts of the vegetable were collected, properly washed, chopped into homogenous pieces, and divided into four portions. The first portion was kept as the raw (control). The remaining portions (second, third, and fourth, respectively) were divided into boiled, sun dried, and boiled + sun dried.

| Sample treatment
For the preparation of the boiled vegetable, about 200 g of the second portion (homogenous pieces) of the vegetable was immersed in water (about 500 ml) and boiled for approximately 10 min after which they were removed and the water was drained. For the preparation of the sun-dried vegetable, about 200 g of the third portion (homogenous pieces) of the vegetable was sun-dried, while for preparation of the boiled + sun-dried vegetable, about 200 g of the fourth portion (homogenous pieces) of the vegetable was immersed in water (about 500 ml) and boiled for approximately 10 min.
Thereafter, they were removed from the water, water was drained, and then sun dried.
The raw and processed (boiled, sun-dried, and boiled + sun-dried) vegetables were then homogenized prior to analysis.

| Proximate analysis
The method of AOAC (1984) was used for the determination of moisture, ash, protein, lipid, and crude fiber contents of the raw and processed vegetables. Total carbohydrate was obtained by difference.

| Mineral assay
The atomic absorption spectrophotometer (Analyst 200, Perkin Elmer, Waltham, MA, USA) was used for the analysis of Mg and Ca, while the flame photometric method was used for the analysis of potassium (AOAC, 1984) in the raw and processed vegetables.

| Phytochemical analysis
The method of AOAC (1984) was used in the determination of saponin and tannin, the gravimetric method of Harbone (1973) was used in the determination of alkaloids, flavonoids, and anthocyanin, while the method of Pearson (1976) was used in determination of the phenolic content of the raw and processed vegetables.

| Assay of vitamins
The vitamin C content of the raw and processed vegetables was determined using the titrimetric method (Onwuka, 2005), while vitamin A was assayed using the method of Maciej and Krzysztof (2007).

| Titratable acidity
A measured quantity (5 g) of the raw and processed vegetables was homogenized with 25 ml of distilled water. The mixture was then titrated with 0.1 M NaOH to pH 8.1 and the results were reported as % of malic acid (AOAC, 1990).

| Statistical analysis
Data generated were analyzed statistically using the Statistical Package for Social Sciences (SPSS) version 17.0. One-way analysis of variance (ANOVA) was used for the comparison of means. Results were considered to be significant when p < .05.

| RESULTS
The results of the proximate composition of the raw and processed A. montanus leaves are presented in Table. 1. As shown in the table, the moisture, crude protein, lipid, fiber, ash, and total carbohydrate contents of the raw A. montanus leaves were 59.15, 1.85, 2.32, 3.76, 2.04, and 34.65 g/100 g, respectively.
Although boiling of A. montanus leaf significantly increased (p < .05) its moisture content in comparison with the raw, sun drying of the vegetable or combination of boiling and sun drying resulted in significant decrease (p < .05) in its moisture content compared with the raw.
The crude protein content of the boiled or boiled + sun-dried A. montanus leaves significantly decreased (p < .05) when compared with the raw. On the contrary, the protein content of the sun-dried leaves significantly increased (p < .05) when compared with the raw.
The crude lipid content of the boiled or boiled + sun-dried A. montanus leaves significantly decreased (p < .05) when compared with the raw, while the lipid content of the sun-dried leaves was not significantly different (p > .05) from that of the raw.
The crude fiber content of the boiled or boiled + sun-dried A. montanus leaves significantly decreased (p < .05) when compared with the raw. On the other hand, the crude fiber content of the sun-dried leaves significantly increased (p < .05) when compared with the raw.
The crude ash content of the boiled or boiled + sun-dried A. montanus leaves significantly decreased (p < .05) when compared with the raw. On the other hand, the ash content of the sun-dried leaves was not significantly different (p > .05) from that of the raw.
The total carbohydrate content of the boiled leaves significantly decreased (p < .05) when compared with the raw. On the contrary, the total carbohydrate content of the sun-dried and boiled + sun-dried leaves significantly increased (p < .05) in comparison with the raw.
The phytochemical composition of the raw and processed A. montanus leaves are shown in Table 2. As shown in the table, the raw vegetable contained considerable amounts of saponin. However, when the raw vegetable was processed, the saponin content of the boiled, sun-dried, and boiled + sun-dried A. montanus leaves significantly decreased (p < .05) when compared with the raw. Table 2, the raw A. montanus leaves contained considerable amount of alkaloids. However, boiling, sun drying, or combination of boiling and sun drying of the vegetable resulted in significantly decreased alkaloid content (p < .05) compared with the raw. Table 2 indicated that boiling or combination of boiling and sun drying resulted in significant decreases (p < .05) in the tannin content of the A. montanus leaves compared with the raw. In contrast, sun drying alone significantly increased (p < .05) the tannin content of the vegetable when compared with the raw.

Results shown in
T A B L E 1 Proximate composition (g/100 g) of raw and processed Acanthus montanus leaves 6.01 ± 0.13 mg/100 g vitamin C, respectively.
The titratable acidity (TA) in the raw and processed vegetables is shown in Figure 1. As shown in the figure, the raw vegetable contained 0.25 ± 0.04% TA, the boiled vegetable contained 0.09 ± 0.02% TA, the sun-dried vegetable contained 1.09 ± 0.08% TA, and the combination of boiled and sun-dried vegetable contained 0.05 ± 0.00% TA.
There were significant decreases (p < .05) in the TA of the boiled and boiled + sun-dried vegetable compared with the raw, but significant increases (p < .05) in the TA of the sun-dried vegetable compared with the raw.
T A B L E 2 Phytochemical composition (g/100 g) of raw and processed Acanthus montanus leaves

| DISCUSSION
Values obtained for most of the parameters on the proximate composition of A. montanus leaves were higher than previous reports of Nnamani et al. (2015) on the proximate composition of this plant except moisture in which the values obtained were lower than that reported by Nnamani and colleagues.
The decreased moisture content of the sun-dried and boiled + sundried vegetable could be attributed to loss of water during the drying process.
The decreased protein content of the boiled or boiled + sun-dried vegetable suggests denaturation of some of the cellular proteins during the process of boiling of the vegetable (Khairul, Khan, Sarkar, Nurul, & Sarkar, 2013 Crude fiber represents the portion of food not used up by the body but mainly made up of cellulose with a little lignin (Lilian, 2002).
Diets rich in crude fiber have also been shown to be beneficial as they delay the digestion of starch to simple sugars through inhibition of pancreatic amylase, which is an important factor in the management of diabetes . The decrease in the crude fiber content of the boiled or boiled + sun-dried A. montanus leaves could be attributed to some soluble carbohydrates present in the vegetable that may have leached into the boiling water. However, the increased crude fiber content of the sun-dried vegetable suggests increased release of some insoluble carbohydrates from their food matrix. The current study reveals that sun drying of A. montanus leaf increases its crude fiber content.
The decrease in the ash (total mineral) content of the boiled or boiled + sun-dried vegetable could be attributed to the leakage of some minerals into the boiling water. On the other hand, the nonsignificant difference in the ash content of the sun-dried vegetable compared with the raw suggests that sun drying of this vegetable retains its mineral content.
The increased total carbohydrate content of the sun-dried and boiled + sun-dried vegetable is simply due to their loss of moisture during the drying process, whereas the decreased total carbohydrate content of the boiled vegetable is due to increased moisture content during the boiling process.
Saponin possesses the potentials of precipitating and coagulating red blood cells. Other characteristics associated with saponin include formation of foams in aqueous solutions and cholesterol-binding properties and bitterness (Awa & Eleazu, 2015). The considerable amount of saponin in the raw vegetable suggests that it might possess these properties. The decreased saponin content of the boiled, sundried, and boiled + sun-dried A. montanus leaves indicates that the processing methods used in this study decreased the saponin content of A. montanus leaves.
Alkaloids are the most efficient therapeutically significant plant substances. Pure isolated alkaloids and their synthetic derivatives are used as basic medicinal agents because of their analgesic, antispasmodic, and antimicrobial properties (Awa & Eleazu, 2015). The considerable amount of alkaloids in the raw A. montanus leaves suggests that the raw vegetable could possess any or all of these therapeutic properties. Although boiling, sun drying, or their combination decreased the alkaloid content of this vegetable, the amounts obtained which could be considered to be high could be one explanation for the medicinal/ therapeutic usage of the boiled/decoctions of this vegetable in folklore medicine.
Tannins are water-soluble high-molecular-weight phenolic compounds found in many plants that are important in herbal medicine due to their wound healing properties (Eleazu, Eleazu, Awa, & Chukwuma, 2012) and antioxidant properties.
Flavonoids and phenolic acids are known to possess antioxidant activities due to the presence of hydroxyl groups in their structures and their redox properties (Eleazu, 2016;Ukwe & Ubaka, 2011). In addition, flavonoids as the largest group of phenolics found in fruits, vegetables, and other plant parts have been linked to reducing the risk of major degenerative diseases .
Therefore, high amounts of these polyphenols in the raw vegetable could have contributed to the hypoglycemic, α-amylase, and α-glucosidase inhibitory properties of this vegetable as previously reported (Ogundajo et al., 2016;Ukwe & Ubaka, 2011).
It is noteworthy that one of the ethnomedicinal uses of the boiled form/decoctions of this vegetable is in the management of diabetes mellitus due to its ability to inhibit these brush border enzymes as supported by experimental studies (Ogundajo et al., 2016). Although boiling decreased the amounts of these polyphenols in this vegetable, the amounts obtained for most of these polyphenols in the boiled vegetable which could be considered to be high could have contributed to the hypoglycemic action of the boiled form/decoctions of this vegetable as used in folklore (Ogundajo et al., 2016;Ukwe & Ubaka, 2011). In addition, the findings of this study revealed that sun drying of this vegetable resulted in increased levels of these polyphenols in A. montanus leaf, a finding that may be of benefit to people with type 2 diabetes mellitus.
Anthocyanins belong to the class of flavonoids commonly found in plants that confer them with color and antioxidant activities (Karau, Njagi, Machocho, Wangai, & Kamau, 2012;Mary, 2004;Zahid et al., 2015 Titratable acidity is a quantification of the hydrogen ions that are consumed by titration with a standard base to an end point. It gives an idea of the amount of acid that is present in a solution. The decreased TA in the boiled and boiled + sun-dried vegetable when compared with the raw suggests that boiling of the vegetable or its combination with sun drying decreased the acidity of the vegetable.
On the contrary, the increased TA in the sun-dried vegetable suggests increasing dry matter contents of the samples or release of some organic acids during the process of sun drying that may have increased the acidity of the vegetable. While the increased TA in the sun-dried vegetable could cause unpleasant taste in the vegetable, this increase is desirable as together with its low moisture content it will make the sun-dried vegetable uninhabitable for microorganisms, thereby increasing its shelf life.
To the best of our knowledge, this is the first holistic documentation on the proximate, phytochemical, minerals, vitamins, and TA contents of A. montanus leaves and the effect of processing on these components of this vegetable.

| CONCLUSION
The study revealed that sun drying of A. montanus leaves retained the calcium, potassium, and lipid contents of this vegetable, while it increased its protein, total carbohydrates, crude fiber, tannin, flavonoid, phenol, anthocyanin, magnesium, and TA contents. Finally, the reduced moisture content of the sun-dried vegetable together with the increased TA will make the sun-dried vegetable uninhabitable for microorganisms, thereby increasing its shelf life.

CONFLICTS OF INTEREST
None declared.