Traditional vegetable preservation technologies practiced in Acholi subregion of Uganda improves mineral bioavailability but impacts negatively on the contribution of vegetables to household needs for micronutrients

Abstract The impact of traditional African preservation methods on the contribution of vegetables to household micronutrient needs (Recommended Dietary Allowance: RDA) has largely remained unquantified. Using Acholi subregion of Uganda as a case area, this study examined using the predominant vegetables consumed in fresh and preserved forms (cowpeas—Vigna unguicullata, okra/lady fingers—Abelmoschus esculentus, Malakwang—Hibscus cannabinus, and eggplants—Solanum melongena), the effect of major traditional vegetable preservation methods (sun drying, boiling and sun drying, and salting and sun drying) on the contents of micronutrients (vitamin A, iron, zinc, calcium, magnesium, and phosphorus), the levels of antinutritional factors (total polyphenols, oxalate, tannins, and phytate), bioavailability of iron and zinc, and the contribution of vegetables to the cumulative annual household RDA for micronutrients. Laboratory analysis showed that all the preservation methods, except the sun drying method reduced the contents of micronutrients by 20%–82% (p ≤ .05). The contents of antinutritional factors reduced by 1%–80% while in vitro bioavailability of iron and zinc increased by 21%–296% (p ≤ .05). Nutritional computation revealed that except for calcium, the preservation methods combined, reduced the contribution of the vegetables to cumulative annual RDA for other micronutrients by 28%–60%. These results demonstrate that improvements in bioavailability of essential nutrients (iron and zinc) by traditional preservation methods investigated are associated with significant loss of micronutrients which culminates in significant reduction in the contribution of cultivated vegetables to household RDA for micronutrients. Traditional African preservation methods should be optimized for nutrient retention.


| INTRODUC TI ON
The significance of adequate consumption of fruits and vegetables to human health and nutrition has received great attention globally. This is largely because of their high contents of micronutrients and bioactive compounds that are essential for nutrition and health (Arimond et al., 2018;Sivakumar et al., 2018). The increase in occurrence and prevalence of diet-associated noncommunicable diseases such as cancer, diabetes, obesity, and high blood pressure has also created renewed attention to the need for adequate consumption of fruits and vegetables (Lin et al., 2018;Rinaldo, 2020). As a strategy to meet micronutrient requirements and to contribute to achieving other health outcomes (e.g., reduced level of cholesterol, blood pressure maintenance, weight control), the Food and Agriculture Organization (FAO) and World Health Organization (WHO) recommends that adults should consume at least 400 g of fruits and vegetables per day (FAO & WHO, 2004). Despite the FAO and WHO recommendation on fruits and vegetables intake, micronutrient deficiency is still an important nutritional constraint, most especially in developing regions of the world and rural areas in particular (Fongar et al., 2019). sub-Saharan Africa ranks highest in terms of micronutrient deficiency. This is well evidenced by statistics which show deficiency rates of 29.1% for vitamin A (Bailey et al., 2015), 43% for iron (Engle-stone et al., 2017) and 26% for zinc (Wessells & Brown, 2012) in the subcontinent.
In many sub-Saharan African countries such as Uganda, because of poverty and less functional market systems, rural households rely largely on own production to support food needs (Akombi et al., 2017).
In most situations, such households depend on vegetables because grain stocks hardly support year-round food needs while staple food available in the market is often too expensive for them to afford (Boukary et al., 2016). On the other hand, and also due to poor socio-economic conditions, resource-constrained households in the subcontinent, majority of which are concentrated in rural areas, hardly access animal source foods such as meat and meat products, and poultry and poultry products (Boukary et al., 2016). This is notwithstanding the fact that animal-based foods are a good source of essential micronutrients with high degree of bioavailability (Akomo et al., 2016). These situations indicate the importance of vegetables, which are easily cultivated and take a short time to mature (Rybak et al., 2018), to household nutrition and micronutrient intake in particular, among poor rural communities in the subcontinent.
Vegetables contain substantial quantities of micronutrients such as vitamin A, iron, calcium, zinc, magnesium, and phosphorus, and when consumed in adequate amounts as recommended by WHO (2003), can contribute effectively to prevention and alleviation of micronutrient-associated medical conditions such as night blindness, iron deficiency disorders, osteoporosis (weak bones), and weak teeth (Akomo et al., 2016). Other than micronutrients, vegetables are also a good source of fiber and a variety of non-nutrient phytochemicals (such as phenols, phytates, saponins, oxalates, and tannins), when consumed in small quantities, are believed to confer health benefits including anti-inflammatory, antioxidant, anticancer, lipid lowering, and blood pressure regulation actions (Lin et al., 2018). However, these phytochemicals, when present in large quantities, confer antinutritional properties and thus become antinutritional or antinutrient factors instead. These antinutrient factors, through their direct action and or via their metabolic products reduce bioavailability of essential nutrients such as iron, calcium, zinc, and protein (Bazaz et al., 2016;Burgos et al., 2018;Gwamba et al., 2019).
In many sub-Saharan African countries such as Uganda, production of vegetables is seasonal due to seasonal agricultural patterns dictated by weather. Therefore, fresh vegetables are largely unavailable during off-season periods. Seasonal nature of agricultural production and high level of perishability hinder year-round availability of fresh vegetables. In addition, the presence of antinutritional factors in them also reduces micronutrient bioavailability (Samtiya et al., 2020). While preservation generally solves the problem of perishability and availability (Shen et al., 2018), it inevitably affects nutritional and antinutritional contents to various extents (Gwamba et al., 2019) and the extent largely depends on the preservation method used (Bulbula & Urga, 2018). Information on the impact of food preservation on nutritional quality of vegetables is largely based on experiments conducted under controlled laboratory conditions (Adeleke et al., 2017;Burgos et al., 2018;Gwamba et al., 2019).
In the context of rural development and in the context of developing countries such as those in sub-Saharan Africa, results derived from such laboratory-controlled studies may not be easily applied because of technological lacuna in rural areas. In practice, rural communities in such countries largely apply traditional methods to preserve vegetables (Rybak et al., 2018;Santhi & Sengottuvel, 2016).
However, limited information exists on how African traditional vegetable preservation methods as practiced under rural conditions affect the contribution of vegetables to household requirements (RDA) for micronutrients. This question partly has implications on outcomes of micronutrient nutrition among African rural communities. The likelihood of the impact draws credence from undesirable statistics on micronutrient intake in African countries. For instance, among rural households in Uganda, deficiency rates of essential micronutrients are high and stand at 15.8% for vitamin A, 54% for iron, and 31.7% for calcium among children (Nabeta et al., 2015;UBOS & ICF, 2018). Therefore, using Acholi subregion of Uganda as a case area, the objective of this study was to examine under real rural set-up, based on the predominant vegetables that are consumed in fresh and preserved forms, the effect of traditional vegetable preservation methods on the contents of micronutrients, the levels of antinutritional factors, mineral bioavailability, and the contribution of vegetables to the cumulative annual household RDA for micronutrients.

| Study area
The study was conducted in Acholi subregion of Uganda. The subregion was purposively selected because of the high state of micronutrient deficiency rates experienced in the recent past (UBOS & ICF, 2018), and previous studies on vegetable consumption had been carried out in it (Anywar et al., 2014;Okidi et al., 2018). The subregion is made up of eight districts (Gulu, Kitgum, Agago, Pader, Amuru, Nwoya, Omoro, and Lamwo). The main agro-ecological zone in the subregion is savannah grassland characterized by wet and dry seasons with an annual rainfall of approximately 1,500 mm (ACF, 2007). The main economic activity in the subregion is agriculture with over 90% of the population engaged in subsistence farming (UBOS, 2016). The main crops cultivated are maize, sesame, cassava, beans, rice, and a variety of vegetables. The subregion has an area of 28,500 km 2 and a population size of about 1,500,770 people (UBOS, 2016). The subregion is bordered by South Sudan to the north, Karamoja subregion to the east, Bunyoro subregion to the west, West Nile subregion to the southwest, and Lango subregion to the south (Figure 1).

| Analysis of micronutrients
The following micronutrients were analyzed: vitamin A, iron, zinc, calcium, magnesium, and phosphorus. Vitamin A, iron, zinc, and calcium were selected because of their public health importance (Stevens et al., 2013;WHO, 2020

| Analysis of antinutritional factors and mineral bioavailability
The following antinutritional factors were analyzed: total polyphenols, tannins, total oxalates, and phytates. These antinutritional factors were selected because they are the major types that affect bioavailability of nutrients from plant-based foods (Gwamba et al., 2019;Nadeem et al., 2010;Sinha & Khare, 2017). The content of total phenols was determined spectrophotometrically at 725 nm according to the method previously applied by Burgos et al. (2018) and Škerget et al. (2005), and the results expressed as μg of gallic acid equivalents per gram of dry mass (DM) of the extract (μg GAE/ gDM). The content of tannins was also determined spectrophotometrically but using the Folin-Denis quantitative method as previously applied by Ejikeme et al. (2014). Tannic acid (Sigma-Aldrich Chemicals) was used to prepare the calibration curve while optical density was measured at 700 nm using the Spectrum 23A spectrophotometer (AAF-6701; Shimadzu Corp.). The concentration of tannins was expressed as tannic acid (mg/100 g). Total oxalate content was determined using ion exchange HPLC chromatographic method previously applied by Savage and Dubois (2006). The standard curve (range 1-20 mg/100 ml) was developed using oxalic acid (Sigma-Aldrich). Chromatographic separation was carried out using a 300 × 7.8 mm Rezex ROA ion exclusion organic acid column (Phenomenex) attached to a cation H + guard column (Bio-Rad) held at 25°C. The mobile phase used was an aqueous solution of 25 mM Sulphuric acid. Phytate content was determined using the colorimetric method (Latta & Eskin, 1980). Finally, bioavailability of iron and zinc was determined using in vitro digestion method as previously applied by Kumar et al. (2017) and Kiers et al. (2000).

| Contribution of fresh and preserved vegetables to household micronutrient requirements
A cross-sectional survey employing individual household questionnaire was applied. It involved participants' recall of the quantity of vegetables that were consumed in fresh or in preserved form over a consumption period stretching from April 2017 to March 2018.
Participants were women and were purposively selected because they are usually involved in food preparation at home in rural settings (Doss et al., 2018;Johnston et al., 2018). The sample size (n) of the households that participated in the study was determined using sample size table for a given population according to Krejcie and Morgan (1970) considering household population of 296,335 households in Acholi subregion (N = 296,335). Based on the sample size table, a sample size of 382 households was considered representative of Acholi subregion. To select the participating households, a multi-stage sampling procedure was used. First, one subcounty was randomly selected from each of the eight districts followed by random selection of two parishes from each of the selected subcounty and one village from each of the selected parish. Finally, 23-24 households were randomly selected from each village. Table 1 provides information on the subcounties, parishes, and villages from which participating households were selected.
Individual household questionnaire was used to collect data on consumption of vegetables identified under Section 2.2. The questionnaire was structured to collect data on amounts produced, amounts consumed, frequency with which they were consumed, duration over which they were consumed, and overall consumption in a year, clearly segregated by duration for which fresh or preserved forms were consumed. In addition, provision was provided for information on household composition by number, age, physiological state, and gender. To ensure data quality, the questionnaire was pretested among 82 households in Alede and Obiya villages in Gulu district, and adjusted for clarity. The Alede and Obiya villages were not selected for the actual study to avoid bias. Actual data collection took place in March 2018 using four research assistants that were trained on individual household method (IHM) for food security (Seaman & Petty, 2009). During data collection, a food weighing scale was used to provide accurate estimate of the quantities of vegetables consumed in each household. All the data collected was checked and verified daily for completeness and eligibility.

| Data analysis
One-way ANOVA was used to determine the effect of the traditional preservation methods studied on the content of micronutrients, antinutritional factors, and level of bioavailability of iron and zinc.
The means were segregated using Tukey's honestly significant difference test at 5% level of significance. To determine the contribution of vegetables to household micronutrient intake, the recommended dietary allowance (RDA) of all members in a given household was computed for a day for the selected nutrients considered in this study using RDA tables from Brown et al. (2011).
RDA for households was calculated as an aggregate of individual requirements (Okidi et al., 2018). The daily requirements were then standardized to the period of consumption (Brown et al., 2011

| Effect of traditional preservation methods on micronutrient content of vegetables
Data on micronutrient content of vegetables as affected by the various traditional preservation methods investigated are presented in

| Effect of traditional preservation methods on antinutritional factors and nutrient bioavailability
Results showing the effects of various traditional preservation methods on the levels of antinutritional factors in vegetables are presented in

| Effect of traditional preservation on the contribution of vegetables to household micronutrient requirements
Data showing the effect of various traditional preservation methods on the contribution of vegetables (all vegetables combined) to household requirement (RDA) for vitamin A, iron, calcium, zinc, magnesium, and phosphorus are presented in Figure 2.
The effect was dependent on type of nutrient and preservation method applied. Sun drying significantly reduced the contribution of vegetables to household RDA for vitamin A by about 35% (p < .05) but had no effect on the contribution toward the intake of mineral micronutrients (p > .05). All the other preservation methods significantly reduced the contribution of vegetables to household RDA for all the micronutrients to various degrees except for calcium.
Generally, in situations where reductions were significant, the extent was more for boiled and sun dried vegetables than those preserved using salting and sun drying. Boiling and sun drying reduced the contribution to RDA for vitamin A, iron, zinc, magnesium, and phosphorus by 93%, 83%, 75%, 58%, and 94%, while the salting and sun drying reduced the contribution by 85%, 77%, 85%, 60%, and 85%, respectively.
The combined effect of the three traditional preservation methods on the contribution of the vegetables to household RDA for vitamin A, iron, zinc, phosphorus, magnesium, and calcium is presented in Figure 3. The three traditional methods reduced the contribution of vegetables to household RDA for vitamin A, iron, magnesium, and phosphorous (p < .05) but marginally for calcium (p = .05). The reduction was highest for magnesium (61%), followed by vitamin A (60%), phosphorus (54%), iron (32%), and zinc (28%) in decreasing order of magnitude.

| Effect of traditional preservation methods on micronutrient content of vegetables
Fundamentally, food preservation is an essential element that affects the utilization dimension of food and nutrition security (Amit et al., 2017;Bourdoux et al., 2016). It is pertinent for global food supply but critical particularly in localities where food production is dependent on natural weather conditions and seasonality (Ayua & Omware, 2013). Nonetheless, it is important to appreciate that for food preservation to effectively contribute to food and nutrition security, the preservation methods applied should not compromise substantially the nutritional content of the foods. Literature is replete with information on standardized preservation methods such as freezing, canning, pasteurization, refrigeration, high hydrostatic pressure, and microwave cooking (Barrett & Lloyd, 2012). In rural set-ups typical of many developing countries, traditional preservation methods are dominantly used Santhi & Sengottuvel, 2016) and the manner of their application vary from location to location dictated by culture and tradition (Alowo et al., 2018;Asogwa et al., 2017). Due to their significance in offsetting seasonality constraints in food production, the efficacy of  (Sheahan & Barrett, 2017), and therefore, the significant reduction in mineral contents of the boiled and sun dried samples could be largely due to leaching (into the boiling water) although mineral contents in the leachate was not measured. Sun drying did not significantly affect the mineral micronutrients. This is because sun drying largely reduces the moisture content of vegetables but does not affect the mineral contents which are stable to heat . This is also well-illustrated with the results from this study which shows that sun drying alone had no effect on mineral contents of the vegetables. Losses of mineral micronutrients in water used during cooking through leaching have been demonstrated by various laboratory studies (Babalola & Alabi, 2015;Lewu et al., 2009), and because of this fact, it is normally recommended that water used for cooking vegetables should constitute part of the meal and should not be discarded (Oluoch et al., 2012). In the context of vegetable preservation through a process of boiling and sun drying, the final stable product comes out in a dry form. A key question that still needs to be answered is how to recover, under rural conditions, the minerals lost in the leachate to be added back to the final dry product. This is a potential subject for future research.
Significant loss of vitamin A from vegetables was registered following the application of boiling and sun drying process. Vegetables contain largely β-carotene (the precursor of vitamin A) which is largely insoluble in aqueous system (Musa & Ogbadoyi, 2012).
Therefore, losses recorded for vitamin A cannot be attributed to leaching as adduced for the case of mineral micronutrients, but rather due to heat inactivation during boiling and photo-oxidation during the drying process. β-carotene is heat labile (Oluoch et al., 2012) and sensitive to photo-oxidation. Therefore, the combination of heat treatment and sunlight could have caused oxidation of the carotenoids in the vegetables to a compound with no beta-carotene activity (Musa & Ogbadoyi, 2012). On the other hand, moist heat treatment of vegetables (blanching) is a pretreatment action essential for inactivating endogenous enzymes (such as lipoxygenase) that cause off-flavors during storage (Workneh et al., 2014).
Industrially, blanching is well optimized for nutrient retention for conventional vegetables such as carrots, peas, and beans (Miglio et al., 2008). However, in the context of this study, whereas boiling prior to sun drying could be essential for inactivating enzymes, it is apparent that the process as applied traditionally leads to excessive loss of essential micronutrients (25%-82%). Future studies should optimize the traditional heat treatment process to minimize nutrient losses from vegetables. Interestingly, it seems reduction in micronutrient content of vegetables subjected to boiling and sun drying is not a general phenomenon. This is because, whereas results of other studies conducted under laboratory conditions using chaya leaves, Cnidoscolus aconitifolius (Adanlawo & Elekofehinti, 2012;Babalola & Alabi, 2015;Oduse et al., 2012), concur with the findings from the current study, the work of Lewu et al. (2009) indicated noneffect based on experiment conducted using Colocasia esculenta.
Salting and sun drying method also caused significant losses of vitamin A and mineral contents of the vegetables but the effect was second to boiling and sun drying method in terms of magnitude. With respect to mineral micronutrients, sun drying alone had no effect on their contents in vegetables. This implies that the low levels of mineral micronutrients observed in salted and sun dried vegetables were caused by the action of salt. Sodium a major ingredient in table salt causes tissue weakening and destruction (Miglio et al., 2008). Thus, reduction in the levels of minerals could probably be attributed to drip loss caused by tissue weakening and injury.
However, empirical evidence is necessary to confirm this assertion by assessing the extent of the drip and the amount of minerals, if any in the drip pool. This is another potential subject of future research. Loss of β-carotene in vegetables due to photo-oxidation is well known (Kolawole et al., 2011). Results from this study shows that loss of vitamin A was more from vegetables subjected to salting and sun drying than those exposed to sun drying process. This suggests that salt had an effect. However, the mechanism of β-carotene loss due to action of salt is largely unknown. This is another potential subject for future research.
Among all the preservation methods investigated, sun drying was the most effective in ensuring retention of mineral micronutrients in the vegetables. This result contrasts sharply with findings from other studies. For instance, Zoro et al. (2015) reported higher content of mineral micronutrients in vegetables while results of Musa and Ogbadoyi (2012) and Oni et al. (2015) illustrated an inverse situation. It is important to appreciate that the results of these studies cannot directly be compared with the findings from the current study due to methodological disparity. In the current study, all measurements were conducted and reported on a dry weight basis while in other studies the bases for nutrient content measurements were not explicitly indicated. It was clear that the extent to which various preservation methods reduced the contents of micronutrients varied with the type of vegetable. This indicates that the rate of loss of micronutrients from the vegetables varies from species to species. The observed variations could be due to differences in botanical architecture of various plant species. Future studies aimed at optimizing the traditional preservation methods for optimal nutrient retention should take due consideration of interspecies differences.

| Effect of traditional preservation methods on antinutritional factors and nutrient bioavailability
The occurrence of antinutritional factors in vegetables limits bioavailability of mineral micronutrients from them (Mepba et al., 2007).
Thus, innovations in food processing to improve bioavailability of essential minerals such as iron and zinc have been explored (Ertop & Bektaş, 2018;Mbah et al., 2012). In those explorations, limited attention has been paid to traditional processing methods such as those investigated in this study. Interestingly, the results of this study have demonstrated that the three traditional preservation methods significantly reduced the contents of the four antinutritional factors in all the four vegetable species used. This illustrates the potential of the traditional food preservation technologies to address not only the seasonality and perishability constraints, but also to improve mineral nutrition among rural households. These results are in line with but also in disagreement with the findings from other studies depending on the preservation method. With regard to sun drying method, some studies have reported reduction in the level of tannins, total oxalates, and phytates in vegetables (Abiodun & Adepeju, 2011;Matazu & Haroun, 2004;Musa & Ogbadoyi, 2012;Oni et al., 2015) and are in agreement with the results of the current study. However, the levels of reductions reported in the quoted studies differ with the levels recorded from the current study. These differences could be due to differences in how the traditional technologies are applied in different localities dictated by tradition as well as taste and preferences accorded to the end-products (Barrett & Lloyd, 2012;Oni et al., 2015). The differences could also be due to differences among vegetable species or cultivars used occasioned by differences in botanical micro-structural arrangements. For instance, Oni et al. (2015) Zoro et al. (2015) because an increase in phenolic content would suggest de novo synthesis of polyphenols during the drying process, a process which was not measured in either studies. In addition, for both studies, all measurements were conducted and recorded on dry weight basis. On the other hand, if measurements were made on wet weight basis, the reported increase would be due to concentration effect as the water evaporates from the vegetables (Lewu et al., 2009).
Boiling and sun drying, and salting and sun drying reduced all the antinutrients but the decline was more in the boiled and sun dried, followed by the salted and sun dried and least for the sun dried forms. This trend suggests that moist heat and salt had substantial negative effects on the antinutritional factors investigated because the combined effect of boiling and sun drying or salting and sun drying was much higher than sun drying alone. For the boiling and sun drying process, during moist heat treatment of the vegetables, their cell walls might have been destroyed which caused leaching of the contents (Adeleke et al., 2017). The antinutritional factors investigated are water soluble (Oni et al., 2015) and could have been lost through leaching into the boiling water. Similarly, other studies (Babalola & Alabi, 2015;Luo & Xie, 2013) reported comparable results when Vicia faba L. and C. aconitifolius were subjected to the boiling and sun drying process. In addition, as adduced by Blaabjerg et al. (2010) andElMaki et al. (2007), moist heat exposure at a temperature of 40-55°C degrades inositol hexaphosphate to the pentaphosphate or lower molecular weight forms thus lowering its concentration. More so, the phytic acid content decreased during boiling probably because insoluble complexes between phytate and other compounds were formed thereby reducing the amount of free phytates (Mazahib et al., 2013).
Superimposing the results of nutrient retention on the results for antinutrients provides an interesting scenario. On nutrient retention, sun drying was the best, followed by salting and sun drying, and least with boiling and sun drying. However, for antinutritional factors, the reverse was true. Generally, in food preservation, the aim among other issues is to inactivate antinutritional factors but observing minimal nutrient losses (Adeleke et al., 2017;Barrett & Lloyd, 2012).
This implies that traditional preservation technologies should be optimized. Future studies should consider this aspect. Antinutritional factors investigated in this study such as phenolic acids have free radical scavenging potentials and as such could therefore contribute to lowering cellular aging process in human body (Xu et al., 2017) and enhance immune function (Pereira et al., 2016). Because the preservation methods investigated substantially reduced the content of antinutritional factors examined, it is plausible that traditional preservation methods impacted on the antioxidative potential of the vegetables used in the study, but empirical evidence is required. This is another potential subject for future studies.
Iron and zinc are among the trace minerals of greatest concern when considering the nutritional value of vegetarian diets (Ertop & Bektaş, 2018;Sobiecki et al., 2016) and nutritional needs of women of reproductive age and children in developing countries (Sharma & Dhandoria, 2019;Wessells & Brown, 2012). This situation provides a strong rationale for improving bioavailability of iron and zinc from plant-based foods. There exists an inverse relationship between bioavailability of essential nutrients such as iron and zinc and the contents of antinutritional factors in plant-based foods (Adetuyi et al., 2011;Afify et al., 2011). Therefore, processing/preservation technologies that reduce the content of antinutritional factors are expected to improve bioavailability of iron and zinc (Barrett & Lloyd, 2012). Results of this study have demonstrated that in vitro bioavailability of iron and zinc was highest in the boiled and sun dried than in salted and sun dried, and least in sun dried samples compared to fresh samples. This is in tandem with results on antinutritional factors which show that their contents were most reduced by boiling and sun drying, followed by salting and sun drying and least by sun drying. Looking at the results from the element of nutrient retention through inactivation of antinutrients to bioavailability, it is apparent that boiling and sun drying is the best at improving bioavailability of iron and zinc but also causes most significant loss of micronutrients. Therefore, improvements in bioavailability of iron and zinc can be attributed to reduction in the levels of antinutritional factors investigated in this study. A key question that remains unanswered is how the improved nutrient bioavailability of micronutrients can be balanced against losses. The same scenario applies to the salting and sun drying method. On the other hand, improvement in in vitro bioavailability of iron and zinc was not uniform across all the vegetables investigated, but varied among them. This suggests variability factor based on the botanical differences among the plant species used in the study. Improvements in bioavailability observed in this study are comparable to that of Samia et al. (2005) and in some situations much better than those reported by Ertop and Bektaş (2018) and Gupta et al. (2006) in studies conducted under controlled laboratory conditions. This again indicates the potential of traditional preservation technologies at improving bioavailability of mineral micronutrients from plant-based foods and further justifies the need for the current study. Above all, lack of harmony in the level of results reported in the current study and others reported before implies that more work still needs to be done in order to fully understand how traditional food preservation technologies impact on nutrient content, content of antinutritional factors, and bioavailability of nutrients from vegetables under rural conditions. In addition, future studies should address the issue of bioaccessibility, an important aspect of nutrition which was not considered in this study.

| Contribution of cultivated vegetables to household micronutrient nutrition
In rural areas in developing countries, vegetables are the main source of micronutrients because animal source foods, despite having high-quality nutrients in terms of bioavailability, are too expensive to be included regularly in the diet (Boukary et al., 2016;Oryema et al., 2015). Despite their importance, the significance of vegetables to household nutrition in rural areas is largely recognized in qualitative terms. However, the contribution of vegetables to household nutrient needs in terms of the RDA for micronutrients has not been adequately quantified. Okidi et al. (2018) showed that wild foods contributed more than adequate amount of vitamin A and iron to household nutrition in Acholi subregion of Uganda, however, for cultivated vegetables information lacuna exists. This information is necessary for fostering regular vegetable consumption to enable adequate intake of essential micronutrients.
Generally, the results of the current study show that the four main cultivated vegetables used in the study contributes between 14% and 20% of the household RDA for essential micronutrients when consumed in the fresh form. However, when processed, the contribution reduces to various degrees for various micronutrients except for mineral micronutrients from vegetables processed using sun drying method. However, as observed for other aspects investigated in this study (nutrient retention, content of antinutritional factors, and bioavailability of iron and zinc), boiling and sun drying have the most negative effect on the contribution followed by salting and sun drying. In totality, whereas preservation methods investigated improve bioavailability of nutrients from the vegetables, they potentially reduce the contribution of vegetables to household RDA for essential micronutrients ostensibly due to nutrient losses. The observed substantial reduction in the contribution of the preserved vegetables to the RDA may not only be due to nutrient losses due to preservation but consumption of preserved vegetables sparingly over a longer duration of time. It also suggests that those vegetables that had higher nutrient losses dominated vegetable intake during the lean season. This implies that whereas, food preservation in principle, is essential for reducing food wastage and overcoming scarcity during off-season (Kiremire et al., 2010), traditional preservation technologies as applied by rural households compromises nutrition security due to the observed high level of nutrient losses especially for boiling and sun drying, and salting and sun drying.
As already noted under Sections 3.1 and 3.2 of this paper, it is important to reiterate the opinion that traditional preservation methods require optimization to minimize nutrient losses to enable rural people take advantages associated with improved nutrient bioavailability from preserved vegetables. A critical search through literature reveals that this is the first time the effect of traditional preservation technologies on the contribution of cultivated vegetables to household RDA for the micronutrients has been studied.
However, from a methodological point of view, it is apparent that not all vegetables that can potentially be preserved were included in the study. Therefore, the 14%-20% contribution reported in the current study may be an underestimation. This is because food choice is largely determined by taste and other sensory appeals but rarely informed by nutritional content nor nutritional quality (Flyman & Afolayan, 2006;Mavengahama et al., 2013). It is plausible that other vegetables not included in the study could contain nutrients in superior quantities although not regularly preferred by rural households.
Future studies should extend the scope of vegetables types.

| CON CLUS IONS
The results of this study have demonstrated that traditional African vegetable preservation technologies have mixed effects (desired and undesired). Whereas the ultimate desired outcomes of any food preservation strategies are to extend shelf life, improve nutritional quality and guarantee safety, in the current study, improvements in bioavailability of essential nutrients (iron and zinc) were highly associated with significant loss of nutrients instead, culminating in significant reduction in the contribution of cultivated vegetables to household RDA for micronutrients. Therefore, traditional African vegetable preservation methods should be optimized for nutrient retention to enable households derive more micronutrients from vegetables. RUFORUM played no role in the design, analysis or writing of this article. The authors are grateful to the district, subcounty, and local council leaders, research assistants, and rural communities of Acholi Sub-region of Uganda for their active participation during data collection.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.

E TH I C A L R E V I E W
Ethical clearance was obtained from Gulu University Research Ethics Committee (GUREC 008-18).

I N FO R M E D CO N S E NT
Informed consent was sought at household level from the eligible study participants who were assured of confidentiality of the information provided, and their names were not recorded on the questionnaire. Participants were free to ask questions and terminate the interview if they so wished.

DATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study have already been included in the manuscript. Raw data are available from the corresponding author upon reasonable request.