Phytochemical Changes in Root Vegetables during Postharvest Storage

Abstract

Root vegetables contain phytochemicals that are essential for human nutrition, in addition to offering desirable health benefits such as anti-oxidative, anti-cancer, and immunomodulatory activities. The quantity and stability of these phytochemicals vary greatly among root vegetable cultivars and landraces. Besides, freshly harvested root vegetables deteriorate rapidly thus causing significant losses in their quality attributes. To minimize these losses, various postharvest technologies have been assessed and shown efficacy in prolonging the shelf-life of stored vegetables. However, postharvest technologies may contribute to deterioration of nutrients and/or accumulation of toxic compounds such as glycoalkaloids. Therefore, this chapter summarizes information that has been reported on the influence of varied pre-storage treatments and storage systems on the quality of root vegetables. Quality attributes that are highlighted include changes in: root vegetable morphology such as sprouting, dehydration, and greening; phytochemical content of phenolics, flavonoids, glycoalkaloids, alkaloids, glycosides, and terpenoids; and nutritional content of carbohydrates, protein, vitamins, and carotenoids.

1. Introduction

Root vegetables are important components of a balanced diet for human health because of their richness in many biologically active compounds. These compounds, collectively termed phytochemicals or functional nutrients, provide numerous desirable health benefits such as anti-oxidative, anti-cancer hypoglycemic, anti-microbial, and immunomodulatory activities beyond basic nutritional benefits [1]. These compounds include terpenes, polyphenols, glucosinolates, phytoestrogens, and carotenoids [2, 3, 4, 5]. They form part of a plant’s secondary metabolite profile, and often accumulate as part of the plants’ response mechanisms to abiotic and biotic stress factors [6, 7, 8, 9]. Besides, some of the compounds are widely used as natural colorants in the textile, food, and drug industry. The quantity and stability of these phytochemicals vary greatly among root vegetable cultivars and landraces. Besides, root vegetables deteriorate rapidly after harvest with significant losses in their morphological (e.g., weight loss, sprouting, greening, and shriveling), phytochemical (e.g., deterioration of phytonutrients and/or accumulation of toxic compounds), physiological (e.g., softening of the tissues, color changes caused by the synthesis of new pigments or destruction of others) and biochemical (e.g., increased rate of respiration) attributes [10, 11]. To produce high quality processed vegetable products, majority of farmers are not only growing new, promising root vegetable varieties, but also minimizing these losses by preserving the root vegetable quality during storage. Various post-harvest technologies, including use of varied pre-storage treatments and storage systems, have been assessed and have shown efficacy in prolonging the shelf-life of fresh root vegetables [11, 12, 13, 14, 15, 16, 17, 18, 19, 20]. Therefore, this chapter summarizes information reported on the effect of post-harvest technologies on the quality of the commonly utilized edible root vegetables that include beetroot (Beta vulgaris subsp. vulgaris L.), carrot (Daucus carota L.), cassava (Manihot esculenta Crantz), potato (Solanum tuberosum L.) and radish (Raphanus sativus L.).

2. Phytochemical and nutritional composition of root vegetables

2.1 Beetroot

Beetroot (B. vulgaris subsp. vulgaris L.) (Family: Chenopodiaceae) also known as red beetroot has fleshy root that is commonly consumed in form of supplemental juice, powder, bread, gel, oven-dried, pickled, pureed or jam-processed across different food cultures. Its bioactive phytochemicals include phenolics and carotenoids, and betalains hydrosoluble nitrogen containing pigments (e.g., betacyanin that are red-violet and betaxanthin that are yellow-orange) [21]. Betalains form 70–100% of the total phenolics and have antioxidant and anti-inflammatory properties and anticarcinogenic potential [22]. Antiviral and antimicrobial effects of betalain pigments have also been reported [23]. Other antioxidant compounds include rutin, epicatechin, and caffeic acid. The sugar comprises mainly of sucrose (91.6%) [24], with a small and relatively similar proportion of glucose and fructose [25]. Red beetroot also contains dietary fiber, minerals (e.g., potassium, sodium, iron, copper, magnesium, calcium, phosphorus, and zinc), and vitamins (e.g., retinol, folate, ascorbic acid, and B-complex [26]. Moreover, beetroot contains a substantial amount of both non-essential and essential amino acids.

2.2 Cassava

Cassava (M. esculenta Crantz) (Family: Euphorbiaceae) is a starchy fibrous root crop used for traditional desserts, salad dressing, soup thickening, binding agent in sausages, high fructose syrup, and textile industries [27]. The sweet type of cassava root can be boiled or roasted and eaten as fresh root, or minimally processed into various products. Cassava contains alkaloids, as well as flavonoids that have antioxidant and hypolipidemic effects [28], and glycosides that are potent for heart disease [29]. It has low content of nutrients such as protein (<2%), fat (<0.2%), and fiber (<4%) [30]. Most of the carbohydrate is present as starch (>32% of fresh weight) with smaller amounts of free sugars (less than 1% of fresh weight). It has rich dry matter (>80%) thus making it a good source of energy [27]. The root also contains minimal amount of micronutrients (e.g., iron, potassium, magnesium, copper, zinc, and manganese. It also has some anti-nutrients and toxic substances (e.g., cyanogenic glucosides), which together with their breakdown products (cyanohydrins and free hydrocyanic acid [HCN]) that are formed during processing, can inhibit the digestibility and intake of major nutrients [31]. The yellow root varieties contain a significant concentration of β-carotene (up to 1 mg 100 g⁻1,dry-weight) [32].

2.3 Carrot

Carrots (D. carota L.) (Family: Apiaceae [Umbelliferae]) play a major role in human nutrition, because of their high dietary value [33]. Phytochemicals present comprise mainly of phenolic compounds (e.g., phenolic acids, such as p-hydroxybenzoic, caffeic, and chlorogenic; and flavonoids such as anthocyanins), carotenoids (a precursor to vitamin A formation, which is involved in vision, cell differentiation, synthesis of glycoproteins, mucus secretion from the epithelial cells, and overall growth and development of bones) [34], polyacetylenes, and ascorbic acid. These chemicals aid in the risk reduction of cancer and cardiovascular diseases due to their antioxidant, anti-inflammatory, plasma lipid modification, and anti-tumor properties [35]. Phenolic acids are the potentially bitter compounds found in carrot peels. The moisture content of carrot varies from 86 to 89% [36]. They are good source of carbohydrates and minerals like calcium, phosphorus, iron, and magnesium, and also contain protein (0.9%), fat (0.2%), carbohydrate (10.6%), crude fiber (1.2%), and total ash (1.1%) [36].

2.4 Radish

Radish (R. sativus L.) (Family: Brassicaceae) has high nutritional value and is consumed in salads, or cooked or salted together with other vegetables. The roots can also be processed as dried or canned pickles. The extracts of radishes have been used to treat stomach disorders, urinary tract infections, hepatic inflammation, cardiac disorders, and ulcers [37]. Various reports have recorded the antimicrobial, anticancer, antioxidant, and anxiety reducing properties of radishes. Phytochemicals present in radish include alkaloids, reducing sugar, flavonoids, glycosides, cardiac glycosides, tannins, saponin, protein, amino acid, terpenoids, and steroids [38]. Anthocyanin pigments provide the red color of roots, while a high potential to form isothiocyanates contributes to the pungent flavor and distinctive taste [39]. Radish is low in calories but has a high content of vitamin C which helps to build tissue, blood vessels, bones and teeth. Other vitamins (e.g., B6 and folate) and minerals (e.g., potassium, magnesium, calcium, iron, zinc, copper, sodium, and phosphorous) are also found in radish roots. Also, radish has fiber and roughage that is effective in the treatment of constipation.

2.5 Potato

Potato (S. tuberosum L.) (Family: Solanaceae) is a staple food that is often cooked or processed into edible products such as fries and chips. It contains several phytochemicals such as phenolics, flavonoids, polyamines, and carotenoids. These phytochemicals have beneficial effects on human health hence they are highly desirable in diet. The phenolics, together with amino acids present in potatoes, confer anti-oxidant protection towards tissue damage, reactive oxygen species and diseases like atherosclerosis, diabetes mellitus, renal failure, and cancer [40]. Nutritionally, potato has complex carbohydrates (>20%) in the form of free sugars (with glucose and fructose as the principal monosaccharides and sucrose as the major disaccharide, crude proteins (>2.5%), crude fats (0.1%), crude fiber (0.6%), vitamins, and water (74%) and along with minerals (>0.8%), amino acids, and trace elements that include potassium, sodium, iodine, and magnesium, folic acid, pyridoxine, vitamin C, and iron. Additionally, potatoes contain glycoalkaloids, which are toxic steroidal glycosides and anti-nutritional substances. Although they play a role in plant resistance to bacterial and fungal diseases and pests, when in excessive amounts, they worsen taste, and a concentration above 200 mg·kg⁻¹ of fresh weight has toxic effects on the human body [41].

3. Pre-storage treatment of root vegetables

Freshly harvested root vegetables are metabolically active, and therefore still undergoing the physiological and biological processes of senescence and maturation. The rates of these processes are influenced primarily by the produce temperature. To prolong postharvest nutritional and quality (e.g., appearance, texture, and flavor) attributes, freshly harvested produce is often exposed to one or several optimal pre-storage treatments that often work by; slowing down the senescence and maturation processes, reducing/inhibiting development of physiological disorders and growth of decay producing microorganisms, restricting enzymatic and respiratory activity, inhibiting water loss, and reducing ethylene production. Physical treatments include; heat [42], irradiation [19], coatings [15], pre-cooling [18] and curing [43].

Heat treatment methods that have been applied on carrot and potato include hot-water (sometimes accompanied by brushing), hot dry air, and steam [44, 45, 46]. These treatments activate or deactivate enzymatic activities that result in reduced effects on the phytochemical and nutrient content, besides reducing chilling injuries and controlling decay. The treatments can be of short (up to 1 h) or long-term (up to 4 days) duration, but they have high energy costs. Gamma irradiation and short wave ultraviolet radiation have been used to effectively inhibit growth and development of sprouts and microbial pathogens on potato [47, 48]. However, their use is still subject to strict legislation. Paraffin wax coat is often used in combination with the exclusion of oxygen or submerging roots in water or storing in an anaerobic environment that can inhibit the streaking of the cassava xylem tissue [20, 49].

Pre-cooling is the removal of field heat in the produce immediately following harvest by using methods that include; Hydro-cooling (i.e., submerging crops in cold water), forced-air cooling (i.e., cold air is directed directly through the crop at high velocity), room cooling (i.e., placing crops in a cold room where cold air passing through a fan) and package icing (i.e., placing crushed ice directly on top of the produce). The choice of the method depends on the product type and factors such as the airflow rate, air, and produce temperature, relative humidity, and the packing configuration [50]. Bunched beetroots (with tops) are pre-cooled to optimal level of below 4°C within 4–6 h of harvest, while mature beetroot are pre-cooled to below 5°C within 24 h after harvest. Forced-air cooling, prompt washing, and hydro-cooling in chlorinated water to under 5°C are essential to maintain carrot freshness. Bunched radish is often hydro-cooled in chlorinated water to an optimum level of 0–4.5°C [51]. Radish can also be pre-cooled using the package icing technique. Generally, room cooling has low or no cost involvement. Forced-air cooling has the risk of root desiccation. Hydro-cooling permits faster cooling but offers the moisture which some pathogens require to penetrate the skin of root vegetable [52]. Hence, it is recommended that washed root vegetables should be dried at room temperature before storage. Curing process, which is hardening the skin of potatoes and cassava under temperature and RH conditions that facilitate wound healing, depends on cultivar and on whether they are destined for industry or home consumption.

Surface treatment chemicals that include maleic hydroxide, α–naphthalene acetic acid, methylester, isopropyl N-(3chlorophenyl carbamate) chlorpropham) (CIPC) and 1, 2, 4, 5 tetra chloro-3nitrobenzene are applied on potatoes. These chemicals inhibit meristematic cell division and delay sprout development. Nevertheless, there are safety concerns about the potential toxic and carcinogenic properties of CIPC and its metabolites [53]. Safer alternatives include hydrogen peroxide plus (HPP) [54], 1,4-dimethylnaphthalene (1,4-DMN), essential oils, and ethylene [55]. Gaseous treatments include ozone that has been evaluated for postharvest disease control and other storage uses on potatoes and carrots [56, 57]. However, additional research is needed to define the potential and limits of effective use of ozone for postharvest treatment of whole and minimally-processed vegetables and fruits.

4. Storage systems of root vegetables

Most fresh cassava cultivars deteriorate within 2–3 days after harvest and therefore, processing the roots into storable forms (through sun drying and fermentation) at the farm level is a better option for extending the shelf life and eliminating some toxic compound like cyanide. Some of the commonly used traditional methods include; coating of root with a paste of mud or earth, in-ground storage, field clumps, storage in a box or trench with alternative layers of moist sawdust or wood shavings and cassava roots, and storage in plastic bags [49, 58, 59, 60]. Advanced methods that are mainly used on export produce include cold storage/refrigeration at lower temperature range of 0–4°C, freeze drying, and modified atmosphere packaging [20, 49]. Nonetheless, financial and technical constraints have limited the use of advanced methods in many developing countries. Although a number of researches have been conducted on beet, carrot, and radish roots focusing on the potential uses as minimally processed ready-to-use, fresh-cut produce, or as ingredients of processed foods, information on the maintenance of freshness and quality of whole roots during storage is very limited [61]. However, freshly harvested bunched (with tops) and topped beets, carrots, and radish are usually stored between layers of moist sand, leaves, or sawdust in a box in cool place with condition of 0–4°C and 90–95% relative humidity (RH) [62]. The success of Controlled Atmosphere (CA) storage technology requires that the precise levels of CO₂ and O₂ gases are achieved and maintained within the storage facility. Where CO₂ level is too high and O₂ level too low, then the root vegetables may be irrevocably damaged [63]. However, there is little or no benefit from controlled atmosphere storage of these vegetable roots.

Potatoes are used for seed, ware, and processing and hence, storage requirements vary depending upon the purpose for which potatoes are to be used. Seed potatoes are required only at the time of planting; therefore, they need to be stored for longer. Generally, seed potatoes are stored at low temperatures (2–4°C). However, most local farmers store seed potato in simple and low-cost diffused light stores (DLS) that use natural indirect light with good ventilation to control excessive sprouting and to produce sprouts which are short, stout, green colored and with higher vigor [64]. Ware and processing potatoes are in demand throughout the year and hence both short- and long-term storage are needed. Ware and processing potatoes are stored at higher temperatures (8–12°C at 85–90% RH) [65]. Majority of smallholder farmers use traditional storage methods of ware potato. These include piling on the floor or corner in houses, dark stores, DLS, covering potato deep in the soil, stacking tubers in sacks, and heaping potato tubers under the tree shades. Majority of these methods allow keeping potato in good quality for a short period of 2–5 weeks only, depending on the potato variety [66]. Ambient ware potato storage units are also in use and can maintain the marketability of ware potato up to 9 weeks. While advanced ware potato storage methods like evaporative cool storage and cold storage exist, they are not used in most developing countries due to high costs.

5. Effect of pre-storage treatment and storage conditions on quality of root vegetables

Freshly harvested root vegetables deteriorate over a short period of time if not handled appropriately. Several morphological, biochemical, and physiological changes that are essential to the root tissue occur. For example, increased respiration rate, moisture loss and desiccation, spoilage caused by fungi and bacteria, earthy odors and flavors, sprouting and development of white blush on damaged surfaces, softening of tissues, color changes resulting from the synthesis of new pigments and destruction of others, and changes in the phytochemical and nutritional composition. Most of these changes are temperature-dependent.

Postharvest quality changes in carrot includes weight loss, bitterness, bacterial deterioration, rooting, and sprouting. The carrot has low metabolic activity at low temperatures, and can be stored for 6–8 months without loss of quality under optimal storage conditions of 0°C temperature and 98% RH [45]. Table 1 provides a summary of some quality changes that occur in root vegetables when exposed to varied pre-storage treatments. Post-harvest hot-water treatment (50°C for 1 min) can be used for preserving their β-carotene and vitamin C content, although for carrots not destined for storage (Table 1) [45]. Pre-storage treatment involving carrot exposure to Ozone atmosphere 50± 10 nL/L treatment at 0.5°C and ≥95% RH recorded reduced severity of watery soft rot and gray mold fungal diseases, and blotches of discolored brown periderm tissue after a storage period of 180 days [56]. Similarly, reduced severity of fungal watery soft rot disease was observed on carrots that were exposed to 5 s pre-storage treatment of steam under 0.2 MPa pressure and 70°C prior to storage at 0.5°C for 60 days, and an additional 14 days at 20°C [46]. A UV-C (0.88 kJ/m2) treatment of carrot at 10°C and 90% RH resulted in reduced severity of watery soft rot and gray mold fungal diseases on carrots stored for 15 days (Table 1) [67].

Root vegetable	Pre-storage treatment	Effect on root quality	References
Carrot	HW; 50°C for 1 min	β-carotene and vitamin C levels remained unchanged	[45]
	Ozone atmosphere 50 ± 10 nL/L; 0.5°C; ≥95% RH; 180 d	reduced severity of watery soft rot and gray mold diseases; blotches of discolored brown periderm tissue	[56]
	ST (0.2 MPa pressure); 70°C; 5 s application time; 0.5°C; 60 d + 14 d at 20°C	reduced severity of fungal watery soft rot disease	[46]
	UV-C (0.88 kJ/m2); 10°C; 90% RH; 15 d	reduced severity of watery soft rot and gray mold diseases	[67]
Cassava	PW coat;55–65°C; few seconds; 60 d	prolonged shelf life	[49, 68]
Potato	Ethylene treatment; continuous application	sprouting inhibited; increased reducing sugars content	[69, 70]
	HPP treatment for 10 h; 180 d at 10 ± 1°C	complete sprout suppression	[54]
	UV-C light; 20°C; 80% RH; 40 d (in dark)	reduction in sprout length and development up to 20 d	[48]
	GI at 10, 30, and 50 d after harvest; 8 and 16°C; 150 d	early irradiation and higher irradiation levels had decreased sprouting, percent weight loss and specific gravity; delayed irradiation had decreased ascorbic acid and contents of reducing and non-reducing sugars; decrease in ascorbic acid at higher storage temperature	[47]

Table 1.

Summary of studies on effects of pre-storage treatments on the quality of selected root vegetables.

d: days of storage; EC: edible coating (carboxy methyl cellulous and cellophane); PW: paraffin wax; FT: fungicide treatment; HPP: hydrogen peroxide plus; GI: gamma irradiation (0, 50, 100, and 150 Gy); HWT: hot water treatment; ST: steam treatment; min: minute; RH: relative humidity; and UV-C: ultra violet light (intensities 0.0, 3.4, 7.1, 10.5, and 13.6 kJ m⁻²).

Generally, when plants are subjected to postharvest abiotic stresses they synthesize secondary metabolites, such as phenolic compounds. Table 2 summarizes the effect that different storage systems on root vegetables.

Root vegetable	Storage condition	Effect on root quality	References
Beetroot	CS; 5°C; 196 d	decreased betanin and increased isobetanin content in first 140 and 98 d, respectively	[71]
	CS; 30 d	increased total dry matter content after 30 d, and a decrease after 120 d; increased total phenolic content	[72]
	CS; 1 ± 1°C; 90–95% RH; 210 d	retained total soluble sugar content after 30 d, and a decrease after 120 d	[61]
	CS; 120 d	increased nitrate content after 30 d with no further change	[73]
Carrot	CA 5 kPa O2; 5 kPa CO2; 4°C; 8 d	increased phenolic (chlorogenic acid) content; maintained overall visual quality	[74]
	CS; 1°C; 120 d	increased polyacetylenes (falcarinol, falcarindiol and falcarindiol-3-acetate) content	[75]
	CS; low, moderate and high O2 condition	Decreased vitamin C in high and moderate O2 conditions; vitamin C content retained at low O2	[76]
	CS;4°C; 25°C; 140 d	increased β-carotene after 3 d at 4°C, and 14 d at 20°C	[77]
	Freezing	decreased vitamin C content	[78]
	20°C; 48 h	increased phenolics content	[79]
	CS;4°C; 25°C; 140 d	high anthocyanins content retained at 4°C than at 25°C	[76]
	Temperature range 7.5–37.5°C	decreased β-carotene contents after 8 d	[80]
Cassava	In-ground storage; >12 months	increased fibrous and woody content, reduced extractable starch; increased susceptibility to pathogen attack	[49, 58]
	Curing and clumps; >90 d	no internal discoloration, reduction in hydrogen cyanide content, increased soluble sugars, softening of central root core	[75]
	Boxes with moist sawdust; 28 d	roots remained healthy	[60]
Potato	CS; 3°C; 90 d	increased phenolic (chlorogenic, caffeic and sinapic acids) content	[81]
	Curing
	CS; 4 °C; 120 d	decreased vitamin C	[82]
	Pit/ underground storage	increased respiratory heat that promoted growth and development of potato sprouts	[83, 84]
	Store (ACS, GTHS, DLS); 9.7–19.4°C; 65–93 RH; 56 d	increased weight loss, greening, shrinkage, and reducing sugar content in DLS; decreased starch content with increasing storage time	[85]
	4 or 10°C; 42 and 92 d	increased glycoalkaloids content at low temperatures within 14 d	[86]
	CS 4°C; also room temperature and 15°C; 90 d	all phenolic acid content increased with storage time, except para-coumaric acid which decreased at 4°C; decreased vitamin C content	[87]
	CA; 5 or 10°C; gas mixture (0.5% CO2 and 21.0% O2 or 9, 6.4, 3.6, or 0.4% CO2 all combined with 3.6% O2); 100 d	Complete sprout inhibition at 9.4% CO2 with 3.6% O2 at 5°C; decreased weight loss; healthy skin maintained	[63]
Radish	CS airtight; 1°C; 45 d	reduction in total aliphatic glucosinolates after 5 d	[88]
	CS; 0°C; 120 d	reduction in sulforaphene isothiocyanate concentration	[89]
	Curing and packed in micro perforated HDPE film in plastic crate	decreased weight loss in cured and non-cured samples; maintained total soluble solids level, and flesh and skin firmness of cured; reduced black spot disease severity and shrinkage	[90]
	Temperature; 20 ± 2°C; without leaves	increased weight loss after 3 d	[91]
	Temperature 1, 5, 10°C; 10 d	increased weight loss at 10°C	[92]

Table 2.

Summary of studies on effects of storage system on the quality of selected root vegetables.

d: days of storage; CS: cold storage; CA: controlled atmosphere; RH: relative humidity; h: hour; ACS: ambient charcoal store; GTHS: grass thatched hut store; DLS: diffused light store; and HDPE: high-density polyethylene.

In a study where carrots were stored for 48 h at 20°C, a significant increase in the phenolic content was found [79]. In [76], study results showed that when black carrots were stored at 4°C retained a high level (53.4–81.0%) of anthocyanins than samples stored at 25°C for 20 weeks (7.8–69.3%). Similarly, in a study that investigated the effect of controlled atmosphere on baby carrots revealed that that controlled atmosphere of 5 kPa O2 and 5 kPa CO2 significantly increased the phenolic content, particularly chlorogenic acid [74]. Slight variations in α- or β-carotene have been demonstrated when carrots were stored at 0°C for 6 months [93]. According to Imsic et al. [77], storing carrots at either 4°C or 20°C resulted in increases in (all-E)-β-carotene of 20.3% after 3 days at 4°C and 34.4% after 14 days at 20°C, respectively. In contrast, another study [80] reported that β-carotene contents were reduced after 8 days of storage at different temperatures, by 46% (7.5–8.5°C), 51% (17–21°C), and 70% (22–37.5°C). Significantly high concentrations of polyacetylenes (falcarinol, falcarindiol, and falcarindiol-3-acetate) were documented in whole carrots that were refrigerated for 4 months at 1°C [75]. This indicates that polyacetylenes were produced during postharvest storage or there was little degradation in intact carrots after cold storage [75]. In a separate study [76], the level of vitamin C in baby carrots reduced during cold storage in high and moderate O2 conditions but under a low O2 atmosphere, baby carrots retained the highest amount of vitamin C. Freezing also has a negative impact on vitamin C content of carrots as in [78] where a decrease of 4.1% was recorded. Also, prolonged storage duration has been shown to lower the concentration of vitamin C from 15 to 49% [94]. Similarly, Kjellenberg et al. [95] noted that during storage, there is a decrease of glucose and fructose and a development of polyacetylenes, which causes a reduction of soluble sugars. The decreased sugar content is linked to the development of harsh, and oily flavor in carrots during prolonged storage [96].

Significant differences have been observed in the content of main beetroot betcyanins (betanin and isobetanin) during cold storage of red beetroot at 5°C for 196 days [71]. The content of betanin in red beetroot peel decreased in the first 140 days of cold storage and then slightly increased (Table 2). In terms of isobetanin, until 98 days, an increasing trend and afterward, up to 140 days of storage, a light decrease were noticed [71]. In [72] it was shown that after 1 month of cold storage, there was an increase in beetroot water loss that resulted into increased total dry matter content. The total phenolic content of beetroot in cold storage increased after 4 months of storage. In another study, the nitrate content in beetroot increased significantly after 1 month of cold storage [73], and no further significant increase was observed after a cold storage period of 4 months. In [61], there was no significant change in total sugar content of 11 beetroot genotypes stored under optimal cold storage conditions at 1 ± 1°C and 90–95% RH for 7 months. Additionally, beetroots retained their levels of total soluble sugar contents after 1 month of cold storage, and the levels decreased after 4 months of cold storage. Therefore, it is beneficial to use these beetroot genotypes freshly or within the first month of storage, when a high sugar content is desired.

Freshly harvested cassava roots transpire and loose moisture, which reduces their quality during storage. Cassava undergoes postharvest physiological deterioration (PPD) once the roots are separated from the main plant. As a result of this mechanical damage (wounding), the roots respond with a healing mechanism that initiates about 15 min after damage, and fails to switch off in harvested roots [97]. This is observed as a blue/black or brown discoloration of the vascular parenchyma (vascular streaking) within 24–72 h of harvest [20] rendering it unpalatable. As described by Beeching [98, 99], this mechanism involves an oxidative burst of the superoxide radical (O₂−), which is followed by the further production of reactive oxygen species, altered gene expression, and the accumulation of secondary metabolites. This physical and biochemical change is often followed by microbial deterioration and root tissue softening.

Traditional methods have been shown to extend cassava shelf life for few days during which, the cyanide, moisture, and starch content in the root decreases while the ash, sugar, crude fiber as well as the acidic content increases with the length of storage [49, 58, 59]. Generally, temperatures of about 20–30°C and low RH between 65 and 80% encourage deterioration [100]. Therefore, storage of cassava under high relative humidity and limited oxygen conditions, for example, in polyethylene bags, storage boxes, and coating with paraffin wax, can reduce water loss and oxidative stress [101]. Besides, Rickard [102] reported that at 80–90% RH, cassava roots showed a typical wound-healing response with periderm formation occurring in 7–9 days at 35°C and 10–14 days at 25°C.

Prolonged in-ground storage has been shown to increase the size of the root. However, the roots become more woody and fibrous, and with decreased palatability and amount of extractable starch, besides becoming more susceptible to attack by pathogenic microorganisms (Table 2) [49, 58]. Covering cassava roots with paraffin wax by dipping the root in paraffin wax (at a temperature of 55–65°C for a few seconds) after treatment with fungicide has been reported to prolong shelf-life of cassava roots up to 2 months (Table 1) [49, 68].

Cassava roots can also be stored for 2 weeks between 0 and 4°C without any internal deterioration, and after 6.5 months of storage, the part of the root without decay usually is in excellent condition for human consumption [49]. At temperatures above 4°C, roots develop the PPD symptoms more rapidly and have to be discarded after 2 weeks of storage [49]. A study on the effect of the total carotenoids content in cassava root on the reduction and delay of postharvest deterioration showed that cassava roots kept at 10°C and 80% RH can remain fresh till after 2 weeks [100]. Thus, higher carotenoid can reduce or delay the onsets of PPD and extend the shelf life of the root. Wijesinghe and Sarananda [103] reported that freezing of fresh cassava for up to 8 weeks resulted into loss in water and increase in dry matter, that was driven by the high vapor pressure deficit created between the product and the low RH in the refrigerator environment. Consequently, the water stress which remained of acceptable eating quality although none remained as good as the freshly harvested ones.

Physicochemical parameters that determine quality of radish are well maintained in lower storage temperature of 0°C hence it is recommended for extended storage period of radish. Studies by Chandra et al. [90] showed that weight losses of radish roots were remarkably lower (<3%) in radish packed with micro perforated HDPE film in plastic crate and that cured then packed in micro perforated HDPE film in PC while the cured sample maintained its total soluble solids, and flesh and skin firmness. Both samples also recorded lower scores of black spot, surface shrinkage, and fungal infection incidence (Table 2). About 1.3% weight loss of unpacked topped radish root was noted after 9 days of storage at either 5°C or 10°C [104]. However, severe weight loss of radishes (about 52%) was noted just after 3 days of storage when radishes were stored at room temperature (20 ± 2°C) without leaves [91]. In a related study, a weight loss of about 2.5% was noted when whole radishes were stored at 1°C or 5°C for 10 days, whereas this loss reached nearly double when they were stored at 10°C [92]. These results indicate that storage temperature greatly affects the fresh weight retention ability of radish. In another study, the concentration of isothiocyanate sulforaphene and myrosinase activity were measured in two radish cultivars, namely “Chungwoon plus” (CP) and “Taebaek” (TB), during storage at 0°C for 4 months. After the storage period, the sulforaphene concentrations in the CP and TB radish cultivars decreased by 81% and 40%, respectively [89]. Also, the myrosinase activity decreased in both cultivars which subsequently decreased the formation of sulforaphene [89]. Glucosinolates, lipid-soluble vitamins E and K contents, and primary metabolites were measured from topped radish root stored at 1°C for 90 days. The results indicated that the tested storage conditions had no effect on the concentration of aliphatic glucosinolates present in radish [88].

Use of low temperatures (i.e., 2–4°C) and potato sprout inhibitors are the widely used storage treatments on freshly harvested potatoes. Curing potatoes allows the formation of a protective layer (wound periderm) over areas of potato that could have been damaged during harvesting. Curing has been reported to limit weight loss and to prevent the penetration of pathogenic microorganisms. In a study in which the effects of various curing and storage conditions (i.e., duration, temperature, and RH) on the quality of two potato cultivars, “Moonlight” and “Nadine” were investigated, high curing RH (93%) led to significantly lower skin browning, shriveling, and weight loss in both cultivars, and significantly lower incidence of rot in “Nadine” than low curing RH (62%) [105]. A 7 days curing at >90% RH and at least 15°C was recommended.

Sprout inhibitors (e.g., ethylene) and treatments to inhibit microbial establishment on harvested potatoes are reported to be effective but with varied secondary effects on potatoes. Previous studies have reported that ethylene application can either shorten or delay potato dormancy period depending on both treatment duration and concentration [106]. Furthermore, a continuous application of ethylene [69] and/or early application (applied after appearance of first sprouting) [70] is reported to prevent potato sprouting. However, this can also increase the content of reducing sugars (primarily glucose and fructose) in potato, thus limiting its use for processing potatoes. High levels of reducing sugars in processing potatoes causes cold induced sweetness [107] that is responsible for the dark brown color on processed potato products that gives a bitter taste [108]. In a separate study, a single treatment with HPP or CIPC resulted, after 6 months of storage at 10 ± 1°C, in sprouting rates of 61% and 58%, respectively, vs. 87% in the untreated control [54]. From preliminary experiments in [48], potatoes exposed to UV-C light at five different intensities (0.0, 3.4, 7.1, 10.5, and 13.6 kJ m⁻²) and stored in the dark at 20°C and 80% RH for 40 days showed reduction in sprout length and development up to 20 d. However, this effect diminished during storage. Also in [47], potatoes exposed to gamma Irradiation (0, 50, 100, and 150 Gy levels) on different dates (10, 30, and 50 days after harvest) were studied during prolonged storage at 8 and 16°C. Results indicated that indicated that early and higher irradiation levels significantly decreased sprouting, percent weight loss and specific gravity of potato. However, the loss of ascorbic acid, and reducing and non-reducing sugars significantly increased by delay in irradiation whereas the sugars and ascorbic acid content was decreased by irradiation. Higher storage temperature (16°C) caused greater loss of ascorbic acid. A delay in irradiation and storage at high temperature was not recommended [47]. A study in [83] showed that potatoes buried at deeper depths (overground/pit storage) accumulated a lot of respiratory heat. This heat has been reported to promote potato sprouts [84]. Sprouting results in remobilization of storage compounds mainly starch and proteins as sprout tissue is built from the potato reserves. This increases the rate of respiration as well as evaporation [109], and consequently weight loss. Also, vitamin C is adversely affected by sprouting [47]. Sprouting and sprout growth contributes to formation of toxic glycoalkaloids (TGA). This involves the buildup of chlorophyll beneath the peel, a process known as “greening” [110]. Greening in potatoes is associated with the TGA solanine accumulation. In a study where six potato cultivars were analyzed for TGA content after 6 and 14 weeks of storage at either 10 or 4°C, results indicated that the exposure of some cultivars, to low temperatures within 2 weeks of harvest resulted in a relatively rapid accumulation of TGA to levels close to or exceeding the recommended safe maximum level of 200 mg of TGA per kilogram of fresh weight [86].

Nevertheless, storage of potatoes at low temperatures has been shown to significantly decreases the content of vitamin C [87, 111, 112], although in [82] it was noted that even with the decrease in vitamin C, a significant amount was still retained. The decrease in vitamin C content is attributed to its use, in the potato, as an antioxidant compound in response to oxidative stress caused by low temperature storage. In [87], an evaluation for antioxidant parameters of potatoes stored at room temperature, 15°C and 4°C for 90 days showed that all phenolic acid content increased with storage time, except para-coumaric acid which decreased at 4°C. Similarly, a separate study showed an increase in chlorogenic acid, caffeic acid, and sinapic acid content during storage of potatoes for 90 days at 3°C [81].

High temperatures can also influence respiration rate, development of decay causing organisms, greening, and shrinkage in stored potatoes [113]. A study on the effect of three store types (ambient charcoal-cooled, traditional grass thatched hut and diffused light store [DLS]) on the quality of potatoes stored for 56 days at temperatures 9.7–19.4°C and RH 65–93%, results showed increased weight loss, greening ,and shrinkage in potatoes stored in DLS (temperature 16.15–19.35°C; RH 65–89%) [85]. Additionally, there was a significant increase in the reducing sugar content. The starch content in potato samples from the three stores decreased with increasing storage time.

6. Conclusion

The quality of root vegetables deteriorate gradually during storage in response to several endogenic factors and environmental conditions. Processes such as transpiration, respiration, senescence, and maturation, as well as attacks by pathogens lead not only to quantitative and quality losses. The range of pre-storage treatment methods and storage conditions that are commonly used have been reported to be safe and effective at mitigating several postharvest deterioration in root vegetable. Nevertheless, they can also influence the quality of freshly harvested root vegetables. Therefore, choice of appropriate treatments and storage conditions that do not significantly decrease the quality of stored produce should be considered.

Acknowledgments

Authors thank the Kenya Climate Smart Agriculture Project (KCSAP) under Project Title “Climate-smart adaptive technologies for sustainable production and postharvest management and utilization of ware potato in Kenya” that provided the background and interest on storage of root vegetables. Dr. Viola Kosgei, Moi University (Kenya), Department of Chemistry and Biochemistry, for the discussions on carotenoids.

Conflict of interest

The authors declare no conflict of interest.