State of Knowledge Report

State of Knowledge on Quality Traits of Fresh, Boiled & Fried Sweetpotato

Nairobi, Kenya, December 2018

Linly BANDA, CIP, Nairobi, Kenya

Tawanda MUZHINGI, CIP, Nairobi,Kenya

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This report has been written in the framework of RTBfoods project.

To be cited as:

Linly BANDA, Tawanda MUZHINGI. State of Knowledge on Quality Traits of Fresh, Boiled & Fried Sweetpotato. 2018. Nairobi (Kenya) RTBfoods Project Report, 15 p.

Image cover page © Dufour D. for RTBfoods.

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CONTENTS

TABLE OF CONTENTS

Table of Contents ................................................................................................................................ 2

1 Composition and structure of Sweet Potato .................................................................................. 3

1.1 Composition .......................................................................................................................... 3

1.2 Structure ............................................................................................................................... 4

2 Processing conditions .................................................................................................................. 6

3 Sensory analysis and consumer preference ................................................................................. 7

4 Product characterization and relationship with sensory evaluation ............................................. 10

4.1 Evolution of composition and structure with processing ...................................................... 10

4.2 Instrumental Texture assessment and relationship with sensory evaluation ........................ 11

4.3 Relationship between composition and sensory evaluation ................................................. 12

5 References ................................................................................................................................. 13

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1 COMPOSITION AND STRUCTURE OF SWEET POTATO

1.1 Composition Sweet potato (Ipomoea batatas) is an important root crop consumed in much of Sub Saharan Africa (SSA) (Stathers et al., 2013). It has great potential to fight malnutrition and enhance food security. The fleshy roots are the most consumed part and have a high nutritional value; they provide starch, minerals, dietary fibre, vitamins, carotenoids and antioxidants (table 1). Sweet potatoes have a lower glycemic index of 63–66 as compared to potatoes (65–101), making them a good option for diabetics (Selvakumaran et al., 2017).

Table 1. Chemical composition and nutritional value of raw sweet potato root per 100g, average values (Source: USDA, 2009)

Nutrient Value per 100g Water 77.28 g Dry matter 25 g Energy 359 KJ Protein 1.57 g Fat (total lipid) 0.05 g Ash 0.99 g Carbohydrate 20.12 g Dietary fibre 3 g Calcium (Ca) 30 g Iron (Fe) 0.61 mg Magnesium (Mg) 25 mg Phosphorus (P) 47 mg Potassium (K) 337 mg Sodium (Na) 55 mg Vitamin C 2.40 mg Pantonthetic acid 0.8 mg Vitamin B6 0.21 mg Vitamin A 14 187 IU (8509 µg)

The sweet potato root is composed mainly of carbohydrates (80 to 90% of the dry matter content), with starch being the major component, making up to 80%. Dry matter (DM) content varies significantly amongst varieties and from one location to another. Kagimbo et al., 2017 reported a DM content range of 27.40% to 43.50% for ninety two germplasm studied in Tanzania. Of these, seventy two were local varieties and twenty were from CIP, Peru. DM content of 23.5 – 35.2% was reported for Ugandan varieties in a study carried out to determine the inheritance of DM content (Shumbusha et al., 2014). Tumwegamire et al., 2011 evaluated ninety East African varieties for DM content and reported a variation of 19.4 – 38.3%. DM content is commonly determined through the oven drying method (Kagimbo et al., 2017 and Shumbusha et al., 2014), which relies on heating a known weight of sample until constant weight is reached. The specific gravity method (weight in air – weight under water) and the freeze drying method (Sato et al., 2018) are alternatively used.

Sweet potato is a good source of polyphenols particularly phenolic acids and anthocyanins (cyanidin and peonidin), which are responsible for the purple colour of purple fleshed varieties (PFSP). These are quantified spectrophotometrically or by high pressure liquid chromatography (HPLC). Orange fleshed sweet potato (OFSP) varieties are an excellent source of carotenoids, most importantly β carotene which has 100% pro vitamin A activity. Deep orange varieties can contain up to 20 000 μg of β carotene per 100 g fresh root weight. OFSP has emerged as an important crop for alleviating Vitamin A deficiency in children and pregnant women in SSA.

Various methods are employed in the determination of the composition of sweet potato; the most popular ones being analytical biochemical procedures such as the Kjeldahl analysis for protein determination, Soxhlet extraction for fats, Atomic Absorption Spectrophotometry (AAS) for mineral

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composition, microscopy for starch composition. Near-InfraRed Spectroscopy (NIRS) has emerged as an alternative method for the quick determination of various chemical constituents such as protein, mineral and vitamin content (Tumwegamire et al., 2011).

1.2 Structure The enlarged, starchy, tuberous sweetpotato root generally has a smooth surface, but it is common for some varieties to show irregular defects (figure 1a). The roots vary in shape (fusiform to oblong or pointed oval) and size according to the genotype and environmental influence. The skin and flesh colours range from white, cream, yellow, orange and purple.

Figure 1. Structure of (a), sweet potato roots and (b), transverse section of sweet potato root.

The transverse section of a sweet potato root (figure 1b) reveals the skin (periderm), cortex (cortical parenchyma), cambium ring and the central parenchyma. The cortex is filled with large starch cells (figure 2), whose precise structure varies from variety to variety. In a study by Tumuhimbise and others, 2009, the storage parenchyma of four raw OFSP varieties was found to be composed of polyhedral cells with a diameter of approximately 98 μm. The cell wall of the raw OFSP was smooth and intact. In crude sweet potato, thickness (average 2.5 μm) of cell walls, which show a very thick middle lamella, is two times that of potato cell walls (Valetudie et al., 1999). The parenchyma of the sweet potato contained several intercellular spaces which were approximately 6.8 μm in size.

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Figure 2. Micrographs (a–d) of storage parenchyma tissue of raw OFSP for varieties; Ejumula, SPK004/6/6, SPK004 and SPK004/6, respectively, stained with Periodic Acid Schiffs (PAS), Bar=50 μm (Tumuhimbise et al., 2009).

The parenchyma cells are filled with starch granules that are round, bell shaped or polygonal as observed under the scanning electron microscope (SEM) figure 3, ranging in size from 2 – 50 μm (Kitahara et al., 2017), although 100µm granules have been reported. The wide angle X ray diffraction pattern of the granules reveals type A and C crystalline structure.

Figure 3. Photomicrographs of native sweet potato starch as observed in (a) an optical microscope and (b) SEM (Rocha et al., 2010).

The sweetpotato starch is made up of 20 – 30% amylose to 70 – 80% amylopectin, although up to 38% amylose has been reported (Zhu and Wang, 2014). The amylose content and amylopectin molecular structure (branch chain length and pattern) predominantly contributes to the structural and functional properties of both potato and sweet potato starch (Zhu and Wang, 2014). Most studies have reported a narrow range of variation in sweet potato amylose content, 18.6 – 20.7% in seven Japanese varieties (Kitahara et al., 2017), 16.5 – 18.5% in four Thailand varieties (Soison et al., 2015) although China, which accounts for about 90% of worldwide sweet potato production with an annual production of 117 million tons has a wide range of varieties and an amylose variation of 13.33 to 26.83% in eleven commercial cultivars (Abegunde et al., 2013). The amylose content has also been reported to vary depending on the flesh colour, with orange fleshed varieties exhibiting higher values than purple and pale fleshed varieties (Noda et al., 1997, Jangchud et al., 2003, Abegunde et al., 2013 and Soison et al., 2015).

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2 PROCESSING CONDITIONS There has been a great improvement in sweet potato research and development, and this has transformed the crop from a simple staple food to an important commercial crop with multiple uses such as a snack, ingredient in various foods and complementary vegetable. The common processing methods are boiling, steaming, frying, roasting and baking, each cooking method leads to different changes in the quality attributes of sweet potato. This report will focus on boiled and fried sweet potato. In both cases, the initial step is sorting, to select only disease free roots, followed by thorough washing to remove all soil and dirt from the surface of the root. Commercially, disinfection is carried out by soaking in a hypochlorite solution with warm water, while this is not a common practice in home cooking. The cleaned roots are peeled using mechanical abrasion, knives, steam or treatment with lye (sodium hydroxide solution). The use of steam is associated with a partial loss in cell structure through an increase of pressure inside the root, whereas lye peeling only separates the skin by breaking hypodermal and epidermal cell walls (Oladejo et al., 2014). Abrasive/mechanical peeling of sweet potato is difficult to achieve due to the irregular shapes of the sweet potato, thus resulting in high losses. At the home level or small processing companies, the use of knives is very common, although its time consuming and also results in significant losses. Once peeled, the sweet potato roots are cut into smaller pieces and cooked. In other processing methods, peeling is only done after cooking.

Boiled sweet potato products

In Sub Saharan Africa, sweet potatoes are traditionally prepared by washing and boiling at 95 – 100°C for 25 – 40 minutes, depending on the cultivar, while submerged or partly submerged in water (Oke and Workneh, 2013). The cooked roots are peeled and consumed as a snack, or incorporated into other dishes such as stews and pies. In some communities, the roots are washed and sun dried to increase shelf life, then boiled after reconstituting by adding water. OFSP varieties have been widely promoted in SSA to help combat vitamin A deficiency but are not preferred in some communities because of the moistness/softness or tendency to become soggy after boiling. This has led to the emergence of several value addition processes in which the OFSP is formulated into more acceptable and shelf stable products. The OFSP is boiled/steamed and mashed into a puree that is incorporated into a wide range of products such as juice, jam, baby foods or as a substitute for wheat flour in making bread, biscuits, and cakes (Magnaghi et al., 2015). Besides OFSP, other flesh colour varieties have also been processed into puree.

Fried sweet potato products

Frying enhances the flavour and reduces the moisture content of sweetpotato roots, thus can results in more shelf stable products. Fried sweetpotato products include French fries, crisps (chips), wedges and other products in which puree or flour is used as an ingredient, such mandazi (fried doughnut) and chapatti (flat, unleavened bread) (Wheatley and Loechl, 2008), common in East Africa. French fries, wedges and crisps are the most popular fried products on the market due to the convenience and storability. These are potential nutritional snack foods when produced from cultivars high in β carotene and ascorbic acid (Ali et al., 2012).

The processing begins with selection of roots, washing, disinfection and peeling. It is common to blanch the peeled sweet potatoes before further processing, this is to improve the appearance and colour by minimizing browning reactions during frying, and also to reduce oil uptake and improve the firmness of the fried product. The sweet potatoes are dried in hot or cold air so as to reduce the moisture content prior to frying. The release of too much water into the oil during frying leads to quick oxidation and shortens the life span of the frying oil. Pre drying at 70°C for 30 minutes without blanching has been reported to enhance the sensory characteristics of crisps/chips, while blanching followed by pre drying enhances the texture and brittleness of crisps but increases oil absorption (Abdulla et al., 2014). Finally, deep frying is carried out at high temperatures so as to limit the rate of oil uptake and to produce a crispy external bite in fries, or a crispy, brittle chip, while slightly lower temperatures are employed for wedges, which are generally softer. For French fries, roots are washed, peeled, cut into the desired shape, usually strips, blanched, dried and deep fried in various vegetable oils such as sunflower and

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palm oil (Fetuga et al., 2014), soya bean oil (Ali et al., 2012) at 160 – 185°C for 5 to 10 minutes until a light brown colour is achieved and the oil stops bubbling.

Figure 4. Process flow chart for making sweet potato crisps. Adapted from Fetuga et al 2014.

Crisps are peeled and sliced into 1.2mm – 1.5mm discs which are blanched in 1% w/v NaCl or citric acid for a few minutes, drained and fried in various vegetable oils at 140 to 180°C over a period of 3 to 12 minutes (Fetuga et al., 2014).

3 SENSORY ANALYSIS AND CONSUMER PREFERENCE

The success of a newly introduced sweet potato variety or processed food depends largely on its acceptance by the target consumers. Cooked products present a large diversity of structural, sensory and functional properties according to the method and intensity of cooking-processing. Sensory analysis aims to describe a product’s taste, texture (mouth-feel), flavour, colour and overall appearance using well understood descriptors. This is achieved by using a trained panel of individuals to come up and agree on a set of descriptors (lexicon). Consumer acceptability is based on the product’s sensory and utilisation (such as cooking quality) characteristics and preference differs greatly between communities (Tomlins et al., 2004). An acceptability survey is carried out by assessing overall liking of a product using a hedonic scale which can be from 1 (dislike extremely) to 9 (like extremely). This is achieved by interviewing a large number of untrained individuals from the community where the product is being introduced. Consumer preference has been important in the adoption of β-carotene-rich orange flesh sweet potato (OFSP) in Sub-Saharan Africa, which is important in the battle against vitamin A deficiency. Sensory evaluations of sweet potato in East Africa suggest that profiles for traditional cream-fleshed and new OFSP cultivars differ substantially over the sensory spectrum (Tomlins et al., 2012) and this can have a negative impact on the uptake of nutritionally superior OFSP. Consequently, eating quality, predominantly flavor and texture, must be taken into account, alongside nutritional quality for the development of successful cultivars (Low et al., 2015 and Lagerkvist et al., 2016).

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Texture is an important sensory attribute that determines consumer acceptance. Texture descriptors include grainy/smooth, moist/dry, soft/hard (Tomlins et al., 2004). It has been reported that communities in Eastern Africa generally prefer firm products with high dry matter (Tomlins et al., 2004) as compared to Southern African communities. Flavour can include aroma (odour) and taste (sweetness) (Laurie et al., 2012). Sweetness is generally accepted in Southern Africa, where sweet potatoes are mostly consumed as a snack whereas less sweet products are preferred in Western Africa for consumption as a staple. Colour also plays a major part in consumer acceptance, with some communities preferring white fleshed varieties (WFSP) over the OFSP. Boiled sweet potatoes usually have a higher sweetness compared to fried products, such as crisps.

Table 2: Lexicon for descriptive sensory analysis of boiled sweet potato (adapted from Leighton et al., 2010)

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Leighton and others, 2010, studied the sensory attributes of some boiled OFSP and WFSP varieties from South Africa (table 2). Their findings reveal that OFSP had an earthy aroma, associated with damp soil and slightly undercooked potatoes, and a less intense sweet potato aroma, in comparison to the WFSP. OFSP was also found to be sweeter, with butternut/carrot/pumpkin flavour characteristics. In terms of texture, the OFSP cultivars had a less moist texture on appearance and first bite, whereas WFSP had the most moist and fibrous texture of the five cultivars tested. OFSP was characterized by a denser and more adhesive texture than that of WFSP. In terms of colour, the Resisto cultivar had the darkest orange colour, which is an indication of high beta-carotene content.

Table 3. Sensory attributes with definitions and techniques for evaluation in sweetpotato fries (adapted from Sato et al., 2018)

French fries and crisps are fried products with completely different sensory attributes compared to boiled products. There are several attributes that are important for acceptance of sweetpotato fries (table 3). Fries have an outer crispness and an inner moistness that is typically preferred by consumers whereas crisps are expected to be brittle-crunchy. Colour (appearance) is also an important sensory attribute, intense browning is not acceptable, but only a slight brown/gold colouration. Another key factor in fried products acceptance is the oil content; consumers have become conscious of the effects of diet on health so less oily fries and crisps are acceptable.

In a study by Ali et al., 2012 in Nigeria, sweet potato fries and crisps of three cultivars were prepared and subjected to sensory evaluation by a 45-member, semi- trained panel made up of individuals who were familiar with the quality attributes of the products. Panellists evaluated coded samples presented in a random order for appearance, taste, texture and overall accept- ability using a 5-point hedonic scale where 1 – dislike extremely and 5 – like extremely). For the sensory attributes like appearance, taste and texture, the crisps scored higher than French fries. From this study, consumers enjoyed a crunchy texture as compared to the soft texture of French fries. Crisps had a higher acceptability of 3.5 whereas French fries had a mean acceptability score of 2.8.

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4 PRODUCT CHARACTERIZATION AND RELATIONSHIP WITH SENSORY EVALUATION

4.1 Evolution of composition and structure with processing

Cooking of foods leads to the improvement of microbiological and organoleptic quality, destroys toxins and antinutritional factors, increases digestibility and nutrients bioavailability (Ikanone and Oyekan, 2014). Cooking can be detrimental to the micronutrients but beneficial to macronutrients. Structural changes in sweet potato processing are mostly due to changes in starch, since it is the major dry matter component. Varieties with high DM content are known to develop a firm and mealy texture after cooking, although those with low DM content have a soggy texture after cooking (Sato et al., 2017).

Starch undergoes numerous physical changes during heating and cooling and these changes are influenced by starch properties such as water content, presence of phosphorus and lipids, amylose-amylopectin ratio, structure and packaging in the granules, and the cooking environment such as heating rates (Zhu and Wang, 2014). During heating in water, at 52 – 72°C starch granules absorb water and swell, while starch components leach out and solubilise. The sweet potato contains a heat-activated (60 – 90°C) β amylase that breaks down starch through hydrolysis of the second α-1,4-glycosidic bond, releasing maltose (Binner et al., 2000). This results in a non mealy texture and increased sweetness. Maltose is hardly detected in raw fresh sweet potatoes, but accumulates in cooked products to a concentration about ten times higher than fructose, glucose and sucrose (Kitahara et al., 2017). Continued heating and water absorption leads to gelatinization and an increase in viscosity. Different flesh colours have been reported to have variations in gelatinization properties, with white flesh reportedly having a lower gelatinization value. Gelatinization is also influenced by the mineral composition and affects the stickiness or tackiness during French fry processing. Microscopy studies conducted immediately after boiling and cooling sweet potato roots revealed that in parenchyma cells, all starch granules lost their shape, regardless of the cooking method, resulting in a sponge-like organization of the starch granule, variable from one cell to the next (Valetudie et al., 2009)

Changes to the cell wall structure also greatly affect the structure of the product. The primary cell wall is largely composed of polysaccharides (cellulose, hemicelluloses and pectins), enzymes and structural proteins (Micheli, 2001). The pectin monomers (galacturonans) are methyl esterified or acetylated. Pectin methylesterases (PMEs) catalyse the demethylesterification, resulting in acidic pectins and methanol. Hydrolysis of the cell wall during cooking has an impact on the resultant texture. PMEs can hydrolyse homogalacturonans in a random fashion, and contribute to cell wall loosening (softening). Linear hydrolysis pattern results in the release of blocks of free carboxyl groups that interact with calcium ions, this contributes to cell wall stiffening (hardening). The degree of pectin methylation and the activity of pectin methyl esterases can thus be used for prediction of cooked texture of sweet potato. Structural modifications of the cell wall depend greatly on the method of cooking (table 4).

Table 4. Variation of some physicochemical characteristics of cooked sweetpotato (adapted from Valetudie et al., 1999)

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The findings by Valetudie and co-workers, 1999, reveal that starch hydrolysis was more advanced in precooked roots cooked as whole, cell damage was lower in precooked roots and the level of pectin solubilised and released during cooking was much higher in cooked sliced roots compared to whole roots, which may be attributed to a larger surface area. The intercellular spaces increased by 25% in boiled sweetpotato, allowing the escape of gelatinized starch into the spaces, whereas in precooked boiled roots, there was no gelatinized starch in the intercellular spaces.

During cooking, there is also an appreciable loss of nutrients such as β carotene, antioxidants and minerals during thermal processing and the loss percentage depends on the cooking time and method. It was observed by Ikanone and co-workers, 2014, that boiling retains more carbohydrate than frying, while frying retains more vitamin C and minerals than boiling (table 5).

Table 5. Effect of cooking on the composition of sweet potato roots. Adapted from Ikanone and Oyekan, 2014.

Composition Raw, fresh roots Boiled roots Fried roots Carbohydrate (mg/100ml)

3.70 3.60 3.48

Vitamin C (mg/100ml) 70.69 19.53 27.64 Iron (ppm) 32.67 17.00 18.00 Zinc (ppm) 43.17 8.33 29.33 Magnesium (ppm) 107.67 105.67 102.67 Sodium 2.00 2.33 2.67 Calcium 28.67 24.67 26.67 Copper 1.00 1.00 0.67

*All values are expressed as a mean for three determinations

4.2 Instrumental Texture assessment and relationship with sensory evaluation

Of the various sensory attributes that are important for consumer acceptance, texture is the key determinant. Sweetpotatoes can be classified into firm, dry and mealy or moist, soft varieties based on the cooked texture. In addition to the sensory evaluation procedure, texture can be assessed using instrumental procedures, specifically, texture analysers. A texture analyser is an instrument that measures the force required to penetrate a sample as a measure of hardness or softness. The harder the sample is, the more force is required to penetrate or cut through. This gives an estimation of how the consumer perceives the texture upon eating the product. Texture analysis can be done in two ways, uniaxial compression to determine the hardness/ softness of a sample or texture profile assessment (TPA), in which double compression is done so as to mimic chewing.

In a study by Laurie et al., 2012, twelve boiled South African varieties were subjected to both sensory evaluation and instrumental texture analysis. For the sensory texture, the panel was asked to score from 0 to 100 for the attributes wateriness (0 –dry, 100 – moist), graininess (0 – smooth, 100 – grainy) and firmness (0 – extremely soft, 100 – firm). The instrumental texture was analysed on a universal testing machine (Model 3344, Instron, Norwood, Massachussets, USA) fitted with a BlueHill software. A 2cm diameter sample was compressed using a 2cm diameter plate, to 50% of its original height in two cycles at a cross head speed of 200 mm min-1. The results of the sensory and instrumental analysis showed significant correlation; instrumental firmness was highly correlated with sensory texture, wateriness had a significant negative correlation with instrumental firmness. This study concluded that taking only instrumental firmness measurements is reliable and will suffice.

Significant correlation between instrumental and sensory texture of sweet potato fries prepared from sixteen genotypes was reported by Sato and co-workers, 2018. Instrumental texture was determined by measuring peak force and overall hardness measurements on a TA.XT2Plus Texture analysis machine equipped with a 2mm cylinder puncture probe and a French fry rig respectively. Peak force

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was determined as the force (N) required to penetrate a French fry using a 5kg load cell set at 3mm/sec pretest and 10mm/sec posttest speed. Overall hardness was determined as area under the curve after using a 50kg load cell at 3mm/sec test speed. The results of the texture measurements correlated positively with each other, thus one measurement will suffice. The instrumental texture also correlated with the sensory derived attributes (table 6).

Table 6. Correlation coefficients (r) between instrumental measurement and sensory attributes of sweetpotato French fries produced from 16 genotypes (Sato et al., 2018).

4.3 Relationship between composition and sensory evaluation

The composition of sweet potatoes, and the cooking method employed, has a profound effect on the sensory attributes of the resulting product. Sensory evaluation assists the researcher to understand how certain chemical or biophysical traits lead to the development of specific sensory attributes. This information is particularly important for selection of new varieties for specific uses; the sensory profile can be predicted from the compositional analysis data.

Table 7. Relationship of composition and sensory attributes of sweetpotato Composition Sensory attribute Relationship Dry matter Texture High dry matter is associated with increased firmness

(Abong et al., 2016, Sato et al., 2018) Alcohol insoluble solids (AIS)

Texture High AIS is positively correlated with overall hardness and crispness(Sato et al., 2018)

Ratio of amylose and amylopectin

Texture Amylose content above 8.5% leads to waxy texture

Total sugar Appearance Reducing sugars undergo non enzymatic browning especially during frying, resulting in darkening (Abong et al., 2016).

Maltose Taste High maltose concentration leads to increased sweetness (Binner et al., 2000, Kitahara et al., 2017)

Amino acids Aroma Gives the typical sweet potato aroma B carotene Colour As the amount of β carotene increases, the orange

colour in OFSP becomes darker. Anthocyanins Colour Responsible for purple colour in purple flesh varieties

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