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Sweet potato (Ipomoea batatas) by-products

Datasheet

Description
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Common names 

Sweet potato, sweetpotato, sweetpotatoes [English]; patate douce [French]; boniato, batata, chaco, papa dulce, camote [Spain] batata-doce, batata-da-terra, batata-da-ilha, jatica, jetica [Portuguese]; patats [Afrikaans]; Süßkartoffel, Batate, Weiße Kartoffel, Knollenwinde [German]; ubi jalar, ketela rambat [Indonesian]; patata dolce, patata americana [Italian]; kamote [Tagalog]; khoai lang [Vietnamese]; 番薯 [Chinese]; શક્કરીયાં [Gujarati]; शकरकन्द [Hindi]; サツマイモ [Japanese]; 고구마 [Korean]; रताळे [Marathi]; सखरखण्ड [Nepali]; Бата́т, сла́дкий карто́фель [Russian]; வற்றாளை [Tamil]; มันเทศ [Thai]

Product names:

  • Sweet potato wastes, sweet potato cannery wastes, sweet potato peels, sweet potato peelings, sweet potato trimmings
  • Sweet potato distillery by-product, sweet potato stillage waste, sweet potato distillers
  • Sweet potato starch waste, sweet potato pulp
Synonyms 

Convolvulus batatas L., Ipomoea apiculata M. Martens & Galeotti

Description 

Sweet potato tubers (Ipomoea batatas (L.) Lam.) are a staple food, an alternative food, or an animal feed ingredient in many countries. Sweet potatoes are consumed fresh, canned or processed (purée, crisps, starch...), and they are used for the production of drinks (alcoholic or not) and bioethanol. Their processing results in numerous by-products which are extremely variable. Like the by-products of the potato (see the Potato by-products datasheet), sweet potato by-products can be raw or result from processes involving heat.

  • Sweet potato peelings / sweet potato peels. This is a sticky, slurry-like product.
  • Sweet potato pulp from starch extraction.
  • Cannery waste are mixtures of peels and chunks of sweet potatoes resulting from cannery processing. They may be raw or heat-treated.
  • Other by-products of food industries include (culled) fries, crisps, flakes, hash browns, crowns, batter, crumbles, nubbins, etc. These products have usually undergone a heating process.
  • By-products of distillery include various slurries and solubles.
  • Culled tubers, or tubers and parts of tubers resulting from screening and other early processes are described in the Sweet potato, tubers datasheet.

Despite the widespread use of sweet potato in food industries and the good nutritional value of sweet potato by-products, information about their use in animal feeding remains scarce. Most of sweet potato by-products are fed primarily as a source of energy, as they contain large amounts of readily digestible carbohydrates. Some are rich in protein (distillery by-products). The main issue with sweet potato by-products is their high moisture content, which limits the distance that they can be hauled as well as their shelf life, as moulds and bacteria develop readily, impairing their safety as feed ingredients (Poore et al., 2000). Drying appears generally too cost-prohibitive for the dried products to be economically profitable (EPA, 1974). Sweet potato peels are the most commonly studied product for livestock feeding purposes, particularly in Sub-saharan Africa and Asia. Some sweet potato by-products are mentioned in the scientific literature under the generic name of "sweet potato waste". Information about other by-products is very limited.

Distribution 

Sweet potato by-products are present wherever sweet potatoes are produced or processed. Estimating the amount of sweet potato by-products is difficult, due to the wide variations in the way sweet potatoes are used. In 2018, the world production of sweet potatoes was 92 million t, most of it being produced in Asia (66%) and Africa (28%) (FAO, 2020). In China, which is the main producer of sweet potatoes, consumption has changed in the past decades. In the 1950-1970, sweet potatoes were mostly eaten fresh (50%) or used for feed (30%), with about 10% used for processing. In 2015, 50% are used for processing (about 25 million t), 30% are consumed fresh, and 10% are fed to livestock. In Africa, sweet potatoes remain largely a food and feed product, and processing is less developed (Ma, 2019).

Processes 

Peeling

Peeling is the first step of sweet potato processing. The amount and quality of the resulting sweet potato peelings depend on many factors including tuber size, tuber maturity, peeling time and peeling process. Physical methods using abrasing rollers or hand peeling result in high peel losses (up to 30-60%). This make these processes unsuitable for industrial production, though they can be used in smaller facilities. Chemical processing using lye is more efficient and has been used extensively, but it is no longer a common method in the industry due to issues with equipment corrosion and waste disposal. The most common method in industrial facilities is steam peeling. Steam and pressure are applied a few seconds, which increases moisture and gelatinizes the starch contained in the peels. One benefit of steam peeling is that it has a sanitation effect, anti-microbial or bacteriostatic (EPA, 1974; Truong et al., 2018; Button, 2015; Bouwkamp, 1985).

Starch extraction

In the starch extraction process, the tubers are ground in limewater and the extracted starch is separated from the pulp by washing over a series of screens, bleaching with sodium hypochlorite, and then settling by gravity or centrifugation. The starch can be stored wet or dried by natural or artificial methods (Truong et al., 2018). Starch extraction yield varies from 12 to 28% and depends on many factors, including the process, the variety and environmental conditions, which in turn influences the starch content of the residual pulp (Rahman et al., 2003).

Distillery by-products

Condensed distillers solubles are produced during the production of shochu, a traditional Japanese hard liquor obtained by distillation of sweet potato (Kamiya et al., 2017). However, this product has a high moisture content and is very acid, and is thus difficult to use in animal feeding (Woolfe, 1992).

Other by-products

The production of many sweet potato-based food products, which often involves a heating process (cooking, blanching, frying), results in by-products such as culled fries, culled crisps, hash browns, crowns, batter, crumbles, nubbins etc. These by-products may contain additional oil, which increases their energy content, and other ingredients like salt. The heating process results in gelatinized, readily digestible starch (Bouwkamp, 1985). Information about the use of these by-products in animal feeding is inexistent.

Environmental impact 

Using sweet potato by-products in animal feeding may help to alleviate their environmental burden, as these products are often disposed of in landfills or bodies of water (Akoetey et al., 2017). For example, sweet potato starch production is a major environmental concern in China and Japan as about 6 m3 of wastewater with a high chemical oxygen demand are yielded per ton of sweet potato tuber processed. Disposal of this water is problematic in that it has a high chemical oxygen demand (Xu et al., 2014).

Nutritional aspects
Nutritional attributes 

Sweet potato peelings

Sweet potato peelings have a high moisture content (> 75%) and have a slurry-like appareance when fresh. They contain little protein (about 6% DM), crude fibre (4% DM), and lipids (4%). Depending on the efficiency of the peeling process, they contain variable amounts of fibre, sugars, starch, and pectins. The latter two components can be extracted for industrial purposes (Hamidon et al., 2017; Uzochukwu et al., 2020).

Starch extraction by-products

Sweet potato pulp resulting from starch extraction is poor in protein (< 4% DM) and fat (< 1.5% DM), rich in starch (44-45% and up to 55-65% DM) and pectins (41% DM), with variable amounts of ash (2-10% DM) (Bouwkamp, 1985; Takamine et al., 2019). The amount of fibre seems extremely variable. One source gives relatively low values (2-14% DM, type of fibre unknown; Bouwkamp, 1985) but other authors gives much higher values (NDF 59% DM, ADF 35.9% DM, lignin 3% DM for Takamine et al., 2019; cellulose 44.6% DM, Lee et al., 2004). Sweet potato pulp can be dried or pressed to be directly fed to livestock as an energy feed (Bouwkamp, 1985).

Waste water from sweet potato starch production contains sugars and soluble proteins. Experiments in United States and Japan have shown that treating waste water with acid and heat coagulated these proteins, resulting in a product containing about 50% DM of protein (Dawson et al., 1951; Woolfe, 1992).

Distillery by-products

Condensed distillers solubles resulting from the production of shochu liquor is a syrupy product rich in protein (20-26% DM), sugars (27% DM) and ash (16% DM). It is a good source of minerals (K, Cl, S, P) (Kamiya et al., 2013; Kamiya et al., 2017; Woolfe, 1992). In Brazil, a slurry resulting from ethanol production contained about 18% DM of protein (Rodrigues et al., 2012).

Potential constraints 

Tannins and antinutritional factors

Sweet potato peelings contain phenolic compounds ranging from 1-6 gallic acid equivalent /g DM (Akoetey et al., 2017). The presence of protease inhibitors has been mentioned as a cause for the lower than expected nutritional values of sweet potato wastes (Zhang et al., 2019).

Toxicity

Tubers that are mouldy, rotten, or injured can be contaminated with fungi that produce toxic metabolites (ipomeamarone, 4-ipomeanol) that may cause serious health problems, particularly respiratory ones, in livestock. It is thus recommended to not distribute mouldy sweet potato by-products to farm animals (Wilson, 1973; Poore et al., 2000).

Acidosis and laminitis

Sweet potato by-products rich in fermentescible carbohydrates must be introduced into the diet progressively and well supplemented in forage and protein in order to avoid acidosis and laminitis (MSU, 2010).

Teeth damage

Sweet potato cannery wastes have a low pH and a high lactic acid content that can severely damage cattle teeth, by eroding enamel, releasing calcium, and blackening incisors. This can be solved by increasing the pH above pH 4 before feeding the animals (Poore et al., 2000; MSU, 2010)

Ruminants 

Sweet potato peelings and wastes

Sweet potato by-products are generally a good source of readily digestible carbohydrates for ruminants.

Cattle

Studies carried out in the United States before 1970 concluded that sweet potato by-products could be fed to cattle but that drying costs were too expensive to make it profitable. Direct feeding of peelings obtained with a low-water method was possible if they were mixed with trim wastes, and allowed to ferment for 24 hours to reduce pH (EPA, 1974).

Sheep and goats

Sweet potato peelings were offered to West African Dwarf bucks at 3% of their body weight (DM basis). The animals fed on sweet potato peelings had lower weight gain than those fed on yam and cassava peelings but higher than those fed on ripe plantain peels. Feed intake was also lower but feed efficiency was similar to that of yam peels (Omeruo et al., 2016). West African Dwarf Goats (8 kg) could be fed a diet comprising 45% concentrate and a mixture of sun-dried sweet potato peels (20-30%) and cashew nut shells (25-35%) (Okoruwa et al., 2015).

Distillers solubles

Cattle

In Japan, condensed distillers solubles from shochu liquor production were fed to Japanese black steers. The concentrate and TDN intake exhibited high values in steers fed diets containing 10% and 20% solubles (DM basis). The solubles had no effect on N efficiency and they increased feed DM digestibility (Kamiya et al., 2013). This product could partly replace (up to 30%) a commercial concentrate in the diet without any adverse effects on kidneys despite its high P and Mg content (Kamiya et al., 2017).

Pigs 

Information about the use of sweet potato by-products in pig feeding is scarce. It was reported that sweet potato starch pulp was palatable and digestible for pigs (Bouwkamp, 1985). In Uganda, a survey reported that sweet potato peelings represented 30% of the fresh root weight and were used for pig feeding. A further study assessed the economical opportunity to process sweet potato wastes with sweet potato vines in order to prepare silage for pigs but no feeding trials assessed the technical feasibility (Asindu et al., 2017).

Poultry 

Sweet potato distillery by-product

In Brazil, a sweet potato distillery by-product with 25% (as fed) of crude protein was assessed in the diet of slow growing broilers. The AMEn of this by-product was reported to be 2732 kcal/kg, a value that was reported to be similar to that of sweet potato meal. However, the broilers fed at increasing (6%, 12% and 18% dietary DM) levels on sweet potato distillery by-product had all lower gain weight, N excretion was increased and energy was not well used. The authors recommended to limit the inclusion of this by-product at 1% (Parente et al., 2014).

Sweet potato waste

In Japan, sweet potato waste from the production of shochu liquor was dried for 6 h at 72°C and fed to broilers at 25-26% inclusion rate, replacing about 50% of maize grain. It was found to be a suitable feed ingredient for broilers as it achieved the same growth performance as broilers fed the maize-based control diet. It reduced meat yellowness and increased in its lipid content (Zhang et al., 2019).

Protein-rich sweet potato pulp

In the Philippines, sweet potato pulp from starch production was treated with urea and fermented with fungus Trichoderma harzianum, increasing its protein content up to 17-20%. Broilers fed with finishing mash incorporated with protein-enriched sweet potato pulp increased their body weight and feed conversion efficiency. The process was found to be cost efficient and safe (Fresco, 2002).

Rabbits 

Sweet potato peelings

International literature on the use of sweet potato peels in rabbit feeding is relatively scarce. Dried sweet potato peels can partly replace of maize grain in complete diets for growing rabbits (Akinmutimi et al., 2008; Ibrahim et al., 2018). Depending on experimental condions, the inclusion rate may be increased up to 8 and even 15%. However, in the 2 experiments mentioned above, sweet potato peels replaced strictly maize grain point per point without taking into account the modification of the nutritive value associated with the change in the formula (e.g. lowering of protein content). More experiments with well-balanced diets are necessary to know whether a higher proportion of sweet potato peels can be used safely in rabbit diets.

Sweet potato waste

Dehydrated sweet potato waste have been tested in several rabbit trials. Unfortunately, the product is not defined precisely in these experiments. It could correspond to peelings, to pulp from starch processing or to mixtures of different wastes (Nguyen et al., 2016; Nguyen et al., 2018). Mixed with maize husks, sweet potato waste may be included at up to 9% of the complete diet for growing rabbits (Lestari, 2012), but higher proportions were not tested in usual fattening conditions. However, an inclusion rate of 15% was used without problem in an experimental diet for digestibility studies with Rex rabbits The apparent digestible energy of the tested batch of sweet potato waste was 11.5 MJ/kg DM and protein digestibility was 93% (Zhang et al., 2014).. But the margin of error of these determinations was high, particularly for the protein digestibility, due to the low level of proteins in sweet potato waste (2.63% in the experimental batch).

Fish 

Nile tilapia (Oreochromis niloticus)

Sweet potato peelings were used to feed Nile tilapia fingerlings (0.47 g) during 10 weeks. The crude protein of the diets containing the sweet potato peels was set at 31%, within the required protein levels for juvenile herbivorous fish. All levels of inclusion of sweet potato peelings resulted in depressed growth in the fish when compared to control (Omoregie et al., 2009).

Common carp (Cyprinus carpio)

Common carp fingerlings (0.47 g) were fed on isonitrogenous diets containing increasing levels (0, 5, 10, 15, 20 and 25%) of sweet potato peelings during 10 weeks. All inclusion levels resulted in lower fish final weight in lower specific growth rate and in lower apparent digestibility than the control. It was however concluded that it was possible to feed fish on sweet potato peelings up to 15% dietary DM as it could reduced feed costs in the production of farm fish (Faramarzi et al., 2012).

African catfish (Clarias gariepinus)

Sweet potato peelings could be fed to African catfish fingerlings (1.7 g) to replace 50, 75 or 100% of yellow maize meal in their diet. Up to 33.3% (100% maize replacement) of sweet potato peelings could be included but growth parameters were higher at 16.7 and 25% sweet potato peelings in the diet. It was recommended to use sweet potato peelings in order to reduce feed costs and to limit sweet potato peelings as wastes (Solomon et al., 2015).

Nutritional tables
Tables of chemical composition and nutritional value 

Avg: average or predicted value; SD: standard deviation; Min: minimum value; Max: maximum value; Nb: number of values (samples) used

Main analysis Unit Avg SD Min Max Nb  
Dry matter % as fed 28.9   26.8 31 2  
Crude protein % DM 6 3.3 1.5 15.3 12  
Crude fibre % DM 3.9 1.2 2.1 6.5 11  
Neutral detergent fibre % DM 45.5       1  
Acid detergent fibre % DM 7.6       1  
Lignin % DM 2       1  
Ether extract % DM 3.7 1.9 0.9 6 9  
Ash % DM 5.4 2.1 3.2 10.8 12  
Insoluble ash % DM 1.8       1  
Gross energy MJ/kg DM 17.7         *
               
Minerals Unit Avg SD Min Max Nb  
Calcium g/kg DM 2.3       1  
Phosphorus g/kg DM 1.7       1  
Potassium g/kg DM 10.6       1  
Magnesium g/kg DM 1       1  
               
Ruminants nutritive values Unit Avg SD Min Max Nb  
OM digestibility, ruminants % 84.8         *
Energy digestibility, ruminants % 81.7         *
ME ruminants MJ/kg DM 12.2         *
Nitrogen digestibility, ruminants % 55.2         *
               
Rabbit nutritive values Unit Avg SD Min Max Nb  
DE rabbit MJ/kg DM 13.9         *
MEn rabbit MJ/kg DM 13.8         *
Energy digestibility, rabbit % 78.8         *
Nitrogen digestibility, rabbit % 21.7         *

The asterisk * indicates that the average value was obtained by an equation.

References

Akinmutimi et al., 2008; CIRAD, 1991; Dastu et al., 2018; Faramarzi et al., 2012; Ibrahim et al., 2018; Okoruwa et al., 2015; Omeruo et al., 2016; Omoregie et al., 2009; Solomon et al., 2015; Ukanwoko et al., 2020

Last updated on 16/09/2020 11:26:03

References
References 
Datasheet citation 

Heuzé V., Tran G., Hassoun P., Lebas F., 2020. Sweet potato (Ipomoea batatas) by-products. Feedipedia, a programme by INRA, CIRAD, AFZ and FAO. https://www.feedipedia.org/node/5325 Last updated on September 17, 2020, 13:43

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