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Sweet potato (Ipomoea batatas) by-products

IMPORTANT INFORMATION: This datasheet is pending revision and updating; its contents are currently derived from FAO's Animal Feed Resources Information System (1991-2002) and from Bo Göhl's Tropical Feeds (1976-1982).

Datasheet

Description
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Common names 

Sweet potato [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 starch waste
Synonyms 

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

Description 

Sweet potato tubers are a staple food or an alternative food in many countries. The consumption of sweet potatoes by humans, and their processing in the cannery, crisp, puree, distillery and starch industries results in different by-products which are quite similar to potato by-products since the processing techniques are also similar.

As for potato byproducts, sweet potato byproducts would be a burden to the environment, being disposed into landfills or bodies of water if they were not used in animal feeding (Akoetey et al., 2017). For example, sweet potato starch production is a major environmental concern in China and Japan as about 6m3 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).

Sweet potato by-products are numerous and very variable depending on the processing method that yielded them. Their nutritive value for farm animals is thus also variable and it is important to clearly define each of them for optimal use in animal feeding.

It is suggested that sweet potato by-products can be classified in the same way as potato by-products (see potato by-products datasheet), i.e. into raw by-products, cooked by-products and mixtures of both classes. Howhever, some by-producst like sweet potato slurry/filter cake, protein concentrate and mixtures of raw and cooked by-products have not been referred to in the litterature dealing with animal feeding.

Raw by-products
  • Abraded/handcut peel: the peel removed from raw sweet potatoes intended for crisp or french fries processing.
  • Screen solids: small sweet potatoes and slices, trimmings, white waste.
  • Sweet potato pulp from starch extraction (Button, 2015; Bouwkamp, 1985).
Cooked by-products

The peeling process during canning operations produces almost half the raw material (Akoetey et al., 2017).

Sweet potato by-products are generally a good source of readily digestible carbohydrates and protein (sweet potato pulp, distillery byproducts) and of other valuable nutrients like vitamins and phenolic compounds. Their main constraint is the moisture content which limits their transportation and conservation in safe conditions, without mould or bacterial development (Poore et al., 2000).

 

Distribution 

[Worldwide, the sweet potato tuber production was 111, 6 million tonnes in 2017 of which 58 million tonnes were used as food, 38 million tonnes were used as feeds while 9 million were lost (FAO, 2020).] In the USA, it was reported that almost 50% (in weight) of sweet potato were wasted between the farm and the consumer's table (Edmunds et al., 2013). Sweet potato by-products are present wherever sweet potato are produced or processed.

Processes 

Raw products

Resulting from the peeling

The amount of peels resulting from the peeling of sweet potatoes depends on many factors including the size of potatoes, their maturity and the methods of peeling: it was reported to vary from 7% to 13% and even up to 30% under mechanical peeling (Button, 2015; Bouwkamp, 1985).

Abraded peels/handcut peels

Abraded and handcut peels are raw sweet potato peels obtained through abrasion (scrapping) or handcutting in the processing of crisps or french fries. Abrasion and handcutting were reported to remove high amount of material (up to 60%) from sweet potato which means that resulting sweet potato peels could have high DM content (Button, 2015; Bouwkamp, 1985).

Resulting from screenings
Screen solids from process water/potato waste

These are raw small sweet potatoes and slices, white waste, nubbins, hopper box that are removed by screening or settling.

Sweet potato starch

This by-product is recovered in process water after the uncooked potato have been cut, sliced chipped and have released the starchy content of damaged cells. This starch is recovered from cold water. It remains uncooked and makes an unvaluable ingredient for human consumption, it does not seem to be used in animal feeding.

Sweet potato pulp from starch extraction

For starch extraction, sweet potatoes are rasped and washed in cold limewater so that the starch exsudates. Starch extraction yields 5-6% sweet potato pulp at 55%-65% starch, 2-4% crude protein, 0.8 to 1.5% fat, 5 to 14% fibre, 2 to 10% ash and 13 to 15% water. Sweet potato pulp has to be dried or pressed to be directly fed to livestock as an energy feed. The water resulting from the pressing of sweet potato pulp still contains high amount of soluble proteins which could be precipitated by pH lowering and then forming a filter cake that can be mixed with sweet potato pulp. The mixture results in a 10-12% protein-rich product and 9.5 tons of such a product could be recovered from 100 tons of sweet potato extracted for starch (Dawson et al., 1951).

Another protein enrichment could be obtained through sweet potato pulp fermentation and it yielded 2 tons of protein rich product at 50% CP per 100 tons sweet potato pulp (Dawson et al., 1951).

Distillery by-products

Distillers soluble are produced during the production of shochu (Japanese traditional hard liquor from sweet-potato). These distillers have high total digestible nutrients, crude protein and mineral (K, Cl, S, P and Mg) content (Kamiya et al., 2017). Their crude protein content is 26% and the digestibility 60% (Woolfe, 1992). However, due to the high moisture content which is difficult to remove, and the excessive acidity, they are not widely used as feeds (Woolfe, 1992).

Cooked products

The production of french fries or crisps involves steam-peeling, cooking/blanching or deep-frying. Steam peels, lye caustic peels, culled fries, culled crisps, hash browns, crowns, batter, crumbles, nubbins are by-products that have undergone heat treatment and may have been added oil, seasoning salts etc. Heat treatments gelatinize starch that becomes readily digestible and deep-frying in oil increases energy amount (Bouwkamp, 1985).

Resulting from the peeling
Potato feed/steam peel

Sweet potato feed or steam potato results from the peeling process occurring during the production of chips (french fries) or potato dehydration. In this process steam and pressure are used for a few seconds to remove the outer peel from sweet potatoes. Steam increases moisture and gelatinizes starch contained in the peel. Peel is removed by a scrubber. Steam has a sanitation effect as it has anti-microbial effect or bacteriostatic effect. Unlike lye caustic peel, steam peel does not require neutralizing treatment prior to be fed to animals (Bouwkamp, 1985). 

Lye caustic peel

In the lye peeling process, sweet potatoes are washed with a hot (100-102°C) sodium hydroxide (2-10% NaOH) solution and then mechanically peeled yielding about 20-35% solids (Bouwkamp, 1985). Peels resulting from lye peeling are alkaline and require neutralization before use as animal feed. From infomation obtained in potato by-products, it could be assumed that, thanks to chemical treatment, lye caustic swet potato peels have a low microbiological load and do not spoil during storage provided pH remains high (Hinman et al., 1978).

Resulting from screenings/trimming
Culled fries, culled crisps, hash browns, crowns, batter, crumbles, nubbins

These are partially or fully-cooked by-products separated from final products because they do not comply with the size or colour standards of food-grade products. However, they have the same nutritive value as fries or crisp. They are highly energetic feeds containing not only high level of gelatinized starch but also of oil (up to 20%). Those products are considered to have higher energy level than any animal feed but oils and fats. The oil used for deep-frying affects the levels of saturated/unsaturated fatty acids with consequences on rumen microflora/microfauna and on methane emissions. Products cooked in rapeseed oil or sunflower oil may have defaunating effects, reduce methane emissions and thus limit the energy losses in ruminants (Crawshaw, 2004). Those by-products are also seasoned and may contain 0.5-1.75% sodium, the upper limit being an issue in pigs diet (Crawshaw, 2004). Potato hash is a mixture of potato skins, starch, fats and yellow maize obtained after the production of snacks. In South Africa, potato industry yields 50 t/day of potato hash that can be used in animal feeding (Nkosi et al., 2011).

Potato mash/ dried potato meal/ potato flakes/potato granules

Those products are obtained after the cooking of potatoes in the processing of chips or of dehydrated puree, potato flour and in the process of artificial chips like "Pringles" reconstituted from potato puree. Those potato by-products have high energy level, high amount (75%) of highly digestible starch (gelatinized) but also good quality protein, little fibre and some minerals (Crawshaw, 2004). They can be used in pigs diets and in ruminants diets provided the ration has enough fibre to prevent acidosis and enough rapidly available N to improve protein synthesis in the rumen (Crawshaw, 2004)..

 

Nutritional aspects
Potential constraints 

Moulding

Because sweet potato by-products or cull are rich in energy, they can rapidly go mouldy and rotten lidding serious health problems particularly respiratory (Poore et al., 2000). Significant herd losses due to these respiratory diseases have been reported since almost a century in USA, Japan, Australia, Brazil and more recently in UK (Poore et al., 2000; Mawhinney et al., 2009).

Sweet potatoes produce stress metabolites in response to mechanical injury, chemical irritation and nematode and fungal infections (Poore et al., 2000). The most active metabolite seems to be the 4-ipomeanol which develops respiratory distress with an atypical interstitial pneumonia (Poore et al., 2000; Thibodeau et al., 2002; Mawhinney et al., 2009). This toxin is often observed when sweet potatoes are colonized by Fusarium solani mould. For that reason, it is recommended to not distribute mouldy or rotten sweet potatoes or by- products to farm animals.

Risk of chocking, acidosis and laminitis

Choke may occur when cull sweet potatoes are offered not chopped. Consequently, it is recommended to chop culled sweet potatoes before feeding the animals Like other feed rich in readily fermentable sugars, sweet potatoes must be introduced into the diet progressively and well supplemented in forage and protein in order to avoid acidosis and laminitis.

Teeth dammage

Sweet potato cannery wastes offered in free choice to cattle may severely erode and blacken incisors (Poore et al., 2000). The cause is the low pH and high lactic acid content of the waste that both erode enamel and release calcium from the teeth. This can be solved by increasing the pH above pH 4 before feeding the animals.

Ruminants 

Sweet potato by-products are generally a good source of readily digestible carbohydrates for ruminants. However, their moisture content limits the distance that they can be hauled and their shelf life as moulds and bacteria develop readily on moist byproducts, impairing their safety as feed ingredients (Poore et al., 2000). 

Cull or chunks sweet potatoes by-products are energy-rich (mainly starch and sugars) but low in proteins. Most of the nitrogen content is located beneath the skin and in the skin (Walter et al., 1984). Sweet potato peelings contain also phenolic compounds ranging from 1.02-6.21 gallic acid equivalent /g DM (Akoetey et al., 2017).

When cull potatoes, peelings, bruised sections of potatoes are dehydrated, the meal produced is darker than that obtained with good quality sweet potatoes. This feed is less palatable than good quality (Rusoff et al., 1947). Consequently, it is recommended to avoid spoiled potatoes or by-products for feeding cattle, not only for palatability reasons but also for health problems (see Potential constraints).

Cattle

When sweet potato trimmings are used in a complete diet (51% in the diet) in place of corn as energy source for finishing steers (289 kg) for 126 days, daily weight gain is lower (800 vs 1000 g/d) but daily dry matter intake (DMI) is lower (9.4 vs 11.6 kg/d) resulting in an identical feed efficiency of 11.6 -11.8 (Bond et al., 1967). Part of these results may be due to the presentation form of sweet potato trimmings which were not ground the first half of the trial. Some differences are observed on carcass quality with less marbling with sweet potato trimmings diet (Bond et al., 1967). The dry matter digestibility of the diet is also lower with sweet potato trimmings (72.3 vs 77 %).  When dried, sweet potato is included at 50% in a diet for steers of about 317 kg and replace corn grain, the dry matter digestibility of the diet is higher (+ 5 points) or tends to be higher with sweet potato energy source Briggs et al., (1947). Cull sweet potatoes can be used as energy supplement to cow-calf couple grazing tall fescue prairie during winter season in place of cottonseed without any adverse effect (Parish et al., 2019). Condensed distillers solubles can partly replace (up to 30%) a commercial concentrate in a diet of steers (14 days’ duration experiment) without any adverse effects on kidneys due to the high mineral content of this by-products (Kamiya et al., 2017).

Sheep and goat

When dried sweet potato is included at 45% in a diet for lambs (28 kg) and replace corn grain, the dry matter digestibility of the diet is lowered by 2-3 points (Briggs et al., 1947).

General recommendations

According to Poore et al., 2000), sweet potato and wastes (cull or shrunk) must be limited to 20% on the dry matter basis into a diet. They must be introduced gradually into the diet, and cull sweet potatoes must be ideally chopped in order to prevent choke. Spoiled sweet potatoes or by-products must be eliminated before fed to the animals. Cull sweet potatoes must be washed and clean before being ensiled to eliminate soil and mould.

Pigs 

Information about the use of sweet potato by-products in pig feeding is scarce. However, 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 byproduct

Parente et al., 2014

Rabbits 

Sweet potato peelings

International literature on the use of sweet potato peel in rabbit feeding is relatively scarce. However it was clearly demonstrated that dried sweet potato peels may replace one part of maize grain in complete diets for growing rabbits (Akinmutimi et al., 2008; Ibrahim et al., 2018). According to experimental conditions, the incorporation level may be increased up to 8 and even 15%. But in the 2 experiments mentioned above, sweet potato peels replace strictly maize grain point per point without taking in account the modification of the nutritive value associated with the formula modification (e.g. lowering of protein content). Thus more experiments with well balanced diets are necessary to know if a higher proportion of sweet potato peels can be used safely in rabbit diets.

Sweet potato waste

Dry sweet potato waste are considered as a classical ingredient in some rabbit’s experimental diets (Nguyen et al., 2016; Nguyen et al., 2018). Mixed with maize husk, sweet potato waste may be introduce up to 9% of the complete diet for growing rabbits (Lestari, 2012), but higher proportions were not tested in classical fattening conditions. However an incorporation level of 15% was used without problem in an experimental diet for digestibility studies with Rex rabbits (Zhang et al., 2014). According to results of this experiment, the apparent digestible energy of the tested batch of sweet potato waste was 11.5 MJ/kg DM and the protein digestibility was 92.6%. 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 

OMOREGIE et al. (2009) reported that fish fed with sweet potato peels exhibited depressed growth when compared to a control diet (no sweet potato peels). However, when the peels were limited to 5-20% of the diet there was no depression in growth. The crude protein of the diets with the sweet potato peels was about 31%, within the required protein levels for juvenile herbivorous fish

Nutritional tables
Tables of chemical composition and nutritional value 
References
References 
Datasheet citation 

DATASHEET UNDER CONSTRUCTION. DO NOT QUOTE. https://www.feedipedia.org/node/5325 Last updated on August 13, 2020, 17:11

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