Wheat bran is the by-product obtained from the processing of sweet potatoes for the production of starch and other products such as noodles and vermicelli. According to incomplete statistics, in China, 5.5 million tons of wheat bran are discharged each year due to the production of starch. Only a small portion of the wheat bran is used as cheap animal feed, while the majority is directly discarded as waste, causing both resource waste and environmental pollution. Wheat bran generally accounts for 10% to 14% of the raw materials, consisting of three parts: the peel, the stem, and the flesh. The peel and the flesh account for approximately 97.2%, while the stem makes up 2.8%. 

Starch and dietary fiber are the main components of dry potato residue. Among them, the proportion of starch is 43% to 61%, while that of dietary fiber is 16% to 27%. The rest consists of soluble sugars, proteins, ash, and phenolic substances, etc. How to recycle and utilize the waste residue from starch processing of sweet potatoes has become one of the current research hotspots. For example, research and applications have been conducted in animal feed, dietary fiber, pectin, edible fungus culture medium, fuel ethanol, and other fields. 

Processing feed 

Fresh sweet potato residue can be directly used as feed for animals. However, due to its high fiber content and low protein content, it cannot meet the nutritional needs of livestock. Especially, if the protein content in the feed is too low, it will seriously affect the growth and development of livestock. Studies have shown that excessive feeding of sweet potato residue will affect the growth and development of piglets, reduce the sperm vitality of boars, and is not conducive to the growth and development of the fetus in sows. Fresh sweet potato residue contains 80% to 90% moisture and is easily contaminated by microorganisms such as Aspergillus in the natural environment. Feeding it to livestock will affect their production performance. If sweet potato residue is made into silage or protein feed, it can become a high-quality feed for animals. 

The silage-making method for sweet potato residue involves processing 1 to 2 days of fresh, uncontaminated sweet potato residue under a sealed and oxygen-deficient environment. Through the fermentation action of anaerobic lactic acid bacteria, it becomes a coarse feed that is mainly used for feeding ruminants. Due to the high moisture content of sweet potato residue, during silage-making, a certain proportion of other feed should be added to reduce the moisture, such as corn, bran, hay, etc. At the same time, an appropriate amount of urea or ammonium chloride can also be added to increase its protein content and silage quality. 

The production of protein feed uses sweet potato residue as the substrate. Specific strains with high protein content and the ability to effectively degrade cellulose are selected. Through microbial fermentation technology, the protein content in the sweet potato residue is increased, the cellulose content is reduced, and the feed palatability is improved. This significantly enhances the feed value of sweet potato residue. 

Dietary fiber extraction 

The common methods for extracting dietary fiber from sweet potato residue include physical methods, chemical methods and biological methods. The dietary fibers extracted under different processing conditions not only have differences in content and microstructure, but also have an impact on their physical and chemical properties and functional indicators. Soluble dietary fiber is an important indicator for evaluating the physiological functions of dietary fiber. However, in natural dietary fibers, the proportion of insoluble dietary fiber is relatively large, and due to its porous structure, directly adding dietary fibers to food will affect the taste. 

By modifying the insoluble dietary fibers, changing their chemical structure and form, while increasing the content of soluble dietary fibers, the water retention, oil retention, cholesterol adsorption, nitrite ion adsorption and other functional indicators of the insoluble dietary fibers will also improve. For example, treating sweet potato residue dietary fibers with steam explosion technology can significantly increase the content of soluble dietary fibers. 

The research found that when 4% to 8% of micro-fine potato pulp powder was added to the wheat dough, the microstructure of the dough was within an acceptable range and the processing performance was good. Currently, potato pulp dietary fiber is mainly used in low-calorie meal replacement foods, baking food additives, and fiber-based health supplements. 

Pectin extraction 

The content of pectin in sweet potato residue ranges from 9% to 23%, making it a good source of pectin. A study utilized 3.95 mg/mL ammonium oxalate in 375W ultrasonic assistance to extract pectin from sweet potato residue, achieving an extraction rate of 15.48%. Fourier Transform Infrared Spectroscopy (FTIR) was employed to determine the esterification degree of sweet potato pectin, and it was found to be a high-methoxy pectin. 

Glycoalkaloids form gels when the pH is between 2.0 and 3.5 and the soluble solids content is ≥ 55%. Therefore, they are suitable for enhancing the stability of acidic media. Further studies have determined the optimal conditions for enzymatic extraction of glucoalgaloids from sweet potato residue. After extraction, through ultrafiltration concentration and ethanol precipitation methods, the yield of glucoalgaloids reached 17.3%, and the galacturonic acid content of the glucoalgaloids reached 80.0%. The physicochemical properties of the salt-extracted glucoalgaloids are superior to those of the acid-extracted ones, with higher purity and better water solubility, making them more suitable for the food industry. 

As a raw material for edible fungi cultivation media 

Sweet potato residue contains a large amount of important nutrients such as starch and cellulose, and can be used in edible fungus cultivation. Some studies have conducted breeding experiments on rough-skinned shelf-ferns, chicken-leg mushrooms, and shiitake mushrooms by adding different proportions of cottonseed shells and corn husks to the sweet potato residue. The results showed that when the fungus seeds were cultivated using sweet potato residue in combination with cottonseed shells or corn husks at certain ratios, the edible fungi grew well. However, when using pure sweet potato residue for cultivation, although the mycelium grew robustly and the appearance was white and dense, the yield was lower, and the biological efficiency was 35% lower than that of conventional cultivation with cottonseed shells and corn husks. 

An experiment was conducted using whole corn husks and sweet potato residue to build walls for the vertical cultivation of shiitake mushrooms. The results showed that the mushroom spores germinated early, the mycelium grew rapidly, the mushrooms emerged early, and the yield was high. This cultivation method comprehensively utilized the poor water retention and large gaps of the whole corn husks and the extremely high moisture content and poor air permeability of the sweet potato residue. It not only saved the investment cost for mushroom cultivation but also transformed waste into resources and reduced environmental pollution. It provided a new approach for the efficient utilization of sweet potato residue. 

Preparation of fuel 

Sweet potato residue is rich in carbohydrates such as starch, and is an excellent raw material for the production of ethanol and hydrogen. When ethanol is produced using sweet potato residue, the yield reaches as high as 5.52 g/L. Usually, by adopting pre-treatment methods such as ultrasonic or microwave assistance on the basis of the conventional process (microbial fermentation method), the yield of ethanol can be effectively increased. 

The sweet potato residue was treated by the hydrothermal carbonization method, which led to dehydration and decarboxylation reactions at high temperatures. As a result, hydrothermal carbon was obtained. The carbon content of the hydrothermal carbon increased from 46.2% of the sweet potato residue to 67.13%, while the hydrogen content decreased from 6.56% to 5.24%, and the oxygen content decreased from 41.95% to 19.54%. Compared with the untreated sweet potato residue, the ignition temperature, burnout temperature and activation energy of the hydrothermal carbon all showed an upward trend, demonstrating better combustion characteristics than the sweet potato residue. 

However, during the process of preparing carbon through hydrothermal treatment, some inorganic substances and intermediate products will dissolve into the aqueous phase, causing secondary pollution. This requires additional treatment of the aqueous phase before it is discharged into the environment. To address this issue, some researchers further studied the water phase circulation during the preparation of hydrothermal carbon from sweet potato residue, and found that reusing the water phase could significantly increase the yield of hydrothermal carbon, promote the decarboxylation reaction, increase the carbon content and high calorific value, and reduce NOX or SO2 emissions. As a result, the energy utilization of sweet potato residue becomes cleaner and more environmentally friendly. 

Furthermore, the study on the co-thermal hydrolysis of sweet potato residue and low-fat microalgae revealed that although the addition of sweet potato residue did not significantly promote the Maillard reaction between starch and protein to form bio-crude oil, it did promote the formation of bio-char; as the amount of sweet potato residue increased, the nitrogen content in the bio-crude oil decreased, thereby reducing the risk of precipitation during the storage of the crude oil and minimizing the environmental pollution caused by the combustion of the crude oil generating nitrogen oxides. When the ratio of low-fat microalgae to sweet potato residue was 3:1, the energy recovery rate reached 70.8%, which was greater than the theoretical calculated value of 66.96%, indicating that the addition of sweet potato residue has a synergistic effect on the thermal hydrolysis of low-fat microalgae. 

Other applications 

The potato residue can also be used to produce glucose, citric acid, solid fuels, and to manufacture environmentally friendly adsorbents, etc. By using enzymatic hydrolysis, the residual starch and fibers in the potato residue are degraded to prepare high-purity crystalline glucose. After removing impurities and purifying, pure medical-grade crystalline glucose with a purity of 99% can be obtained, with a crystallization recovery rate of 90% and a conversion rate of the potato residue of 83.58%. 

The polysaccharides from sweet potato residue were extracted by ultrasonic-assisted water bath method. The optimal process conditions were a material-to-liquid ratio of 1:17, ultrasonic time of 57 minutes, ultrasonic power of 209 W, and ultrasonic temperature of 60 ℃. The average yield of the polysaccharides from sweet potato residue was 5.926%. The oligosaccharides of pectin and composite fibers were prepared by enzymatic method. The results showed that the main components were fiber disaccharides, fiber trisaccharides, pectin disaccharides, and pectin trisaccharides. 

The sweet potato residue was modified and treated with NaOH solution of pH 12 and anhydrous ethanol, etc., to prepare an adsorbent, which was used for adsorbing methylene blue. The results showed that the modified sweet potato residue had a good adsorption effect on methylene blue. The SEM image revealed that the surface of the modified sweet potato residue was loose and porous, making it a good raw material for biological adsorbents. 

Reference materials: 

[1] Yang Shixiong, Gao Feihu, Li Xue, et al. Research Status and Progress on Efficient Utilization of Sweet Potato Residue [J]. Agricultural Product Processing, 2024, (20): 93-97. 

[2] Jin Yanling, Zhao Hai, Zeng Fankui. Research Progress on the High-Value Utilization of Waste Residues from Potato Starch Processing [J]. Acta Agronomica Sinica, 2021, 11(11): 98-103. 

Author Profile: 

Xiao Nisan, a food technology professional, holds a master's degree in food science. Currently, he works at a large domestic pharmaceutical research company, specializing in the development and research of nutritional foods.