Sweet potato drying and nutrition insights for high volume production

THE SWEET potato has been popping up on menus everywhere. It has become a popular ingredient in breakfast cereal, snack food and bakery products. As a world crop, the vegetable has a big role to play in food security. Last year, a hybrid sweet potato won the 2016 World Food Prize – the “Nobel Prize” of the food and agriculture world—based on the potential it offers malnourished populations. The root crop is also ranked the seventh most important world food for its highly nutritious beta-carotene, polyphenols, ascorbic acid and dietary fiber. With food market trends focusing more and more on nutrition, the sweet potato is gaining visibility in many forms, from purées and juices to dehydrated snacks, and this includes conversion to flours. Combined with a growing trend toward the humanization of animal feed, it’s no surprise to find the sweet potato appearing on many feed labels.

As an ingredient for pet food and feed, the sweet potato is becoming an important selling point. A new report in the World’s Poultry Science Journal (Volume 73, Issue 1 – March 2017) highlights the vegetable’s potential as a feed ingredient for poultry diets compared to corn, recognizing the metabolizable energy value and its impact on the performance of chickens and layers fed at various inclusion rates. Demand for affordable dehydrated sweet potato food-grade products is increasing, manufactured according to the same standards as human food. Hygienically-processed sweet potato flour, due to its shelf-stability, is the most common form, providing the starch necessary to bind feed ingredients for extrusion. Flour is made by dehydrating pulp, shreds and cuts, a conversion process that makes powder. Following is a look at the various drying/dehydrating technologies that can be applied and the drying effect on nutrition.

While the multiple-stage dryer has the largest footprint and capital cost, it offers the best process control and flexibility

Drying/dehydrating technologies

Microwave and vacuum/freeze drying, at the laboratory level, show superior results maintaining product quality and nutrition. However, due to technical and above all, cost factors, are not widely viable for commercial, high volume purposes. Contact drum drying and spray drying of a puree slurry also render good quality and nutrition. However, capacity limits contact drum drying acceptance and high capital and energy cost limits spray drying as a cost-effective solution.

Solar drying is an accepted conventional drying method, most common in third world sources. Solar drying results in the lowest price point per kg of finished product due to lowest investment and energy costs, but lacks in product uniformity, quality, sanitation and does not lend itself to continuous commercial processing.

Rotary drying:  Use of convection rotary dryers, historically, has been utilized for over 50 years primarily for drying waste and spoiled product directly from the field. Usually being located close to the fields, the potato was crudely shredded, dried and used for animal feed.

Today’s rotary niches being explored are drying the pulp after a juicing process. The pulp remaining after the extraction of the high value juice still has sufficient nutrients for use in a conversion to flour. This practice is common in fruit and other vegetable pulp dehydration and like other pulps, significant fry mix-back will be required for flow-ability through the dryer.

Conveyor drying: The default technology for hot air convection drying cut, non-pulp forms of a wide variety of vegetables has historically been the multiple-stage dryer. This equipment offers optimum process flexibility in that drying air temperature, velocity and humidity is variable throughout the drying cycle, lending itself to handling multiple other vegetable and fruit products. Multi-stage provides best process control, is easiest cleaned, allows optimum bed width but has the largest footprint and highest capital cost.

With entry cost being a major consideration for a start-up processor, multiple-pass as well as single-stage arrangements are a consideration especially for dedicated product and/or low capacity lines.

The drying effect on quality and nutrition

Conventional wisdom would lead one to believe that the higher the air temperature, the lower the nutritional value of a dried food. However, Bühler Aeroglide research has proven otherwise. Contrary to what many believe, elevated temperatures have a minimal effect on nutritional values. In some cases, it even enhances them.

To examine this concept, a series of drying tests were performed in conjunction with NC State University’s Food Science Laboratory in Raleigh, North Carolina. The goal was to reach a preliminary finding on the effect temperature had on nutrition and color. The test conditions were 310F for 50 minutes, 230F for 90 minutes, and 180F for 180 minutes (this sample had to be further oven dried after equilibration to allow proper milling to a flour for analysis).

The three samples were processed into powder form for further analysis. Several physical and chemical assays were performed on these samples, including color measurement, moisture content, beta carotene, total phenolics, antioxidant activity and vitamin C content. Color was measured on both cube samples and the powder form.

Interestingly, the 310F sample had the highest carotene, vitamin C and total phenolic contents, and antioxidant activities. This could be due to the differences in sweet potato varieties, growing and storage conditions. Drying at a high temperature resulted in dark browning color cubes and flours. Some research has even shown that the browning reaction can contribute to the overall antioxidant activities.

The results showed nutritional loss of the highest temperature was surprisingly minimal compared to the lowest temperature sample. Also, the color of the highest temperature sample, while being browned on the exterior, became similar to the lower temperature after milling.

The possible reasoning behind this observation is the reduced time the solids of the potato are exposed to an elevated temperature, the less degradation occurs.

High nutrition, low cost ingredient

The ability to use elevated drying temperatures makes high volume dehydration process possible at a nominal equipment entry cost. High-quality dehydrated sweet potato flour can be produced using Bühler Aeroglide’s multi stage, multi pass and single pass conveyor dryers, keeping cost low while offering a source for sanitary food approved product for human and pet food manufacturing.

In the end, the versatility for sweet potatoes growers offer endless options to innovate. As a low cost global crop, the potato has a role to play in food security and innovations are limited only by the imagination of food scientists and process engineers. Bühler Aeroglide partners with customers to engineer food and feed products, offering both the industry-leading technology and the scientific process knowledge needed to bring new products to the market.