Why Complete Utilization of the Sugarbeet is Necessary

Published in the February 2014 Issue Published online: Feb 14, 2014

In addition to sugar and animal feed, sugarbeets can provide many value-added co-products for biofuels, human nutrition, plastics and pharmaceuticals.

In the search for sustainability and profitability of the sugarbeet industry, the utilization of the entire plant is mandatory beyond the traditional roles of harvesting for food or feed and then directly or indirectly returning the residue to the soil, burning for fuel or non-environmental disposal. The composition of Sugarbeet Pulp (SBP) suggests that it could be used to produce several value-added products.

 

Food Ingredients

Dietary fiber is one of the main issues in human nutrition. Fiber products from SBP have been Generally Recognized as Safe (GRAS) since 1991 and are produced with a relatively simple process.

Nutritional data from commercial products show that beet fiber contains around 8 percent protein (by weight) and 67 percent carbohydrates such as hemicelluloses (28 percent), cellulose (19 percent), and pectin (18 percent). Dietary fiber available for human digestion is generally more than 20 percent in sugarbeets. Fiber products from sugarbeets can be either the whole pulp or purified pectin and have a wide range of beneficial effects on human health.

For example, the effect of SBP on cholesterol levels was investigated with positive results. The use of sugarbeet fiber in processed foods is limited by its texture and taste, but it is generally used in meat patties, bakery products, cereals and assorted products that need thickening or bulking agents. One recent health claim is that the pectic oligosaccharides may function as a prebiotic in the human gut. Prebiotics affect the microbial population in the human gut and generally favor bacteria beneficial to human health.

In addition, the phenolic compounds may be extracted and used as antioxidants in food products. Pectin is a cell-wall polysaccharide and is best known for its gelling properties in fruit products. Beet pectin exhibits better emulsifying properties than other sources of pectin. Pectin could also be used as an adhesive, emulsion stabilizer and suspension agent in cosmetics or pharmaceuticals. For example, sugarbeet pectin can be used to suspend colorants such as anthocyanins from berries.

 

Plastics and Composites

Plastics are prevalent in the global market. Most are derived from petroleum, and there are many research efforts to replace petro-plastics with bioplastics derived from renewable resources. In some cases, plant polymers only need extraction before use while others need to be synthesized from small molecules also derived from plants.

SBP was processed in a twin-screw extruder using plasticizers to obtain thermoplastic films. The resulting composite could be characterized as cellulose microfibrils suspended in a pectin matrix. SBP was also used as a polyol source for the production of urethanes. SBP was combined with a biobased polymer, polylactic acid, to form polymer composites that had similar tensile properties to commodity plastics. The SGP could be plasticized and used as a co-polymer rather than as a filler in both PLA and poly(butylene adipate-co-terepthalate).

Pectin, extracted from SBP, is also used in plastic packaging materials. In some cases, pectin can be used to protect active ingredients from thermal shock during processing into thin films for food packaging. Microbial and plant sourced polyesters such as polyhydroxyalkanoates (PHA) are making inroads into the plastics market. Sugarbeet juice was used as a sugar feedstock for the production of PHAs such as poly(hydroxybutyrate). PHB is an important biobased polymer with plastic properties similar to synthetics such as polypropylene. It is also compostable and environmentally friendly. The “carbon source” cost of PHA production, calculated based on Sugarbeet molasses as a sole feedstock, was approximately $1.40/kg. This is very cost-competitive in the plastics market.

 

Conversion to Platform Chemicals

The search for renewable and sustainable fuels has driven the development of technology for complete utilization of biomass. In general, lignocellulosic grasses or woody plants garner all the attention of technology developments to harvest fermentable sugars for conversion to a wide range of platform chemicals; ethanol being the current favorite. The low lignin content and the high digestibility of its carbohydrates make SBP a feedstock candidate for biorefineries. Fermentation requires the breakdown of cell wall networks like pectin and cellulose. The severity of the pre-treatment and the use of different enzyme treatments affect the fermentable sugar yield from SBP.

Some products include: vanillin, pharmaceuticals, surfactants, and polyphenols. Sugarbeet vinasse, leftover from ethanol production, contains Betaine (15 percent) which is used as amphoteric surfactants in personal care products. In order to design an efficient and (relatively) inexpensive conversion of any vegetal matter, one needs accurate knowledge of the structure and composition of the components, the interaction and structure between the components, and the synergistic effects of thermochemical and biochemical treatments to separate and purify the constituents. This is not a trivial matter when designing a production facility to process different crops all with different compositions.

 

Energy Production

Energy production from renewable resources is increasingly in demand. Using sugarbeets as the feedstock, the main target for biofuel production is ethanol. Once extracted, sucrose can be directly fermented into ethanol using any number of traditional, industrial-scale methods.

In contrast, starchy crops need additional processing steps to obtain fermentable sugars. Advances in lignocellulosic bioconversion will allow the use of the beet tops and sugarbeet pulp for bioenergy production. For example, ethanol production was demonstrated using sugarbeet pulp and a mixed enzymatic culture to solubilize pectin and cellulose and then the sugars were converted via fungal enzymes.

In this case, the sugarbeet would become a dual purpose crop: sugar and energy. A combined sugar-ethanol plant was studied introducing beet co-products in various stages in a single-tank hydrolysis and fermentation process. A study on the integration of the storage, hydrolysis and fermentation of SBP has shown that ethanol production can be increased by 50 percent over current methods. Optimization of the biofuel production process is ongoing; for example, there have been recent developments into membrane ultafiltration for increasing the ethanol yields of sugarbeet feedstocks.

Of course, environmental impacts must be considered; for example, water usage is one of the more important factors imported into any model. To be viable as a biofuel, ethanol must have high net energy gain, be competitive in price, provide ecological benefits, and have the ability to be produced on a large scale. The design and introduction of new technologies and the assessment of environmental advantages will increase the sustainability and profitability of the sugarbeet. Regional efforts are prevalanet and aimed at designing ethanol plants with minimal waste generation using locally grown energy crops such as sugarbeet.

Ethanol may be driving biofuel technology right now, but second generation biofuels are being studied such as hydrogen, methane, methanol and butanol. The wet storage difficulty could be leveraged by capturing the biogas produced during ensilage by anaerobic digestion. Biogasification (anaerobic digestion) of the molasses co-product by mixed cultures of microorganisms produces methane and carbon dioxide which can be captured and used as a fuel for electricity generation (300 kWh/tonne raffinate) onsite. Work is continuing to produce methane gas as a biofuel using sugarbeet residues. In addition, there are a variety of pre-treatments that improve the formation of biogases from SBP.

 

Carbon Materials for Removal of Contaminants

Renewable, sustainable energy research and concerns regarding the production of greenhouse gases has driven the need for the utilization of biomass in so-called carbon-neutral processes. Contamination of water sources by toxic substances is an ongoing environmental and health concern. Using agricultural materials have some advantages over conventional processes including low cost, regeneration of biosorbents, and potential recovery of heavy metals.

The binding capacity of SBP makes it more valuable in the market. Biochar is produced using high temperatures to burn biomass leaving only carbon (carbonization).

Biochar has been shown to enhance soil fertility and water holding capability and sequesters carbon in the environment. Biochar also has potential as a low-cost absorbent as it shows high affinity for heavy metals. While any biomass can be carbonized, economic studies suggest that agricultural wastes would be more suitable. Biochar from sugarbeets has been shown to sequester phosphates, nitrates, and heavy metals especially Cr(VI).

 

Conclusions

Utilization of agricultural commodities in sustainable, economic and ethical ways is essential in the competitive global marketplace. The value-added coproducts of sugarbeet are summarized in Figure 1. The complete utilization of the sugarbeet should be examined on the basis of a biobased economy in order to select the optimal parameters for the industry. One must also consider the future impact of global climate change on the sugarbeet crop. Life cycle models will give researchers and leaders insight into the environmental, economic, political, and social value of sugarbeets.