Fermented sweet potato biomass a sustainable ingredient for aquafeeds

Rex Ferdinand Traifalgar, University of the Philippines Visayas, Maria Theresa Valdez, Tarlac Agricultural University, Leodevico L. Ilag, Plentex Limited

The contribution of aquaculture to global food production is expected to increase, therefore the need for sustainable and cost-effective feed ingredients will also rise. It has been projected that aquaculture production will reach 105 Mt in the year 2029. However, this projected growth is dependent on feed inputs, which account for about 50-60% of the total production cost. Feed protein supply requirements for the expansion of aquaculture are considered the limiting factor that significantly affects the sustainability and economic viability of the industry.

Currently, feed proteins for aquaculture are sourced from fish and soybean meals. However, utilization of these meals as fish feed has been viewed unfavorably by the public as an inefficient way of resource utilization. Fish should be consumed directly as human food and should not be used as feed for other fish. Furthermore, the food and the livestock industries are competing with aquaculture for soybean meal leading to an increase in prices and erratic supply of this feed material. Moreover, there is also growing concern and trends against the use of soybean because of its association with nonsustainable production processes.

ProEnK: The future of sustainable feed production for aquaculture

To address the global concern regarding the supply of feed ingredients required to sustain the growth of aquaculture, Plentex Philippines lnc., and experts

Figure 1. Weight gain of Pacific white shrimp fed diets containing increasing replacement levels of soybean meal with ProEnk.

Figure 2. Total gut Vibrio load of the shrimp, P. vannamei fed with diets containing fermented sweet potato and the control, with soybean meal as the main protein ingredient.

from the Institute of Aquaculture of the University of Philippines Visayas and Tarlac Agricultural University have teamed up to develop ProEnK, a high protein feed ingredient produced from the fermentation of sweet potato biomass, a byproduct of the sweet potato processing industry. It has been estimated that up to 20% of sweet potato biomass ends up as waste from processing and is not used for human consumption. The group has developed an innovative process of using a cocktail of proprietary microbes that are able to convert agricultural biomass rich in carbohydrate into high-protein biomass that is comparable with soybean meal in protein content but with the absence of omega-6 fatty acids. The developed process requires less water, bioconversion completed within 21 days and transforms sweet potato byproduct biomass with 3% protein into a feed ingredient containing 35-45% protein. This process is cheaper than intensive soybean agronomy that requires a significant amount of water, fetilizers, pesticides and time. ProEnK has the potential to reduce the utilization of fishmeal for aquaculture feed use.

Figure 3. Feed conversion efficiency of milkfish fed diets containing increasing levels of ProEnk.

Figure 4. Omega-3/omega-6 fatty acid ratio of tilapia fed diets containing increasing levels of ProEnk.

This ingredient was found to be 70-90% digestible to milkfish, shrimp and tilapia. Analysis of the fermented biomass indicated an ideal content of carbohydrates, low fat and lower fiber contents. The fatty acid contents are dominated by medium-chain and short-chain fatty acids, which are ideal energy sources and immune activator molecules in aquatic animals. A complete set of essential amino acids required for most aquatic animals were also found to be present in ProEnK.

Trials supporting ProEnK as an aquaculture feed ingredient

To confirm the nutritional and feed value of the ProEnk to aquatic animals, several feeding trials were conducted on Pacific white shrimp, milkfish and tilapia. In the trial with Pacific white shrimp, diets were formulated with increasing substitution of soybean meal (0-100%) in the diets of the treated aquatic animals. The diets were formulated to be isonitrogenous and isocaloric. In the trial with milkfish and tilapia, the diets were formulated containing increasing levels of ProEnK with a corresponding reduction of fish and soybean meals. This was done to determine the maximum inclusion level of ProEnK that could be included in the cultured finfish diet.

Shrimp results

Results from the pacific white shrimp trial indicated that ProEnK could completely replace soybean meal in the shrimp’s diet without affecting the overall growth performance. Moreover, the results indicated that at 50% soybean meal replacement, the growth and feed conversion ratio (FCR) of shrimp is enhanced (Fig. 1). In addition, the fatty acid profiles of shrimp in the best treatment group showed a higher content of omega-3 fatty acids in their tissues. This significant enhancement of growth and a better fatty acid tissue profile was also associated with a low total Vibrio content in the treatments replacing 50% of soybean meal with ProEnK. The results suggested the activation of gut immunity by this dietary ingredient. ProEnk contains a good amount of short and medium-chain fatty acids that are known to activate gut-associated immune responses and are also known as potent antibacterial agents.

Milkfish results

The feeding trial with milkfish also demonstrated a similar pattern. Better growth response and Feed Conversion Efficiency in treatment with the highest inclusion level of dietary ProEnk was observed (Fig. 3). This indicated that ProEnK could be incorporated in up to 50% of the diet and still obtain better growth. It is also worth noting that dietary inclusion of ProEnK led to the improvement of the fatty acid profiles of milkfish. Diet treatments with ProEnk as sole protein source led to a two-fold improvement in the ratio of tissue omega-3 to mega-6 (omega-3/omega-6) fatty acids.

Tilapia results

The milkfish and tilapia feeding trials showed similar results whereby growth indices were similar in diets with the highest inclusion of ProEnK as compared to the control. Prominent improvement in the fatty acid profile of tilapia feed diets with ProEnK could be observed. Almost a three-fold improvement in omega-3/omega6 fatty acid ratio was observed in diets containing the highest inclusion of ProEnK (Fig. 4). The results suggest that utilization of this feed ingredient in fish feed may promote better fatty acid profiles in cultured tilapia.

Conclusions

Collectively, the present data demonstrated the huge potential of ProEnk as a feed ingredient for the aquaculture of tilapia, shrimp and milkfish. The findings also suggest that among these species, dietary inclusion levels of ProEnk appear to influence the omega 3/ omega 6 ratio, an important health biomarker especially for inflammation, which is believed to be central to most chronic diseases. Animals that received the ProEnk diet have a better ratio of these fatty acids indicating less omega 6 fatty acid synthesis. Tilapia has been notoriously associated as unhealthy because of the high levels of omega-6 fatty acids in some studies. The inclusion of ProEnK has the potential to improve the fatty acid profile of tilapia and its image as part of a healthy diet.

The high levels of omega 3 fatty acids in the treatment groups receiving the ProEnk diets suggest that this diet ingredient may have promoted the biosynthesis of omega 3 fatty acids rather than the omega 6 type of fatty acids. Both tilapia and milkfish are known to biosynthesize the long-chain polyunsaturated fatty acids, EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) from 18-carbon fatty acid precursors. Currently, the mechanism and conditions that can potentially induce the biosynthesis of EPA and DHA in tilapia and milkfish are unknown. However, the results from our studies suggest that dietary inclusion of 50% ProEnk in the diet could promote the biosynthesis of EPA and DHA fatty acids in the flesh of tilapia and milkfish.

It is also tempting to speculate that the antibacterial property of ProEnk may limit the degree of bacterial infection and inflammation of the gut. Natural infection and inflammation are known factors to activate the biosynthesis of arachidonic acid, an omega 6 fatty acid. The inhibitory action of ProEnk against gut bacteria could also be a factor in the observed enhancement in performance and better fatty acid profile of these cultured aquatic animals.

ProEnk could be included in the diets of tilapia, shrimp, and milkfish to about 50% by weight without affecting the overall biological performance and feed efficiency of the aquatic animals, with the added bonus of improving fatty acid profiles. This ingredient is sustainable, economical and could reduce the use of fish and soybean meals in aquaculture diets. This feed ingredient is envisioned to play a vital role in the development of sustainable global aquaculture.

References available on request.

More information: Rex Ferdinand Traifalgar

Faculty, Institute of Aquaculture College of Fisheries and Ocean Sciences, University of the Philippines Visayas E: vilag@plentex.com.au