Commercial Production of Black Soldier Flies |Preserving Harvested Larvae : The Life and Times of BSF (Black Soldier Flies)
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Commercial Production of Black Soldier Flies |Preserving Harvested Larvae

by Terry Green on 10/31/13

Scaling up production of BSF grown off of food scrap and agricultural wastes could provide an alternative source of high quality protein and lipid feedstock in reducing demand and consequently over fishing of Menhaden fish stocks off the Gulf Coast and Atlantic oceans  (see Black Soldier Fly Larvae | An Earth Friendly Feedstock?). BSF larvae efficiently assimilate nutrients in biodegradable wastes into insect biomass rich in high quality protein and lipids. Moreover, they are a proven animal feedstock (see Hermetia illucens). Approximately fifteen metric tons (MT) of food scrap fed to Black Soldier fly (BSF) larvae will yield on average about 1 MT of prepupae (see Black Soldier Flies & Recycling | Keeping Organic Leachates in Perspective). Commercial larval production in MT quantities is far in excess of what can be handled practically in backyard lots.  Without a means of stabilizing harvested larvae recovered on a commercial scale, the larvae raised off of food scrap and agricultural waste will rapidly putrefy rendering larvae harvested unmarketable. This blog describes how to produce sun-dried larvae using passive solar energy resulting in stabilized larvae storable on site and suitable for shipping without spoilage in anticipation of marketing commercially farmed larvae to animal feedstock formulators.

In Asia and in some areas along the Gulf Coast, shrimp and cuddle fish, harvested fresh from the sea, are hung up or spread out on concrete, sometimes wire screens, even wooden board platforms, and left to dry in the sun (see, for example, Some history about shrimp fishing and Images for History of Dried Squid). This method applied in drying BSF prepupae was tested as a means of preserving harvested larvae in a stabilized state suitable for storage and shipping.

Figure 1 shows the appearance of BSF prepupae before and after passive drying in the sun using this technique. Prepupae were spread out in large plastic trays built with edges designed to block larvae from escaping from the trays while drying in layers of approximately 1 to 2 cm deep, and covered with mosquito netting  to keep predators (birds, spiders, frogs, rodents, etc.) out of the drying trays.


Figure 1. Images of BSF prepupae harvested from food scrap before (Fig. 1(a)) and following (Fig. 1(b)) drying of the larvae passively in the sun. The moisture content was reduced by ~60% as in Figure 2. Figure 1(b) is an image taken 3 months after the larvae were dried and stored in plastic bags at room temperature (~21 C) in the dark. Copyright (c) 2013 Terry Green, All rights reserved.

Blocking shields placed around the perimeter of the drying trays were necessary. Without blocking shields prepupae quickly seek escape from the sun based upon their intrinsic photophobic nature, but also in response to the rising temperature generated as the sun warms up the drying trays.

The technique appears fundamentally sound and practical. Spreading larvae on clean concrete slabs lined with blocking shields was also effective and may even in the long run prove simpler. Concrete absorbed considerable energy from the sun during the day which radiated off of the concrete as heat and helped accelerate drying of the larvae.

Figure 2 shows the progressive loss of water tracked in approximately six hour intervals following the loss in larval mass while test samples placed in plastic trays were exposed to the sun. Temperatures inside the drying trays reached levels in the range ~ 65 to 75° C (~149 - 167° F) at mid-day.

Figure 2. Loss of water measured by weight loss of Black Soldier fly prepupae harvested from food scrap waste after placing the larvae in plastic drying trays. Measurements were made at approximately 6 hour intervals. Overall loss of water was ~60% of the starting content. Copyright 2013(c) Terry Green, All rights reserved.

Prepupae were moved indoors to guard against moisture condensation in the evening, and brought outside into the sun the following day. It took approximately 3 days of continuous exposure to direct sunlight to drive off all of the moisture (prepupae residual weight, ~40% of the starting weight). Harvested prepupae dried in this manner (Fig. 1(b)) showed no signs of deterioration even stored at room temperature in the dark after three months of storage.

While there are many modern drying techniques available in the food industry including convection, forced air and freeze drying technologies which may likewise work well in processing BSF larvae, dewatering in general requires the input of significant energy and is expensive. Our interest in exploring the drying potential of solar power as an option in preserving BSF larvae is principally based upon keeping the cost of building and operating a large scale production facility as low tech and least costly as practical.

In time, once markets are in place and income streams stabilize marketing BSF products produced off of food scrap and agricultural wastes, it may become economically practical to invest in alternate drying technologies which would likely lead to shorter drying times. Nevertheless, our results show that BSF larvae can be dried efficiently outdoors in the sun.

In these tests we chose to dry the larvae in situ in keeping with pursuing the least complicated and straightforward method of obtaining a final stable product. In feeding trials, we observed chickens quickly devoured the dried larvae without hesitancy and with no observable adverse effects. Larvae dried in this manner feel and have a weight and texture similar to that of dried sunflower or pumpkin seeds.

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