1. Introduction
The current food production system is failing to end hunger by 2030 in accordance with the sustainable developmental goal number 2. In fact, approximately 2 billion people experienced severe and moderate food insecurity in 2019, higher than in 2014 [
1]. However, global food insecurity is mainly associated with limited access to nutritious and sufficient food, food losses and waste [
1], rather than being a consequence of an overall global food shortage [
2].
Approximately 1.6 billion tons of the produced food is lost and wasted annually, from which 1.3 billion tons is edible. This waste leads to high environmental impacts being the third largest CO
2 emitter after USA and China. Moreover, approximately 250 km
3 of fresh water and 30% of the total agricultural land are used to produce food which then is wasted. Furthermore, high losses in biodiversity and natural ecosystem function are associated with monoculture practices and agricultural expansion. In consequence, overall food waste and loss is an important contributor to climate change. While food loss occurs during production, storage and distribution, food waste occurs at the retail or consumer level [
3]. However, the highest waste of food is seen in developed countries at consumer level where approximately 95–115 kg of food/capita/year is wasted [
4].
Following the global tendence, the import of protein sources used in animal production in Denmark is linked to high environmental and socio-economic impacts and consequently to climate change [
5]. Furthermore, Denmark is an important producer of food waste in Europe with an estimate of food waste of around 1.1 million tons generated in 2016 [
6]. In order to address waste production, including food waste, the Danish Government developed the Resource Strategy for Waste Management which aims among other goals to optimize the use of resources, improving recycling and energy recovery across different industries under the Circular Economy paradigm [
7].
New systems, which can recover protein and fat from food waste and further utilize them as feed sources, can help decreasing environmental impacts and food waste at both local and global levels. Such a system is insect bioconversion, which can up-cycle viable nutrients and energy from waste streams such as food waste into high protein and fat larval biomass suitable for animal feed [
8]. Among the most promising insect species for bioconversion of food waste is
Hermetia illucens (Black Soldier Fly (BSF)). BSF is a holometabolous Diptera specie, native to Neotropics and currently spread across the temperate and tropical regions [
9]. BSF is known to only feed during the larval stage, while the adult serves only reproductive purposes and, therefore, is not considered a pest [
10]. In the natural habitat, a female BSF lays eggs in the vicinity of decomposing materials. After emerging from the eggs, the larvae of BSF actively feed on the decaying materials until they reach the prepupa stage, when they migrate outside the decaying materials and find shelter and drier vegetation, where pupation is initiated. Consequently, metamorphosis occurs, and the life cycle is repeated. [
11] Numerous studies find that BSF larvae (BSFL) can successfully bioconvert different low-value organic side streams into high protein and lipid larval biomass that can be used as animal feed [
12,
13,
14,
15,
16,
17]. According to Lalander et al. (2019), the BSFL can be used in the management of different waste streams including: abattoir waste, food waste, human feces and a mixture of: abattoir waste, fruits and vegetables [
12]. Furthermore, the utilization of BSFL for the management of other waste streams including manure (poultry and swine), biogas digestate, vegetable and fruit wastes and municipal waste were previously documented and discussed in multiple review articles [
13,
14]. Similarly, the utilization of BSFL as feed for poultry, swine and in aquaculture is extensively covered and discussed in multiple reviews [
15,
16,
17]. Previous studies have demonstrated that the production of BSFL protein has lower environmental impact than the conventional sources of protein (fishmeal), making this a promising source of sustainable protein [
18]. However, further research and local governmental support in the form of R&D grants are required to fully assess the potential of producing insects on waste streams, such as former foodstuffs, catering waste and organic household waste, not least to document overall safety.
The present study was conducted as part of the Waste, Insects and Circular Economy (WICE) project funded by the Danish Environmental Protection Agency’s Environmental Technology Development and Demonstration Program (MUDP) and was conducted between 2017 and 2019. The project had the scope to demonstrate the potential of up-cycling organic household waste and catering waste into high-quality macronutrients in the form of insect biomass, suitable to be used as a feed source. The nutrient digestibility of BSFL was addressed in a study using mink (Neovison vison (L.)). The mink is a potential consumer of BSFL, since it is outside the food chain and thus legally can be fed insects, but the mink also serves as a model animal for other carnivorous animals such as pets and fish. To achieve the scope of the project, a pilot production of BSFL was established, and a series of experiments for optimizing the production and testing the utilization of BSFL biomass as a feed ingredient outside the food chain were conducted.
4. Discussion
The utilization of food waste as rearing substrate for BSFL has already demonstrated by other authors [
23,
24,
25,
26]. However, most of the studies are either conducted at laboratory scale or during single events and, consequently, could have limitations or be unreliable when implemented at industrial setup [
9]. Consequently, the production of BSFL reared on food waste in an industrial relevant setup (pilot scale) and during multiple batches, conducted during the current project, provides additional information for industrial scale applications.
During the 14 months of pilot scale production, 190 kg DM of food waste was bioconverted into approximately 79 kg of DM BSFL, indicating that the utilization of food waste as rearing substrates for BSFL could be suitable at industrial scale. However, the quality of the rearing substrate such as nutrient composition and DM content is known to influence the development, performance and survival of BSFL [
14]. Cheng et al. (2017) found that a decrease in food waste DM from 30 to 20% can decrease the larval development in approximately 5 days but will also make the separation process (larvae from insect frass) harder [
27]. As discussed by Lalander et al. (2020), a decrease in DM content from 24 to 2.5% can lead to an increase in mortality associated with drowning, from approximately 3 to 81% [
24]. The DM content may decrease the overall production output (larval biomass) because of a high migration rate and high mortality. Dortmans et al. (2017) recommend that in the case of a DM content lower than 15%, the substrate should be dewatered, and an optimal DM of 20–30% should be maintained for BSFL reared on food waste [
28]. Similar to Dortmans et al. (2017), during this study, the DM was maintained between 15% and 23%, using chicken feed as correction substrate. This procedure was found to not only increase the DM content but also to increase the porosity of the substrate, allowing the larvae to feed without risk of drowning. Furthermore, the DM ensured high larval production and short production time (8–10 days). The substrate macronutrient profiles (lipids, proteins, carbohydrates and fibers) are known to have the largest influence on the efficiency of the production and may vary across food waste [
13]. Although the nutrient quality of the food waste was not considered for all the batches, with the exception of B 12 in which protein and fat content was analyzed, the production of BSFL on this substrate was found to be efficient, as indicated by the FCRs.
Overall, the FCR was found to be relatively low after Batch 5 (stabilization of the production) and to have a relatively low variation (1.7–2.9, FCR (DM)). To assess the replicability of BSFL performances during multiple batches (batches: 6–14) in terms of FCR, the coefficient of variation (CV) was estimated to be 19% for the FCR (DM) and 20% for the FCR (DM/FW). Such relatively low CV suggests that the production of BSFL on biopulp can be replicated with high reproducibility. The FCRs obtained during our production were much lower compared to FCRs obtained by other studies [
9]. Thus, the FCR obtained during the production of BSFL on municipal household waste was found to be 14.5. However, as the author discussed, the efficiency of this system was hardly affected by larval mortality associated with zinc contamination and high moisture content in the substrate. [
29] Furthermore, Giannetto et al. (2020) obtained an FCR of 9.3 during the 5th instar and 12.5 at the prepupa stage of BSFL when reared on vegetable wastes [
30].
The overall low FCR is hypothesized to be caused by the high quality of the biopulp with a balanced nutrient content (protein: as a result of being produced from diverse sources of food wastes (restaurants, canteens and households)) and at a high volume (>10 tons/batch). Moreover, the nutrients in biopulp were highly accessible for BSFL because of fractionization and partly stabilized as a result of the fermentation procedure. However, although small FCR and high replicability were obtained during the pilot production, further optimization studies are required before utilizing the biopulp in commercial setups. These studies should aim for ensuring low FCRs and decreasing variation across batches, thus securing high and stable production outputs during commercial rearing. Moreover, further optimization studies should be considered for reducing the operational expense associated with maintenance and handling.
In this study, a feeding experiment was performed to assess the BSFL performance as a result of different feeding management, thus providing insight in optimizing the feeding while reducing the handling. During this experiment, we were able to bioconvert 8.7 kg of food waste (fresh weight) into 550 g larvae and 530 g insect frass (DM basis) per production tray over a period of 10 days. Similarly, Ermolaev et al. (2020) were able to bioconvert approximately 15 kg (fresh weight) of similar food waste into approximately 1 kg BSFL and 1 kg of insect frass (DM basis), respectively, per production tray over a period of 21 days [
31], indicating that the production of BSFL on food waste can be achieved in different setups.
The production performance of BSFL associated with two and five feedings was found to be similar, except for PCE which was higher as a result of multiple feedings. Such results indicate that a decrease in handling, without affecting the production performance, can be obtained by reducing the feedings to two events. Banks et al. (2014) found that feeding the larvae every second day will result in smaller larvae and faster development time compared to one feeding, which leads to the production of larger larvae during a longer period. However, the increase in larval weight and development time is believed to be primely affected by alteration of substrate quality in case of one feeding. [
32] The BSFL are known to have the highest feed intake during the latest stage of development during which reserves are accumulated as fats for ensuring successful metamorphosis to adults and for increasing the chance to reproduce [
33]. However, in the case of low-quality substrates, as in the case of aged materials, the larvae are able to develop compensatory feeding in order to ensure that required nutrients for further development are ensured [
32]. Thus, by administrating in the beginning and at a later stage (day 6 in this experiment), when larvae are in later development stages, the producer can ensure a fine balance between larval biomass output and the production time. Both biomass reduction and PCE were found to be high, with values over 50%, indicating that high food waste reduction and protein recovery can be obtained as a result of bioconversion with BSFL.
The potential of BSFL for feeding carnivores was addressed by evaluation in mink. Mink are characterized by having a high demand for amino acids, a short gastro-intestinal tract with no caecum and thus a short digesta passage time little affected by microbial activity [
34]. These factors result in the need for high-quality and highly digestible feedstuffs. Studies have shown that the mink is a suitable model animal for evaluation of small intestinal protein digestibility in dogs [
35,
36] and digestibility in rainbow trout [
37] and Atlantic salmon [
38], and thus the present results will be applicable for formulating pet food and fish feed with BSFL. The current results on total tract apparent digestibility point towards BSFL as excellent sources of crude protein and fat. The current findings of 86.2 and 90.4% exceed or match those found recently for fish meal by Tjernbekk et al. (2019) of 82.9 and 90.5% apparent total tract digestibility of crude protein and fat, respectively, as well as the commonly used poultry meal with 74.8 and 81.0% apparent total tract digestibility [
39]. The carbohydrate digestibility, however, was low, and this may be caused by the chitin content of BSFL being accounted as carbohydrate in the proximate nutrient analysis and chitin being resistant to digestion by the animals’ own enzymes, thus depending on microbial fermentation [
40] which is low in mink.
Although the bioconversion of food wastes into high-quality BSFL can be successfully used as feed for pet food, the utilization of food waste as rearing substrate for insects is subjected to a series of constrains and requires further clarifications. The classification of whether a certain substrate is waste or not has large regulatory implications regarding whether it is legal to use as a feed material for farmed animals. Since 2017, seven species of insects, including
H. illucens,
Tenebrio molitor and
Musca domestica, have been defined as farmed animals by the European Commission and are, therefore, subject to legal requirements regarding general food law and feed hygiene to be compliant on the EU market. At present, former foodstuffs containing meat and fish, catering waste and organic household waste are not legal to use as feed for insects in the EU. However, the EU regulatory framework for farmed animals has been constructed for vertebrates (e.g., fish, poultry, pigs, and cattle), and insects are, as invertebrates, very different from the relatively small group of animals that constitute and dominate current animal farming. For instance, when it comes to feed safety some of the relevant species in insect farming appear to have biological competences that can overcome some of the regulatory barriers that have been implemented to secure the food production system. There is increasing evidence that
H. illucens has the ability to perform biosanitation on substrates that contain pathogen microorganisms [
41]. The biological mechanism behind this ability appears to be caused by a high enzymatic activity in the larval gut as well as an intersectional difference in pH throughout the gut (ranging from approximately pH 2 to 8). Additionally, this is further supported by various defensive mechanisms including different antimicrobial peptides (AMPs) that help to control potentially harmful microorganisms [
29,
42]. Moreover, several studies provide support of detoxification of certain pollutants [
43] as well as degradation of some mycotoxins [
44], while there is also evidence of bioaccumulation of certain metals [
45]. These biological competences are as such not unique only to
H. illucens but have also been observed in other species like
M. domestica [
46].
Currently, the European insect producers, organized by the trade association IPIFF, aim to get former foodstuff containing meat and fish legalized as feed for insects, and there is hope regarding the biosanitation potential of certain insect species such as H. illucens. However, a lot of evidence still needs to be provided, not least at demonstration scale, prior to obtaining approval in the EU for using insects fed on organic wastes for application as, for example, pet food or fish feed.