Elsevier

Food Microbiology

Volume 91, October 2020, 103535
Food Microbiology

The microbial safety of seaweed as a feed component for black soldier fly (Hermetia illucens) larvae

https://doi.org/10.1016/j.fm.2020.103535Get rights and content

Highlights

  • Drying seaweed at low temperatures retains its nutritional properties.

  • Nutritional benefits of insect larvae can be improved by dietary inclusion of seaweed.

  • Lower drying temperatures risks pathogen carry-over into insects destined for feed.

  • Black soldier fly larvae fed seaweed can become contaminated with human pathogens.

  • Industry-wide regulation needed before seaweed-fed insects promoted as feed and food.

Abstract

Farmed insects can offer an environmentally sustainable aquafeed or livestock feed ingredient. The value of black soldier fly (Hermetia illucens) (BSF) larvae could be improved by enrichment in omega-3 through the dietary inclusion of seaweed. However, the industry practice of drying seaweed at low temperatures to retain nutritional properties may benefit the survival of human pathogenic bacteria, particularly if the seaweed has been harvested from contaminated water. Here we have demonstrated that E. coli and E. coli O157:H7 died-off in seaweed dried at 50 °C, although both were detected in the dried powder following 72 h storage. V. parahaemolyticus fell below the level of detection in stored seaweed after drying at ≥ 50 °C, but L. monocytogenes remained detectable, and continued to grow in seaweed dried at ≤60 °C. Therefore, drying seaweed at low temperatures risks pathogen carry-over into insects destined for animal feed. BSF larvae reared on an artificially contaminated seaweed-supplemented diet also became contaminated by all four bacteria present in the supplement. Water quality at seaweed harvesting sites, seaweed desiccation, and insect rearing practices, represent critical points where development of regulatory standards could achieve targeted control of pathogenic hazards.

Introduction

Seaweed meal is a recognised animal feed substrate in the EU (Reg (EC) 68/2013; EC, 2013). It can provide a supplementary source of energy, proteins, minerals, lipids, vitamins and antioxidants (with bioactive value) for livestock and aquaculture, and most recently, for the mass production of insect larvae (Rajauria, 2015; Liland et al., 2017). The concept of insect protein as a sustainable animal feed ingredient has gathered increasing acceptance across Europe and is now permitted in aquafeed within the EU (Reg (EC) 893/2017; EC, 2017). Recent innovative efforts to combine these two ingredients into aquaculture feed for farmed carnivorous fish has seen advances in the mass production of seaweed-fed black soldier fly larvae (BSFL), Hermetia illucens (L.) (Diptera: Stratiomyidae) (Belghit et al., 2018; Swinscoe et al., 2019). The benefit of feeding insect larvae with seaweeds includes utilizing a renewable feed resource that does not compete with sources of human food or require land use, additional water or industrial fertilization. In Europe, seaweed for animal feed is typically wild harvested from coastal marine waters (Makkar et al., 2016); however, wild harvested seaweeds can also become colonised by human pathogenic bacteria e.g. species of Vibrio and strains of Escherichia coli (Quilliam et al., 2014; Mahmud et al., 2007, 2008). Molecular methods have detected Salmonella enterica ser. Typhimurium, V. parahaemolyticus and E. coli O157:H7 on the farmed kelp Saccharina latissima, and potentially toxin-producing, spore-forming Bacillus licheniformis and Bacillus pumilus have both been isolated from the cultivated kelps Alaria esculenta and S. latissima (Barberi et al., 2019; Blikra et al., 2019). In addition, Cladophora (a freshwater species of macroalgae) has been shown to harbour E. coli, Campylobacter, Shigella, Salmonella and C. botulinum (Byappanahalli et al., 2009; Ishii et al., 2006). Therefore, before seaweed supplements in BSFL diets can be advocated for mass-reared insect production, critical control points (CCPs) during the production of seaweed-fed BSFL must be identified (Swinscoe et al., 2019) in order to guarantee safety of this novel animal feed if it is to enter the human food chain (Reg (EC) 183/2005; EC, 2005).

Standardised processing methods in the feed and food industries are key to product quality and safety, but such a system is currently lacking in the seaweed industry. There are also no microbiological standards for seaweed meal in the EU, and those for insect processed animal proteins (PAPs) in feed are limited to maximum levels of Clostridium perfringens, Salmonella spp and Enterobacteriaceae (Reg (EC) 142/2011; EC, 2011). There is limited evidence for Listeria spp. being present on freshly harvested seaweed (Banach et al., 2020). However, its ubiquity in food and feed processing environments (Carpentier and Cerf, 2011) and resulting opportunity for contamination of feed materials warrants inclusion of L. monocytogenes in inactivation studies of processing effects on seaweed-associated pathogens. Although processing-based interventions for controlling microbial contamination of seaweeds have been explored, e.g. washing and drying (del Olmo et al., 2018; Hyun et al., 2018), the full range of potential microbiological hazards associated with seaweed entering the feed and food chain are not necessarily controlled by existing industrial practices, or accounted for by current feed hygiene regulations.

Typical post-harvest processing of seaweed for animal feed involves (i) washing to remove visible epiphytic flora and fauna; (ii) reduction of bulk and water activity (aw) by hot air drying, which inhibits microbial growth and biochemical degradation; (iii) milling, packaging and storage at room temperature for up to one year. Washing seaweeds however, fails to eradicate coliforms or V. parahaemolyticus, and E. coli can replicate on seaweed during desiccation and storage (del Olmo et al., 2018; Mahmud et al., 2008). Importantly, the higher the seaweed drying temperature, the greater the nutritional loss of the seaweed biomass. The industrial drying of seaweeds therefore needs to be balanced between using a temperature that can sufficiently desiccate the seaweed and destroy bacterial contaminants against potential nutritional losses. Nutritional loss occurs through the denaturation of proteins, oxidation of lipids and the loss of anti-oxidant activity in the seaweed product (Stevant et al., 2018; Lage-Yusty et al., 2014; Moreira et al., 2016; Gupta et al., 2011).

Insect farming to produce animal feed is still a nascent industry in the EU but it is widely acknowledged that the microbiological safety of insects is fundamentally influenced by the hygienic status of their feed (Van der Spiegel et al., 2013). Autochthonous bacteria and allochthonous opportunistic bacteria (including human pathogens) colonise insects either parentally or horizontally from their environment, and are harboured in the insect gastrointestinal tract (GIT), which together with the mouthparts and body surface is the main niche for insect-associated bacteria (Schlüter et al., 2017). Commensal, food spoilage and human pathogenic bacteria, including Enterobacteriaceae, Pseudomonas spp. and Clostridium sp. have been isolated from BSFL (Jeon et al., 2011; Wynants et al., 2019). Thus, good manufacturing and hygiene practices (GMP and GHP) specific to each insect species, the feed substrate, the life stage at harvest, and the production environment need considerable development as CCPs emerge at which pathogens may be introduced, persist or replicate in the insect product (Van Raamsdonk et al., 2017). Therefore, the aims of this study were to: (1) Determine colonisation dynamics of a range of human pathogenic bacteria on a combined mixture of submerged brown, red and green seaweeds in an intertidal simulation of exposure to a wastewater pollution event. (2) Evaluate the effect of typical industrial processing practices (washing, drying and storage) on the survival of bacteria attached to seaweeds. (3) Assess the survival dynamics of these bacterial contaminants when fed to BSFL as a powdered seaweed feed supplement. (4) Identify CCPs where feed manufacturers can target control of bacterial hazards during production of seaweed feed and its application as a feed supplement for the mass rearing of BSFL.

Section snippets

Bacteriological safety of processed seaweed (Experiment 1)

A model system of postharvest industrial processing of seaweed was developed involving sequential stages of washing, drying, milling and storage. Sampling for bacteriological quality was conducted at key stages of the process.

Seaweed material

Living, attached intertidal seaweeds of the species Laminaria digitata (Hudson) (Phaeophyceae), Fucus serratus (L.) (Phaeophyceae), Palmaria palmata (L.) (Rhodophyta) and Ulva lactuca (L.) (Chlorophyta), together with seawater from the surf zone, were collected at low tide

Background bacteriological status of seaweed and seawater

E. coli was not detected on the freshly harvested seaweed used in both Experiments 1 and 2, and was present at a very low concentration (<10 CFU 100 ml−1) in the seawater from which the seaweed was harvested. Total heterotrophic bacteria were present in low abundance on all species of seaweed and in seawater, the highest concentrations being detected on L. digitata and in seawater (data not shown).

Bacteriological safety of processed seaweed

After 24 h in a rotating incubator at room temperature (20.5 °C ± 3 °C), concentrations of E. coli

Bacteriological safety of processed seaweed

This study demonstrates that the current post-harvest processes of washing and drying seaweed intended for animal feed can fail to eradicate (and can even encourage the survival of) E. coli and selected human pathogenic bacteria if seaweed has a high level of contamination at the point of seaweed harvest. The inadequacy of these manufacturing practices therefore, can result in a dried seaweed product in which human pathogenic bacteria can persist during storage, although survival will be

Conclusion

Ensuring production of safe novel animal feed ingredients requires understanding of both the specific bacterial hazards associated with the novel ingredients, and the response of those bacteria to abiotic and biotic processing stresses. Persistence in seawater, and rapid colonisation of brown, red and green seaweeds, by some key human pathogens, indicates that water quality at seaweed harvesting sites should be managed as part of Good Agricultural Practice (GAP) as the first line of defence to

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This PhD project was joint funded by the University of Stirling, and the Institute of Marine Research (Norway). We thank Prysor Williams (Bangor University) for providing the strain of E. coli O157:H7, Kieran Jordan (Teagasc Food Research) for providing the strain of L. monocytogenes, and Craig Baker-Austin (CEFAS) for providing the strain of V. parahaemolyticus.

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