Physicochemical properties and phenolic content of honey from different floral origins and from rural versus urban landscapes
Introduction
Honey produced by honey bees (Apis spp.) can promote many beneficial biological processes in humans, including: antioxidant (Aljadi and Kamaruddin, 2004), antibacterial (Gomes, Dias, Moreira, Rodrigues, & Estevinho, 2010), antidiabetic (Bobiş, Dezmirean, & Moise, 2018), anti-inflammatory (Shah & Amini-Nik, 2017) and antimicrobial activity (Gomes et al., 2010), as well as radical scavenging (Meda, Lamien, Romito, Millogo, & Nacoulma, 2005), and wound healing (Efem, 1988). Of these, food antioxidants have been shown to reduce oxidative stress and thus prevent oxidative damage (Pisoschi & Pop, 2015), and are thus of great interest for promoting human health and wellbeing. The constituents of honey that are responsible for antioxidant effects are flavonoids, phenolic acids, ascorbic acid, catalase, peroxidase, carotenoids, and products of Maillard reactions (Gheldof & Engeseth, 2002). Phenolic acids and flavonoids have been comprehensively researched in honey (Dimitrova et al., 2007, Ferreira et al., 2009, Kečkeš et al., 2013, Khalil et al., 2011, Pyrzynska and Biesaga, 2009) and can be used to assess honey quality. Several studies have shown that antioxidant activities are strongly correlated with the concentration of total phenolics. This was confirmed for seven single origin Italian honeys (Rosa et al., 2011), four types of Romanian honeys (Mǎrghitaş et al., 2009) and seven types of Slovenian honeys (Bertoncelj, Dobersek, Jamnik, & Golob, 2007). A study of Portuguese honeys demonstrated that the phenolic content of honey is not only responsible for its antioxidant properties, but also for its anti-microbial effects (Estevinho, Pereira, Moreira, Dias, & Pereira, 2008).
It is widely agreed that the botanical origin of honey has the greatest influence on its phenolic content and thus antioxidant activity, and some floral honeys are perceived to have a higher quality than others. For example, some-single origin honeys (i.e. from a single plant taxa, e.g. Manuka honey (Leptospermum scoparium) from New Zealand and heather honey (Calluna/Erica spp.) from northern Europe) have a higher retail price than blended honey (a mixture of two or more honeys) (National Honey Board, 1991). The purity of the honey also plays a role in its quality and Manuka honey has varying UMFs (Unique Manuka Factors) whereby the designated factor, price and potential health benefits correlate positively (Beitlich, Koelling-Speer, Oelschlaegel, & Speer, 2014).
In this study, we focus on honey produced in Ireland. The main feature of the Irish honey market is that most of the production comes from small private producers: according to the Federation of Irish Beekeepers, there were 2800 registered beekeepers in Ireland in 2014 and the average number of hives per beekeeper was three (Federation of Irish Beekeepers, 2015, personal communication). Given the maritime (relatively cool and damp) climatic conditions of Ireland, the production of honey is limited, and the majority of Irish beekeepers only harvest honey once a year. Despite this, trade in honey increased in value and tonnage in Ireland between 2009 and 2013. Within these years Ireland imported seven times more honey than it exported (Central Statistics Office, 2013). Increasing consumer preferences for products based on honey has led to the expansion of the honey market, resulting in a greater number of varieties of honey and varieties of honey-based foods. This has also led to increased demand for single-origin honeys (such as heather and ivy (Hedera spp.)) particularly those with perceived health benefits. Being able to identify particular characteristics in honey which are associated with honey’s health benefits, and rapidly assessing these characteristics in Irish honey could help beekeepers to market their honey appropriately and increase its commercial value.
The main aim of this study was to investigate the chemical composition (in terms of total phenolic content) and physiochemical parameters (honey electrical conductivity, moisture, pH, total sugar and colour) of different types of Irish honey and compare these with international honeys. In addition, microbial contamination and microbial stability of Irish multi-floral honey was assessed. The comparison of Irish single-origin honeys (heather, ivy and oilseed rape (Brassica rapa) and multi-floral honeys has to our knowledge never previously been made, nor has a comparison of honey from urban vs. rural hive sites. In addition, the relationship between total phenolic content and physiochemical parameters was assessed to enable rapid assessment of Irish honey quality. Results confirmed that total phenolic content (TPC) could be used to differentiate between honey types. Urban multi-floral honeys had a higher TPC than rural multi-floral honeys. Irish heather honey had the highest TPC of all Irish single origin honeys, and had a higher TPC than Manuka honey. It was illustrated that honey colour correlates with electrical conductivity and with TPC, although ivy honey was an exception to this. While ivy was the darkest Irish honey analysed, its TPC was less than that of heather and Manuka honeys. To our knowledge this is the first time that ivy honey and Irish honey have been comprehensively researched.
Section snippets
Sample collection
One hundred and thirty-one Irish honey samples from Apis mellifera were collected directly from beekeepers between 2013 and 2015, from 78 locations across Ireland (Fig. 1). The majority (124) of samples were multi-floral honeys (55 from urban and 69 from rural locations), three were heather (Erica and/or Calluna spp.) honeys, two were ivy (Hedera spp.) honeys and two were oilseed rape (OSR) honeys. Honeys were classified according to landscape context (urban/rural) (Dijkstra & Poelman, 2014) or
Total phenolic content
The total phenolic content (TPC) of Irish multi-floral honeys (including urban and rural honeys) ranged from 2.59 to 81.10 mg of gallic acid equivalent (GAE)/100 g of honey, (n = 124, Mean ± SD = 23.84 ± 13.07) (Fig. 2(a)). This TPC range is similar to the ranges typically found for European honeys (Kús et al., 2014, Mǎrghitaş et al., 2009). A large proportion of the samples (over 85%) were within the range of 10–50 mg GAE/100 g of honey, (n = 106, Mean ± SD = 23.66 ± 8.62). The variation in
Conclusion
Our findings confirm that the differences in phenolic profiles of honey can be attributed to the floral origin of the honey. Multi-floral honeys from different landscapes (i.e. containing different land uses and therefore a different composition of floral resources) and single origin honeys were demonstrated to contain different phenolic contents. Irish heather honey had the highest TPC of the three Irish single origin honeys whilst OSR honey had the lowest. This study has also confirmed that
Acknowledgements
Financial support was provided by the Irish Research Council, Ireland (Postgraduate Scholarship, S. Kavanagh) and Dublin City University, Ireland (Career Enhancement Award, B. White). The beekeepers of Ireland are gratefully acknowledged.
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