Commercial propolis liquid products : comparison of physicochemical properties and antioxidant and antimicrobial properties : a thesis presented in partial fulfilment of the requirements for the degree of Master of Food Technology at Massey University, Auckland, New Zealand

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Propolis is a resinous substance, which is well-known for its functional properties (e.g. antioxidant, antimicrobial and anti-inflammatory) and collected by honeybees from various plants. Due to its various health beneficial properties, propolis is widely used in many products (e.g. foods, beverages and toothpastes) and liquid propolis extract products are also commercially available as natural healthy supplements. Raw propolis has been broadly investigated, while, there has been much less research on the physicochemical and functional properties of commercial liquid propolis products. This study was thus aimed to evaluate and compare 20 commercial propolis liquid products manufactured in 4 different countries (Australia, China, Korea and New Zealand), in terms of physiochemical properties (e.g. water and ethanol miscibility, colour, pH), chemical composition (e.g. total phenolic and total flavonoid contents) and functional properties (e.g. antioxidant capacity and antimicrobial activity). Besides, all propolis samples were analysed for the detection of heavy metal (e.g. lead, cadmium, and arsenic) and rare earth elements in order to determine the safety and quality of propolis products. Also, the content of salicin in propolis was measured as an indicator of the adulteration of propolis with poplar tree gum. The visual colour of liquid propolis products varied from dark brown, red to green. Almost all commercial propolis samples analysed in this study were more soluble in ethanol than in water, except a propolis sample containing Tween 20 (emulsifier). Most propolis samples were also acidic with pH < 5, whereas, the Korean propolis samples containing potassium carbonate had alkaline pH values. The analysed total flavonoid (TF) content of 19 propolis products matched their labelled values specified on their product packaging. However, some unexpected results were obtained with the TF content being measured to be higher than the total phenolic (TP) content from 4 Korean propolis samples, in which salicin was also detected. This indicates that those 4 propolis products might have been adulterated with poplar tree gum. In terms of the functional properties of propolis, it was found that their antioxidant activity highly corresponded to the TP and TF contents. On the contrary, there was no linear correlation between TP or TF content and antimicrobial activity of the propolis products. Propolis products showed a greater effect on Gram-positive bacteria (S. aureus and B. cereus) than the Gram-negative bacterium (E. coli). Among all propolis samples from the different regions, the propolis samples from New Zealand had a relatively higher TP and TF content and also showed a higher antimicrobial activity than the propolis samples from the other countries. Nevertheless, the content of heavy metal elements (As and Pb) detected was relatively much higher in New Zealand propolis products than that from the other countries. On the other hand, liquid propolis products from Australia contained less heavy metal elements and had the lower possibility of adulteration by poplar tree gum and the stable antioxidant and antimicrobial activities, which seemed to be a better choice among the 20 samples studied in this study. In conclusion, since there is no proper criterion to monitor the quality of propolis, it is necessary to develop a series of indices to evaluate the commercial liquid propolis products, for example, sensory (colour and smell), chemical composition (TP and TF contents), functional properties (antioxidant and antimicrobial activities) and safety properties (heavy metal elements and adulterations).
Figure 2.2 is re-used under a Creative Commons Attribution 3.0 (CC BY 3.0) license. Figure 2.3 (=Siheri et al., Fig 7.3 p.140, in Alvarez-Suarez, 2017) was removed for copyright reasons.