In this study, one-hundred and thirty five samples of unifloral honey from 15 common species-specific Australian floral types and one New Zealand floral type were screened for total antimicrobial activity (antimicrobial activity due to osmolarity, acidity, hydrogen peroxide and additional unidentified factors) and non-peroxide antimicrobial activity (residual antimicrobial activity when hydrogen peroxide is destroyed by addition of catalase during assay) against the reference microbial strain. Staphylococcus aureus, in a seeded well diffusion assay. Total antimicrobial activity was not present in all Australian honey samples and when present this varied significantly from sample to sample.
Non-peroxide antimicrobial activity was detected in samples of only five floral types; and found in only single samples from sources other than Leptospermum honey. Thus, 38 samples of jelly bush (Leptospermum sp.) honey were screened against 15 food spoilage microorganisms (five Bacillus stearothermophilus, three Zygosaccharomyces bailii, two Zygosaccharomyces rouxii, two Pseudomonas fluorescens, Pseudomonas putida, Pseudomonas cepacia and Lactobacillus plantarum) and seven food pathogens (Bacillus cereus (ATCC 49664), Staphylococcus aureus (ATCC 13625) Pseudomonas aureginosa (ATCC 15022), Pseudomonas aureginosa (ATCC 12066), Vibrio parahaemyliticus. Salmonella spp. and Listeria monocytogenes) to determine the spectrum of antimicrobial activity. Most honeys exhibiting antimicrobial activity inhibit the growth of a wide range of food pathogens, but the extent of antimicrobial activity of a particular honey varies with the test species. No generalisations regarding the susceptibility of gram positive versus gram negative organisms to the antimicrobial activity of Leptospermum honey can be drawn. Generally, food spoilage microorganisms were not inhibited by the test honeys beyond the antimicrobial effect of osmolarity and acidity. However, all of the food pathogens (except Pseudomonas aureginosa) strains were inhibited by the non-peroxide antimicrobial activity of Leptospermum honeys.
Colour, moisture, ash, electrical conductivity, specific rotation, proline, HMF, invertase, diastase, carbohydrates (fructose, glucose, sucrose, maltose and turanose), granulation indices, pH, free acidity, lactone acidity and total acidity were determined in 135 samples of 15 species-specific types of unifloral Australian honey and one New Zealand floral type. The analytical methods used in the compositional work (except HPLC carbohydrate analysis) are those used in routine honey control and can be used to determine the quality criteria for honey specified in the European and Codex Alimentarius standards.
In most cases, the unifloral samples of Australian honey would easily meet the regulations of importing countries. Besides higher moisture, ash, acidity and HMF contents and lower glucose and diastase contents, the honey samples assayed in this study are very similar to values obtained in previous studies of unifloral Australian honey. However, the composition profile of unifloral samples of Australian honey show little conformity with the composition profiles of other international honey samples.
Generally, within the temperature range of 0-40 °C, honey varieties investigated in this study were Newtonian, time-independent, and the viscosity over this temperature range can be predicted using an Arrhenius model, if the Arrhenius constants for that particular honey are known. Similarly, viscosity can be predicted using a model based on solute concentration. From the results of this investigation, it can be concluded that carbohydrate concentration is the major factor contributing to honey viscosity. Shear thinning rheological behaviour is reported for the first time in honey samples from the unifloral types, jelly bush (Leptospermum sp.) and manuka (Leptospermum scoparium), and in some narrow-leaved ironbark (Eucalyptus crebra) and stringybark honey samples (Eucalyptus muelleriana).
As antimicrobial activity in honey varied with some honey samples displaying no antimicrobial activity, it seemed likely that there would be chemical or physical differences between active and inactive honey samples. Flow behaviour index and moisture content are the only physico-chemical parameters that consistently differ significantly between active and inactive honey samples when both total and nonperoxide activity are considered.
Moisture content, invertase level, free acidity, lactone acidity and total acidity were all significantly higher, and flow behaviour index significantly lower in honey samples displaying total antimicrobial activity than inactive samples. However, multivariate factor analysis did not reveal any patterns of correlation among any of the physicochemical parameters assayed relative to total antimicrobial activity, and none of the physico-chemical factors were linearly correlated with total antimicrobial activity.
Non-peroxide active honey samples were significantly darker, had significantly higher free and total acidity, HMF, moisture and fructose contents, but significantly lower flow behaviour indices, sucrose contents, pH, invertase and diastase levels than inactive samples. HMF content, free acidity, lactone acidity, total acidity, colour, proline content, moisture content and electrical conductivity revealed strong patterns of correlation relative to non-peroxide antimicrobial. However, the only significant linear correlations were free and total acidity with non-peroxide activity.