The goal of all food processing industries is to produce a high-quality product at the lowest possible cost, which is the result of exposing the raw materials to a series of processes such as heating, cooling, pressure, and mixing. For this reason, food factories must determine the characteristics of the raw materials and control the food and process conditions at each stage of the process so that the characteristics of the final product are as similar as possible to the desired characteristics predicted in advance. Nowadays, significant successes have been achieved by using non-thermal technologies to increase food's shelf life and safety. Among these techniques, we can mention supercritical carbon dioxide, high hydrostatic pressure, cold plasma, ozone technology, and ultrasound which have increased the quality and reduced spoilage in food, sometimes in the successful preservation of food for a long time. Since some foods have heat-sensitive nutrients, these methods can be helpful.
Ultrasound is a technique that can affect the growth capacity of cells that resist the treatment and ultimately reduce the fermentation in honey. Among the practical applications of ultrasound waves are checking the texture, viscosity and concentration of some solid and liquid foods, measuring thickness, surface, and temperature, and determining the composition of fruits, vegetables, meat, dairy products, and other products.
In current study we investigate the effect of ultrasound on the textural, physicochemical, and microbial properties of industrial Iranian honey (produced by Apis mellifera bees). The honey samples were treated using 30 or 42 kHz ultrasound at 20 and 45°C temperatures for 1, 5, and 10 minutes. Then the changes related to HMF, pH, acidity, proline, total microbial count, diastase, moisture, sucrose, fructose, glucose, the ratio of fructose to glucose, ABTS, osmophile phenol, regenerating sugars and total sugars were evaluated after 1, 30, 90 and 180 days.
Water and moisture are essential factors affecting honey regarding quality, stability against fermentation, durability, and crystallization. Silva et al. (2009) reported 16.65% of Portuguese single-flower eucalyptus honey moisture rate [17]. Also, Ozcan and Olmes (2014) results showed the 20-17.1% moisture content in monofloral honeys in Turkey [18], and Chirif et al. (2006) 21 − 15% moisture content in Argentinian honey samples [19]. In this study, our results showed that the amount of moisture in the groups treated with 30 and 42 kHz ultrasound (at 45°C, for 10 minutes) was (12.76 ± 0.94% and 12.57 ± 1.13%) respectively, after 180 days and decreased compare to control group.
Investigations show that the acidity of honey properties can be varied based on the geographical origin where it is produced and offers a different textural nature [20]. In different studies, the pH range for Eucalyptus single-flowered Portuguese honey is 3.83. Our results were consistent with the results of previous studies, and the pH rate in the groups treated with 30 and 42 kHz ultrasound (at 45°C, for 10 minutes) was (27.6 ± 2.26 and 25.33 ± 4.53) after 180 days.
It is essential to check the acidity of honey because it increases in the case of fermentation. Adenkan et al. (2010) presented an acidity of 6.15–41.2 (eq/kg) for Nigerian honey samples [21]. Ozcan and Olmez (2014) showed an acidity of 18.2–47.5 (eq/kg) in Turkish samples [18]. Our results are in agreement with these findings. In the current study, the acidity rate in the groups treated with 30 and 42 kHz ultrasound (at 45°C for 10 minutes) were (27.6 ± 2.26 mEq and 25.33 ± 4.53 mEq) after 180 days.
Evaluating the number of sugars in edible honey helps distinguish nectar honey from honeydew. The higher the fructose-to-glucose ratio, the slower the crystallization of honey [22]. The ratio of glucose (28.83 ± 0.01% and 28.83 ± 0.01%), fructose (45.13% and 44.83 ± 0.12%) fructose to glucose (1.56 ± 0.01% and 1.54 ± 0.01%), and sucrose was (4.69 ± 0.43% and 4.66 ± 0.2%) in the groups treated with 30 and 42 kHz ultrasound (at 45°C, for 10 minutes) after 180 days.
The breakdown product of fructose and glucose is HMF, and its production process depends on many factors such as lower pH, high water content and high temperature. Since honey's pH produced during food processing is higher than honey obtained from flowers, the amount of HMF formation in flower honey is also higher than in produced honey in the factory. HMF content in other studies was 9.41mg/kg on Eucalyptus honey in Portugal, 3.91 (mg/kg) in the study by Hasan (2013) in Iraq, 31.28 mg/kg in the study by Ozcan and Olmez (2014) in Turkey [22]. The HMF rate in the groups treated with 30 and 42 kHz ultrasound (at 45°C for 10 minutes) was (78.96 ± 0.5 mg/kg and 74.35 ± 0.5 mg/kg) after 180 days. Solis-Silva et al. (2017) investigated the effect of ultrasound on bioactive compounds and antioxidant activity in 5 single flower honey samples prepared in Mexico during the storage period [27]. The ultrasound treatments were at 42 kHz for 5, 10, and 15 minutes. The honey under ultrasound treatment had higher levels of phenolic acid, flavonoid, and antioxidant activity than the control sample, and the highest amount was observed in the 15-minute ultrasound treatment. Ultrasound was evaluated as a suitable alternative to the thermal method of honey without changing HMF [27]. HMF content in our samples was higher than in these studies, and this is because of differences in plant species, climate, and soil productivity.
Diastase enzyme activity in honey indicates the freshness of honey. In this study, the highest level of diastase enzyme activity in the group treated with 42 kHz ultrasound (at 45°C, for 10 minutes) after 180 days was (13.71 ± 0.5 DN). Also, in this group, the rate of proline was (229.78 ± 10.12mg/kg). Darvishzadeh et al. (2015) found a proline content of 324-368.7 mg/kg in Iranian honey.
On the other hand, the phenol rate in Gelam, Longan, Rubber tree, and Sourwood monofloral kinds of honey in Bangladesh was 31.72 ± 80.11 (mg/kg) in the study by Krpan et al. on Acacia honey; 199.20 ± 135.23 (mg/kg) in the study by Moniruzzaman et al. (2014) in Bangladesh. Zahir Hussein et al. also studied Gelam and Nenas monofloral honey in Malaysia and reported total phenol levels of 8.47, 41.76, 3.62, and 21.60 (mg/kg) for concentrations of 0.1–0.4, [22]. The total phenol content in our samples was 0.55 ± 0.001 (mg/gr).
Having important antioxidant compounds, honey can protect cells against the harmful effects of free radicals. A comparative study was conducted by Alzahrani, et al, to evaluate the antioxidant activity of three varieties of honey from different plants and geography (manuka honey from New Zealand, acacia honey from Germany, and wild carrot honey from Algeria). Between three varieties of honey, Manuka honey had the highest phenolic content, with 899.09 ± 11.75 mg of gallic acid per kg. Moreover, a strong correlation has been observed between the antioxidant activity of tested honey samples and their total phenolic content [23].
The physicochemical and bioactive properties of different honey from Northwestern Argentina showed the highest antioxidant activity against the ABTS radical cation detected in the darkest honey samples. The highest antioxidant activity was exhibited by multifloral honey (MMS 401) with SC50 values of 10 and 2.73 µg/mL for DPPH as a stable free radical and ABTS, respectively [24].
Scripcă and Amariei (2021), investigated the use of ultrasound to prevent honey crystallization. Untreated honey samples were used as control group for comparison. For the control samples, the smallest changes in hydroxymethylfurfural concentration were in raspberry honey (5%) and the most significant variation was in honeysuckle honey (30%). For the treated samples, the highest variation of this parameter was in Tilia honey (127%) and the lowest variation was related to rapeseed honey (26%). Microbiological quality was higher for treated samples. In the ultrasound-treated samples of acacia honey, honeysuckle honey, meadow honey, yeasts and molds were not observed, but in the control samples, the amount of mold and yeast was evaluated at 10–40 cfu/g [25].
In an investigation by Stojkovic et al. (2020), honey was subjected to heat treatment and nine different ultrasound treatments (at 30, 45, and 60 ºC for 1, 5, and 10 minutes). The results showed that the following parameters changed significantly: water content, pH, electrical conductivity, diastase activity, HMF content, and water activity. Ultrasound led to an increase in total phenol content and antioxidant capacity compared to the conventional thermal method. In most cases, the samples under ultrasound improved antibacterial activity. Heat treatment resulted in a significant decrease in antibacterial activity, while one treated sample under condition of ultrasound 30°C, 5 minutes) showed the best antibacterial activity. Ultrasound treatment, especially at lower temperatures, represents a technique that can maintain and improve the biological properties of honey [26].
Janghu et al. (2017) investigated the optimization of ultrasound power and compared it to the conventional heating method in honey (treatment time: 1–15 minutes, range: 20–100%, and volume: 80 − 40 ml). The samples treated with ultrasound had lower-quality characteristics such as moisture content, pH, diastase activity, HMF content, color parameters, and total color difference. The microbiological results also showed that there were fewer aerobic mesophilic bacteria in ultrasonically treated honey than in thermally processed honey samples, and coliforms, yeasts, and molds were destroyed [27].
In current study, microbiological parameters of Industrial honey (clostridium, total microbial count, mold, and osmophiles) were investigated. The total microbial count in the groups treated with 42 kHz ultrasound (at 45°C for 10 minutes) was (38.33 ± 1.9 CFU/gr) and (74.35 ± 0.5 mg/kg) after 180 days. Also, in this group statistically significant changes in clostridium, mold, and osmophiles were not observed.