How does ultrasound process honey to effectively inactivate yeast, reduce crystallization and increase HMF?

Honey is a high-viscosity product with a unique flavor, aroma, color and texture. Honey is composed of glucose, fructose, water, maltose, trisaccharides and other carbohydrates, sucrose, minerals, proteins, vitamins and enzymes, yeast and other heat-resistant microorganisms and a small amount of organic acids. Honey contains a large amount of tetracycline, phenolic compounds and hydrogen peroxide, which has antibacterial properties.

Enzymes

Honey contains starch digesting enzymes. Amylase is sensitive to heat and easily inactivated, so amylase activity is an important quality indicator of honey. The main enzymes include invertase (α-glucosidase), amylase (α-amylase) and glucose oxidase. Amylase hydrolyzes carbohydrates and is easy to digest. Invertase hydrolyzes sucrose and maltose into glucose and fructose. Glucose oxidase catalyzes glucose to form gluconic acid and hydrogen peroxide. Honey also contains catalase and acid phosphatase. The activity of this enzyme is usually measured as the activity of amylase and expressed as the amylase number (DN). Honey standards stipulate that the minimum DN in processed honey is 8.

Yeasts and Microorganisms

Extracted honey contains undesirable substances such as yeasts (usually sugar-tolerant yeast, Saccharomyces cerevisiae) and other heat-resistant microorganisms. They are the main cause of honey spoilage during storage. High yeast counts lead to rapid fermentation of honey. The fermentation rate of honey is also related to the moisture content. 17% moisture is considered a safe level to inhibit yeast activity. On the other hand, the chance of crystallization increases with decreasing moisture content. The commercial standard yeast count is 500 cfu/mL or less.

Crystallization

Honey crystallizes naturally because it is a supersaturated sugar solution with a sugar content of more than 70% and a relative water content of about 18%. When glucose becomes saturated with the more stable glucose monohydrate, glucose spontaneously precipitates from the supersaturated state by losing water. This results in the formation of two phases - a liquid phase on top and solid crystals below. These crystals form a lattice that holds the other components of honey in suspension, thus forming a semi-solid state. Crystallization or granulation is undesirable as it is a serious problem in honey processing and marketing. Furthermore, crystallization limits the flow of untreated honey from storage containers.

Heat treatment

After extraction and filtration, honey is heat treated to reduce moisture levels and destroy yeast. Heating does help liquefy crystals in honey. While heat treatment is effective in reducing moisture loss, reducing and delaying crystallization, and completely destroying yeast cells, it can also cause product deterioration. Heating significantly increases levels of hydroxymethylfurfural (HMF). The maximum legal level of HMF allowed is 40 mg/kg. In addition, heating reduces enzyme (e.g. amylase) activity, affecting sensory qualities and reducing the freshness of honey. Heat treatment also darkens the color of natural honey (browning). In particular, heating above 90 °C causes caramelization of sugars. Heat treatment also does not completely eliminate heat-resistant microorganisms.

Due to the limitations of heat treatment, research has focused on non-thermal alternatives such as microwave radiation, infrared heating, ultrafiltration, and ultrasonic treatment.

Ultrasonic treatment of honey

Ultrasonic treatment is a non-thermal processing alternative for many liquid foods. Its mechanical power is used for gentle yet effective microbial inactivation and particle size reduction. Most yeast cells are destroyed when honey is exposed to ultrasound. Yeast cells that survive the effects of sonication usually lose their ability to grow. This significantly reduces the rate of honey fermentation.

Ultrasound can also eliminate existing crystals and inhibit further crystallization in honey. In this respect, it is comparable to heating honey. Ultrasound-assisted liquefaction can work at lower process temperatures of approx. 35°C and can reduce liquefaction time to less than 30 seconds. Studies have shown that ultrasound treatment with a frequency of 20kHz can completely liquefy crystals in honey. Ultrasonicated samples remained in a liquefied state for about 350 days (20% compared to heat treatment). Due to minimal heat exposure, ultrasonic liquefaction allows aroma and flavor to be retained to a greater extent. Ultrasonicated samples showed only a low HMF increase and a low decrease in amylase activity. Since less thermal energy is required, the application of ultrasound helps save processing costs compared to traditional heating and cooling. Different types of honey require different intensities of ultrasound and processing time.