Honey is in great demand as food and medicine. Ultrasonic treatment is an effective means of destroying undesirable components such as crystals and yeast cells in honey. As a non-thermal processing technology, it can reduce the content of hydroxymethyl furfural and better retain amylase, aroma and flavor.
Honey is a highly viscous product with a unique flavor, aroma, color and texture. Honey consists of glucose, fructose, water, maltose, trisaccharides and other carbohydrates, sucrose, minerals, proteins, vitamins and enzymes, yeast and other heat-resistant microorganisms and small amounts of organic acids. Honey contains a lot of tetracycline, phenolic compounds and hydrogen peroxide, which has antibacterial properties.
Honey contains starch-digesting enzymes. Amylase is sensitive to heat and easily inactivated, so amylase activity is an important quality indicator of honey. Major enzymes include invertase (alpha-glucosidase), amylase (alpha-amylase) and glucose oxidase. Amylase hydrolyzes carbohydrates for easy digestion. Invertase hydrolyzes sucrose and maltose to glucose and fructose. Glucose oxidase catalyzes the formation of gluconic acid and hydrogen peroxide from glucose. Honey also contains catalase and acid phosphatase. The activity of this enzyme is usually measured as amylase activity and is expressed as amylase number (DN). The Honey Standard specifies a minimum value of 8 for DN in processed honey.
Yeast and Microorganisms
The extracted honey contains undesirable substances, such as yeast (usually Saccharomyces cerevisiae, 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.
Honey crystallizes naturally because it is a supersaturated sugar solution with a sugar content of over 70% and a relative water content of about 18%. When glucose becomes the more stable saturated state of glucose monohydrate, glucose spontaneously precipitates out of the supersaturated state by losing water. This results in the formation of two phases - a liquid phase above and a solid crystal below. These crystals form a lattice that holds the other components of the honey in suspension, creating a semi-solid state. Crystallization or granulation is not advisable as it is a serious problem in honey processing and marketing. In addition, crystallization limits the flow of untreated honey from storage containers.
After the honey is extracted and filtered, it is heat-treated to reduce moisture levels and destroy yeast. Heating does help to liquefy the crystals in the honey. While heat treatment can effectively reduce moisture loss, reduce and delay crystallization, and completely destroy yeast cells, it can also lead to product spoilage. Heating significantly increases levels of hydroxymethylfurfural (HMF). The maximum allowable legal level for HMF is 40 mg/kg. In addition, heating reduces enzyme (eg amylase) activity, affecting sensory quality and reducing the freshness of honey. Heat treatment can also darken (brown) the color of natural honey. Especially heating above 90°C leads to caramelization of the sugar. Heat treatment also cannot completely eliminate heat-resistant microorganisms.
Ultrasonic treatment of honey
Ultrasound is a non-thermal processing alternative for many liquid foods. Its mechanical power is used for gentle and effective microbial inactivation and particle size reduction. Most yeast cells are destroyed when honey is exposed to ultrasound. Yeast cells that survive sonication typically lose their ability to grow. This greatly slows down the rate at which honey ferments.
Ultrasound can also eliminate existing crystals and inhibit further crystallization in the honey. In this respect it is comparable to heating honey. Ultrasonic assisted liquefaction can work at approximately lower process temperatures. 35C, can reduce the liquefaction time to less than 30 seconds. Studies have shown that sonication at a frequency of 20kHz can completely liquefy the crystals in honey. The sonicated samples remained in the liquefied state for about 350 days (20% compared to heat treatment). Ultrasonic liquefaction results in greater retention of aroma and flavor due to minimal heat exposure. Sonicated samples showed only low HMF increases and low amylase activity decreases. Since less thermal energy is required, the application of ultrasonics helps to save processing costs compared to conventional heating and cooling.