Textural and rheological testing of jams treated with high-intensity ultrasonics

Jam is food with intermediate moisture, and it is usually microbiologically stable due to large amounts of sucrose. However, it can be spoiled due to osmophilic acid-tolerant flora, and from the industrial standpoint to ensure the safety of the final product during packaging, hydrogen peroxide is usually used as an antimicrobial agent. This leads to changes in surface appearance such as colour and can also change some textural and rheological properties. Textural characteristics as one component of the sensory properties of food have a very important role in the acceptability of food, as it influences mouthfeel. Mechanical properties of food and foodstuff are specific for the type of the food, but most important are hardness, elasticity, stress, and strain, as well as several others relevant to gel-like materials such as extrusion force, adhesion, spreadability, and stickiness. Those properties have a very important role in further processing steps in the food industry such as pumping, extrusion, drying, baking, and others. Rheological properties such as yield stress and shear stress are also in direct correlation to jam quality and its functionality in the development of final products. Therefore, it is important to retain desired properties while introducing novel food processing technologies for microbial inactivation. Based on the power, amplitude, intensity, and other parameters such as the amount of liquid in the processed system, ultrasonic waves can produce a cavitation effect, which disperses large amounts of thermal and mechanical energy in the system. High-intensity ultrasonic treatment can thus introduce several changes in the cellular structure of treated material, which can consequently exhibit significant changes in textural and rheological properties. Methods for determination of the textural properties of jams and eventual changes after ultrasonic treatment are based on the instrumental texture analysis with various probes such as conical spreadability probe, and back and front extrusion cells. Rheological properties of jams are measured using a viscometer at different temperatures (20 oC to 60 oC) and different shear rates (form 0.1 s-1 to 150 s-1) and calculation of the relationship between shear stress and shear rate is performed using Herschel – Bulkley model. The time dependency of jams at those parameters is calculated based on the kinetic mathematical models by various authors such as Weltman, Figoni, or Hahn. Based on the correlation of ultrasonic parameters such as power and sonication time to measured textural and rheological parameters, optimal processing parameters can be determined to obtain desired log5 reduction of microorganisms and thus ensure microbiological safety, while retaining optimal functional properties of jams. Acknowledgment: This work was supported by the project “Increasing the development of new products and services arising from research and development activities - phase II\”: Development of innovative products to increase food quality \”- KK.01.2.1.02.0282.