###### Abstract  
This work investigated the underlying mechanism of thermo-ultrasonic treatment to improve the nutritional properties and emulsion stability of sea bass fish head broth. The effects of ultrasonication on the processing of fish broth were compared with boiling water treatment. The nutritional properties of fish broth mainly include protein, fat, total sugar, 5\({}^{\prime}\)-nucleotide and free amino acid content. To achieve a similar effect of nutrient extraction, the thermo-ultrasonic treatment required a shorter time (30 min) than boiling water (120 min). The water-soluble protein, fat and total sugar contents were at their maximum at 120 min of the thermo-ultrasonic treatment. In particular, the fat content increased with the time of thermo-ultrasonic treatment from 0.58% to 2.70%. The emulsion structure of the fish soup was characterized by measuring its color and particle size, using optical microscopy and confocal laser scanning microscopy, and determining its storage stability. Thermo-ultrasonic treatment reduced the particle size of the fish broth emulsion and the fat globules became smaller and more homogeneous. Ultrasonication not only accelerated the nutritional and flavor content of the fish head broth, but also reduced the particle size and enhanced the stability of the emulsified system of the fish broth. The fish head tissue was more severely disrupted by the cavitation effect of an ultrasound, and nutrients migrated more and faster. This was mainly due to the cavitation and mechanical breaking force of the ultrasound on the fish head tissue and the fat globules of the fish broth. Altogether, these findings suggest that the thermo-ultrasonic treatment technique is useful for processing nutrient-rich, storage-stable and ready-to-eat fish head broth. sea bass fish head broth; thermo-ultrasonic treatment; nutritional properties; emulsion stability 2024 1  
As a sea fish is rich in proteins, amino acids and unsaturated fatty acids [4], sea fish broth is highly nutritious and very important for maintaining a healthy diet. Fish protein is easily digested and rich in essential amino acids [5]. Studies have shown that fish protein hydrolyses, especially dipeptides and tripeptides, have antioxidant properties, antihypertensive, anticancer, anti-inflammatory and antibacterial effects [6; 7]. Meanwhile, fish broth can be regarded as a complex colloidal system. Sea bass is an economic fish from Southeast Asia that contains high levels of protein, essential amino acids and polyunsaturated fatty acids [8]. However, fish are easily spoiled and are difficult to preserve. For this reason, the development of product processing technology for sea bass broth has become vital [9]. Currently, sea bass broth is often used as a nutritional supplement for pregnant women, the elderly and frail post-operative patients to strengthen their wellbeing [10]. With the development of ready-to-eat foods and fast-paced lifestyles, ready-to-eat fish broths are chosen by many consumers. However, ready-to-eat fish head broth suffers from a low processing efficiency and poor product stability [11; 12]. Ultrasonic technology is one of the most important methods for preparing emulsions [13], and in recent years, ultrasonic-assisted stewing has been developed in meat processing. Ultrasonic processing technology is a non-thermal treatment technology that plays a significant role in controlling, improving and accelerating processes [14]. Ultrasonic processing equipment generates ultrasonic waves, and the medium undergoes compression and expansion cycles at alternating high and low pressures [15]. The action of an ultrasound disrupts the tissue of food materials and accelerates the release, exchange and reaction of substances [16]. Traditional boiling is mainly a function of the transfer of thermal energy. There are reports on combining thermal and ultrasonic waves to improve food processing technology by utilizing the combined effects of cavitation and thermal energy [17].