## 1 Introduction  
A valuable raw material for the production of fruit preserves with increased health-promoting value is blue honeysuckle berry (_Lonicera acerulea_ L.). Recently, this fruit has been gaining popularity in the food industry. It is native to Siberia and northern Asia, but is currently cultivated in many parts of the world [1,2]. The largest plantations are located in China and North Korea (2000 ha), Poland (1800 ha), Canada (1000 ha), Russia (400 ha) and Japan (160 ha) [3]. However, Poland was the world's production leader in the year 2020 [4]. The sugar profile of this fruit is dominated by glucose and fructose, but also contains sucrose and sorbitol. The honeysuckle berry includes other valuable elements and compounds such as calcium, magnesium, fiber, iridoids and polyunsaturated fatty acids. In addition, honeysuckle contains anthocyanins and other polyphenols, as well as vitamin C and E, which together are responsible for high antioxidant activity [2,4,5,6]. A study by [PERSON] et al. [2] indicates that, depending on the variety and location of cultivation, honeysuckle berries contain, in 100 g of dry weight, about 1752-5408 mg of polyphenols, 171-568 mg of anthocyanins and 1628-2855 mg of vitamin C. Among the colorants responsible for the deep, blue color of honeysuckle, the cyanidin-3-O-glucoside dominates, accounting for 79-92% of all anthocyanins [7]. The rest of the anthocyanin compounds include cyanidin-3-O-diglucoside, cyanidin-3-O-rutinoside, pelargonidin-3-O-gucoside and peonidin-3-O-glucose [8]. Numerous health-promoting compounds present in honeysuckle berry and its products contribute to their anti-ageing, cardioprotective, neuroprotective, anti-inflammatory, antidiabetic or antimicrobial properties [1; 2; 5; 6; 7]. In addition, the berry is known to counteract other civilization diseases. Cyanidin-3-O-glucose extracted from these fruits has been shown to be a substance with high potential to inhibit pulmonary carcinogenesis [9], and has a significant role in the antiproliferative of breast and liver cancer cells [10]. Honeysuckle berry consumption, thanks to its richness in polyphenols, can prevent obesity by reducing fat absorption, as well as positively modifying the gut microbiota [11]. Blue honeysuckle berry is characterized by a distinctive, astringent taste; hence, the organoleptic acceptance of raw fruits may be limited. Therefore, it is better accepted by the average consumer in processed form. From honeysuckle berries are produced juices, infusion blends, spreads, smoothies, wines, jams, candies, ice cream, yoghurt, jellies, pastries, and dried fruits [6; 8]. In Poland, popular honeysuckle berry products include preserves like jams or confitures, which can be a rich source of fruits and bioactive compounds in a varied diet. Due to the developing awareness of food quality, alternative methods of food production and processing are being explored in order to preserve the best possible organoleptic features and a significant amount of bioactive compounds. It is important to inhibit or counteract the negative thermally induced biochemical changes that lead to the formation of harmful compounds, such as hydroxymethylfurral (HMF) and overall product quality deterioration, as much as possible. For the production of jams and other preserves, a concentration process under reduced pressure allows simultaneous reduction of temperature and limitation of oxygen effects on bioactive compounds, as well as shortening treatment duration. There have been scientific studies to identify strategies for reducing HMF formation in food using vacuum treatment [12]. [PERSON] et al. [13] reported that strawberry jams obtained by vacuum cooking had a better, more intense red color and from two to nine times less HMF than jams after conventional atmospheric cooking. Vacuum-evaporation-made jam from jujube fruit contained 20.61% more polyphenols compared to open-pan-evaporation jam [14].