Since ancient times, fermented foods and beverages have been a vital part of the human diet, providing significant health benefits (Ansorena & Artiasaran, 2016; Kanwar & Keshani, 2016). Yoghurt, a Turkish word referring to milk curdled with lactic acid bacteria (LAB) or probiotic starter culture, is a fermented dairy product widely consumed globally (Fias, 2006). Lactobacillus delbrueckii subsp. bulgaricus, Streptococcus salivarius subsp. thermophiles and Lactobacilli bifidobacteria are the prominent LAB used in yoghurt production. The fermentation of milk protein by the LAB is responsible for yoghurt’s texture and its characteristic tang taste (Ekere, 2014). The presence of live active LAB in yoghurt also makes it a probiotic product, promoting consumers’ good health and well-being (Guarner & Shaafsma, 1998). According to Ndabikunze et al. (2017), consuming yoghurt could help increase one’s appetite, stimulate bile secretion, and enhance the functions of the pancreas and the liver. In yoghurt production, milk composition, pasteurization treatment and the incubation temperature had been reported as some of the factors that could influence the physicochemical properties of yoghurt (Medeiros et al., 2015). Yoghurt has high nutrient density and may undergo rapid undesirable microbial or chemical changes which could shorten its shelf-life. Consequently, preservatives are often included in yoghurt production. Benzoic and sorbic acids and their respective salts - sodium benzoate (C7H5NaO2) and potassium sorbate (C6H7KO2) - are some of the most commonly used preservatives in food, owing to their favourability to low pH or acidic foods (El-Ziney, 2009; Akbari-adergani et al., 2013; Amirpour et al., 2015). In particular, sodium benzoate and potassium sorbate have been noted to be efficient in inhibiting the growth of moulds, yeast and a huge variety of bacteria (Mroueh et al., 2008). When used as preservatives, sodium benzoate and potassium sorbate are generally within the ranges of 0.05–0.1% and 0.02–0.3%, respectively (Mroueh et al., 2008). The ideal everyday intakes of sodium benzoate and potassium sorbate have been set respectively at 0–5 and 0–25 mg kg− 1 of body weight (EFSA, 2017). Instigated by the effectiveness and affordable cost of these preservatives, some food manufacturers, however, have been observed to exceed the acceptable limit.
Value-adding ingredients (such as fruit or vegetable) may be incorporated into yoghurt to enhance its nutritional, functional or sensory value. For example, Ihemeje et al. (2015) investigated the nutritional profiles of yoghurt samples flavoured with carrot and pineapple, and yoghurt samples spiced with ginger and pepper. The authors found a significant nutritional enhancement in the flavoured and spiced yoghurts as compared to plain yoghurt. Similarly, Teshome et al. (2017) observed a significant improvement in the sensorial and physicochemical properties of mango and papaya flavoured yoghurts. Currently, only exotic fruits such as vanilla, strawberry, peach, raspberry, and banana types of flavoured yoghurts are commercially available. Soursop (Annona muricata) is a juicy, acidic, and aromatic tropical fruit with many therapeutic and nutritive properties (Senadeera et al., 2018). It is highly rich in fructose and vitamins C, B1, and B2 (Omoifo, 2004; Alias, 2009; Badrie & Schauss, 2010). Soursop is an underutilized tropical fruit in West Africa most especially Nigeria. However, since some health-conscious consumers are now reducing the rate of consumption of sugar-containing foods due to certain health concerns such as obesity, diabetes etc. associated with sugar consumption, replacing some percentage of sugar with an aromatic nature fruit such as soursop puree with no or acceptable use of preservatives in yoghurt production could lead to the development of functional yoghurt. Few valuable studies have been conducted on the potentials of soursop in enhancing the nutritional, organoleptic or functional quality of yoghurt. For instance, Senadeera et al. (2018) use pulp of three different varieties of soursop to produce yoghurt under different conditions. It was reported that the addition of soursop pulp to yoghurt increases the antioxidant acitivity and sensory profile of the yoghurt. Sputrayadi et al. (2021) noted that, in addition to enhanced organoleptic properties, the vitamin C and lactic acid levels of sweetcorn milk yoghurt significantly increase with an increasing ratio of soursop fruit extract. Sanusi et al. (2022) studied the kinetic acidification profile of yoghurt produced from soursop puree. Notwithstanding and to the best of our knowledge, the literature is still sparse on the interactive impacts of pasteurization temperature, chemical preservatives (sodium benzoate and potassium sorbate), soursop puree/sugar ratio, and storage duration on the qualities of set-type yoghurt. This study intend to produce set-type yoghurt that would provide valauable information on the impacts pasteurization temperature, chemical preservatives, storage duration and substitution of soursop puree with sugar on the syneresis, water holding capacity, proximate compostion, Vitamin C, total phenolic content, 2, 2 - Diphenyl − 1- picrylhydrazyl (DPPH), lactic acid bacteria emumeration, coliform emumeration, yeast and mould emumeration. This information would be useful is the selection of appropriate processing parameters and conditions that could aid in the production of nutritional and shelf-stable set-type yoghurt. Therefore, in this study aimed to evaluate the impacts of pasteurization temperature, use of chemical preservatives, soursop puree to sugar ratio, and storage duration on the physicochemical, antioxidant and microbial properties of set-type yoghurt.