Introduction
Vegetable oils, such as soybean oil, rapeseed oil, maize oil and sunflower seed oil are widely refined before being consumed or utilized as food supplements. The major steps of refining are degumming, neutralization, bleaching, and deodorization. Among these, deodorization is the final but the most critical step of refining as the final quality of vegetable oils is largely determined by the deodorization process. From an organoleptic point of view, oils should be light in color with a bland taste and a good oxidative stability. Both free fatty acids (FFA) and objectionable volatile compounds are removed from the oil by vacuum steam distillation at elevated temperatures. A fully refined oil contains low levels of FFA (usually<0.05%). Additionally, certain carotenoid pigments are destroyed resulting in a heat bleaching effect. However, micronutrients naturally occurring in oil such as tocopherol, sterol and squalene are partially removed during deodorization. Conventional deodorization temperature is usually 248-250°C or even higher, the heat treatment will induce a series of chemical transformations (oxidation reactions, cis-trans isomerization, cyclization, polymerization, etc.) due to the occurrence of numerous methylene-interrupted ethylenic double bonds (Fournier et al., 2006). Several contaminants such as TFA, 3-MCPD and GE which are considered hazardous are generated during deodorization (Destaillats, 2012; MacMahon, 2014).
Nowadays, more and more attention is paid to the nutritional quality of vegetable oil. Oils should contain levels as low as possible for TFA, polymeric triglycerides, and secondary oxidation products and at the same time being rich in natural antioxidants. Therefore, alternative deodorization methods using milder conditions need to be developed to achieve these new quality requirements.
TFA in the human diet originate partly from natural sources, i.e. from meat and milk of ruminant animals (cows, sheep and goats), but the major part of human TFA exposure is the consequence of industrial food processing, like partially hydrogenation and deodorization of vegetable oils. Refined vegetable oils usually contain trans isomers of oleic acid (C18:1), linoleic acid (C18:2) and linolenic acid (C18:3), which may be formed during deodorization of oils rich in these unsaturated fatty acids (DeGreyt et al., 1996). Fully refined oils contain approximately 1-2% of TFA. There are several adverse biological effects of TFAs on body health. Various clinical studies and epidemiological observations showed that dietary TFA is associated with various health disorders such as diabetes, cardiovascular disease, obesity, breast cancer, prostatic cancer, infertility and coronary artery disease (Willett et al., 1993; Koletzko & Decsi, 1997; Hu et al., 1997; Lopez-Garcia et al., 2005; Micha & Mozaffarian, 2009; EL-Aal et al., 2019; Islam et al., 2019; Shah & Thadani, 2019). Guidelines for restriction or elimination of TFA were proposed around the world since the 1990s (Downs et al., 2013). The World Health Organization (WHO) called for the elimination of TFAs from the global food supply (Uauy et al., 2009). WHO’s European Food and Nutrition Action Plan 2015-2020 suggested that TFAs should be less than 1% of the daily energy intake including TFA from natural origin. In 2018, WHO further proposed to eliminate industrially-produced trans fat from national food supplies, with the goal of global elimination by 2023. Chinese dietary reference intake guidelines (2013) suggested that TFAs from industrial origins should be less than 1% of the daily energy intake. In addition, according to the Chinese National Food Safety Standard for nutrition labeling of prepackaged foods (GB28050-2011) foods that have no more than 0.3 g TFA/100 g (solid) or 100 ml (liquid) can be labeled as having no TFA and can be called zero-TFA.
Many factors affect the overall quality of the oil obtained during the deodorization process, such as the structure of the deodorizer, deodorization temperature, deodorization time, vacuum and stripping steam. Therefore, the objectives of this work are to firstly design and transform a new deodorization system for vegetable oil using precision minimal processing, and then systematically evaluate the effect of this deodorization system on TFA content and soybean oil quality, as well as to provide an industrial alternative for the replacement of conventional high-temperature deodorization to obtain zero-TFA vegetable oil products.