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.