Deodorization temperature optimization
Besides the design of the deodorizer, deodorization temperature is the
most critical factor for oil quality among the four deodorization
influencing parameters (temperature, time, pressure, stripping steam). A
high temperature is beneficial for an efficient stripping of FFA and
heat bleaching, but it also results in a higher loss of tocopherols and
sterols (Hamn et al., 2013). More importantly, a high deodorization
temperature is the most critical parameter for TFA generation. Reported
activation energy values for the thermal cis-trans-isomerization of
linoleic and linolenic acid are rather low, which is an indication that
TFA are relatively easily formed at an elevated deodorization
temperature (Shahidi, 2005). The degree of reaction will vary with the
degree of fatty acid polyunsaturation. The higher the degree of
polyunsaturation, the greater the tendency of the cis-acid to trans-acid
formation. Different studies showed that the relative isomerization rate
can be expressed as follows: C18:3(100)≥C18:2(10)≥C18:1(1) (Hénon et
al., 1999; León-Camacho et al., 2001). Consequently, oils with a
relative high linolenic acid content, such as soybean oil and rapeseed
oil, are more sensitive to cis-trans-isomerization than maize oil and
sunflower seed oil during deodorization. Generally, trans-formation is
negligible below 220°C, whereas it becomes significant between 220°C and
240°C, and exponential above 240°C (Shahidi, 2005). Industrial
deodorizations are usually operated at temperatures between 240°C and
260°C, and approximately 1-3% of TFA are found in full refined oils.
However, limited lab-scale deodorization cannot precisely simulate
industrial deodorization. In order to confirm the industrial
deodorization parameters for zero-TFA vegetable oil production with good
quality, dozens of pilot trials were firstly carried out with soybean
oil among plants to optimize deodorization temperatures of existing
deodorizers (tray column, packed column and soft column). Meanwhile, the
other parameters (deodorization time, vacuum, stripping steam etc.) were
not simultaneously adjusted as variable and remained as usual at their
best according to the actual conditions in each plant. Fig.2 shows
results from the optimization of deodorization temperature in packed
columns (Fig. 2a) or tray columns & soft columns (Fig.2b), and the
relationship between TFA content and deodorization temperature. Since
the oil would stay a long time in tray columns as well as in soft
columns, the results were combined into Fig.2b. In contrast the oil
would stay in packed columns for a much shorter time. The trial
temperature was variable among 220-240°C. Approximately one-third of
trial batches achieved TFA≤0.3%. Interestingly, the majority of them
were deodorized by packed column, while TFA were considerably more than
0.30% with the use of tray columns and soft columns, and increased
rapidly to an average of 1.34% when the temperature was increased to
240°C. Therefore, to achieve the goal of TFA≤0.3%, the upper limit of
deodorization temperature for packed columns and tray columns & soft
columns were significantly different. The temperature for packed columns
was suggested to be no more than 235°C, while the temperature for tray
columns & soft columns should be lower than 220°C. This might be due to
the significant differences between deodorization times in columns with
trays compared to packed columns.
The deodorization temperature was furthermore optimized in this new DCDT
system using soybean oil. The results showed that temperatures for
packed columns and tray columns should be controlled to a maximum of
225°C and 205°C respectively (Fig.3). However, the deodorization
temperature cannot be infinitely low. The volatility of a given
components (FFA, tocopherols, sterols, etc.) is expressed by its vapor
pressure, which increases with increasing temperature. The lower the
vapor pressure, the lower the volatility and, thus, it is more difficult
to remove the components from the oil. For example, a moderate
temperature deodorization was operated in neutralized red palm oil but
found that FFA and all volatile components of odor could not be reduced
although the oil was deodorized for 2 hours at 150°C (Riyadi et al.,
2016). Therefore, to balance low TFA and oil physicochemical quality,
the deodorization temperature for soybean oil in the DCDT system was
calculated to be 225°C for packed columns and around 205°C for tray
columns. The temperatures for rapeseed oil, maize oil and sunflower seed
oil were then respectively optimized and customized, which are shown in
Table 1. Soybean oil and rapeseed oil have similar, but higher linolenic
acid content (respectively 4.2-11% and 5-14%) than the other oils, so
the temperatures for them were designed the same. Due to the lower
content of linolenic acid in maize oil (≤2%) and sunflower seed oil
(≤0.3%), the temperatures for these two oils can be set higher to
further ensure the physicochemical quality of deodorized oils. Comparing
the new zero-TFA oils with refined oils collected from the market ten
years ago (internal unpublished data) it can be shown that TFA content
decreased by more than 90% (Fig.4). Furthermore, comparing zero-TFA
oils with currently available conventional refined oils an 80% decrease
in TFA content is observed (Fig.5).