Table 5.1
Commercially available phospholipases for enzymatic degumming.
Enzyme trade name
Producer
Activity
Lecitase Ultra
Novozymes
Phospholipase A1
Rohalase MPL
AB Enzymes
Phospholipase A2
GumZyme
DSM
Phospholipase A2
Lysomax
Danisco
Lipid Acyltransferase
(type A2)
Purifine
®
DSM
Phospholipase C
5.3
ENZYMATIC DEGUMMING: A MISSING LINK IN THE PHYSICAL REFINING OF SOFT OILS?
133
C
A2
A1
O
X = choline (phosphatidylcholine or PC)
ethanolamine
inositol, link in 1-position
choline
HO
C
H
2
C
NH
2
H
2
X = ehanolamine (phosphatidylethanolamine, PE)
X = inositol (phosphatidylinositol or PI)
X = hydrogen (phosphatidic acid or PA)
O
O
O
−
X
P
O
D
C
O
C
O
R
1
O
CH
H
2
C
H
2
C
R
2
HO
C
H
2
C
N
CH
3
CH
3
CH
3
H
2
OH
OH
O
OH
OH
OH
+
Figure 5.2
Specific activities of the various commercial phospholipases. A1, phospholipase
A1; A2, phospholipase A2; C, phospholipase C; D, phospholipase D.
Theoretically, conversion of 0.1% PL (40 ppm P) leads to formation of
0.036% FFA. With sufficient reaction time (depending on enzyme dosing),
phospholipases A1 and A2 are relatively unselective and will degrade nearly
all phospholipids. LysoMax (Danisco) is a lipid acyltransferase (PL-A2 type)
which transfers FFA released from phospholipids to free sterols, resulting
in the formation of sterol esters. Unlike FFA, sterol esters are not removed
during the refining process and thus represent a limited but real increase
in the refined oil yield. Phospholipase C (PL-C, e.g. Purifine
®
from DSM)
releases the P-containing part of the phospholipid molecule, with formation
of diacylglycerols and phosphate esters as degradation products. Conversion
of each 0.1% phospholipids results in the formation of 0.084% diacylglyc-
erols. Phospholipase C will only react with phosphatidylcholine (PC) and
phosphatidylethanolamine (PE) and has virtually no effect on phosphatidic
acid (PA) or phosphatidylinositol (PI) (Hitchman, 2009).
A general flow sheet of an enzymatic degumming process (basically inde-
pendent of the type of enzyme being used) is given in Figure 5.3. The first step is
the acid conditioning/pH adjustment of the crude or water degummed oil. This
step is required to make the nonhydratable phospholipids more accessible
for enzyme degradation at the oil–water interface and to bring the pH closer
to the optimal pH of the enzyme. Afterwards, the enzyme is added – either
pure or diluted in water. High shear mixing is required to ensure optimal
distribution in the oil. Enzyme dosing depends on the type of enzyme and
on the phospholipids content of the oil, but usually varies between 50 and
200 ppm. The optimal reaction temperature is 50–60
◦
C, while the required
134
CH 5
EDIBLE OIL REFINING: CURRENT AND FUTURE TECHNOLOGIES
Steam
Deodorized Oil
Acid
Caustic
Acid Reaction
Tank
Degumming
Centrifuge
Enzyme
Enzyme
Reaction Tank
GUMS
Wash water
Washing
Centrifuge
Water
ENZYMATIC
DEGUMMED
OIL
Steam
To storage
CRUDE OIL
Steam
Figure 5.3
Typical process flow diagram of a deep enzymatic degumming process. Courtesy of Desmet Ballestra.
5.3
ENZYMATIC DEGUMMING: A MISSING LINK IN THE PHYSICAL REFINING OF SOFT OILS?
135
reaction time mainly depends on the enzyme dosing. While in the past it
was common practice to apply a longer reaction time with a low enzyme
dosage (e.g. 30 ppm enzyme for 6 hours’ reaction), preference is now given
to a shorter reaction time with higher enzyme dosage (e.g. 100 ppm enzyme
for 2 hours’ reaction). This practice is preferred because it increases the
flexibility of the process while keeping the operating (enzyme) cost at an
acceptable level. Finally, the heavy phase (consisting of water and lyso gums
or phosphate esters) is separated by centrifugation from the degummed oil.
Two different types of enzymatic degumming can be distinguished: so-called
enzymatic water degumming and deep enzymatic degumming. Enzymatic
water degumming is typically applied in (soybean) crushing plants. Several
large-capacity plants in South America (Argentina, Brazil etc.) are already
running in this mode. Increased oil yield is the main driver for its implemen-
tation. The expected yield increase depends on the type of oil (P content) and
the type of enzyme used. The highest increase (up to 1.8%) can be expected
when crude soybean oil is enzymatically degummed with PL-C (Hitchmann,
2009; Kellens, 2009); in this case, the oil yield increase is the sum of the dia-
cylglycerols formed and the lower neutral oil entrainment in a smaller heavy
phase (gums fraction). A lower yield increase (1.0–1.5%) will be obtained
from PL-C degumming of crude rapeseed oil or when phospholipase A1 or A2
is used on crude soybean oil (Kellens, 2009). In the latter case, the net oil yield
increase is due to the lower neutral oil entrainment in the gums fraction alone.
An increase in refined oil yield is obviously a very attractive feature
of enzymatic (PL-C) water degumming, but by itself it is not enough to
lead to implementation in all crushing plants. In the overall cost/benefit
analysis of the process, the enzyme cost and side-stream valorisation are
also taken into account. Depending on the value of (lyso-) lecithin, it may
be more profitable for a crusher to apply simple water degumming or
enzymatic water degumming with PL-A1/PL-A2. The latter gives a lower net
oil yield improvement compared to PL-C enzymatic degumming but yields a
lysolecithin side stream that may have value for specific applications.
PL-C enzymatic degummed soybean oil typically still has 100–150 ppm
residual P (mainly present in PA and PC). A significantly better degumming
efficiency (P
<
10 ppm) can be obtained when crude or water degummed
vegetable oils are enzymatically degummed with commercial PL-A1 or
PL-A2. This so-called ‘deep enzymatic degumming’ is already applied in
several industrial plants. In addition to the increased oil yield, the very effi-
cient phospholipid removal – making the degummed oil suitable for physical
refining – is of great interest to refiners. As an alternative option, a combina-
tion of PL-C and PL-A1/PL-A2 can be used for deep enzymatic degumming
(Dayton, 2011; Galhardo & Dayton, 2012). The two enzymes can be added
either separately or as a cocktail, depending on the plant design. Although the
potential advantages of the latter process are well described in the (patent)
136
CH 5
EDIBLE OIL REFINING: CURRENT AND FUTURE TECHNOLOGIES
literature (Dayton & Galhardo, 2008; Gramatikova
et al
., 2011), it is still
rarely applied on industrial scale.
A potential alternative to enzymatic degumming is the direct enzymatic
deoiling of the lecithin fraction resulting from the water degumming of crude
oils. In this patented process (De Greyt & Kellens, 2010), a phospholipase
(e.g. Lecitase Ultra) is added to the wet lecithin and the phospholipids are
degraded into much less hydrophobic lysophospholipids. As a result, 80–90%
of the entrapped neutral oil can be recovered by simple static decantation
or centrifugation (Kellens, 2009; Kellens
et al
., 2010). The recovered neutral
oil (FFA content: 25–30%) can be recycled to the crude or degummed oil
or can be used as such as biodiesel feedstock, while the lysolecithin can
be added back to the deoiled meal. The main advantages of the enzymatic
lecithin deoiling process over enzymatic degumming are the lower enzyme
consumption (
∼
50% less) and the fact that it is applied on a small side stream,
with no impact on the oil degumming/refining process. The process has been
tested successfully on a pilot scale but is currently not yet applied on an
industrial scale.
5.4
Bleaching: from single-stage colour removal
to multistage adsorptive purification
Bleaching was introduced in edible oil refining at the end of the 19th
century to improve the colour of cottonseed oil. Originally, it was a batch
process at atmospheric pressure, in which natural bleaching clay was added
to hot oil with the sole objective of removing colouring pigments. Today
this is no longer the case, and bleaching has become a critical process in
edible oil refining. It has gradually turned from a single-stage ‘bleaching’
into a multistage adsorptive purification process in which a wide range
of unwanted components (soaps, phospholipids, oxidation products, trace
metals, contaminants etc.) are removed prior to deodorisation.
In order to reach this point, a whole series of process improvements was
gradually introduced, with the aim of reducing the overall processing cost
and improving the bleached oil quality. Vacuum bleaching was implemented
first, in order to avoid oxidation and related colour fixation and improve
the oxidative stability. Later, as the capacity of refining plants increased,
bleaching evolved from a batch to a (semi-) continuous process. This evolu-
tion further improved the bleached oil quality and made the process more
energy efficient. Another process improvement was the implementation of
(horizontal/vertical) pressure leaf filters. Initially, plate and frame filters were
used, but these lost favour over the years due to the too high residual oil
content in the spent bleaching earth (typically 35–40%) (Veldkamp, 2012).
Dostları ilə paylaş: |