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J Sci Food Agric 79 :327–332 (1999)
Journal of the Science of Food and Agriculture
Fish silage prepared from different cooked and
uncooked raw materials: Chemical changes
during storage at different temperatures
M Es pe* and E Lied
Ins titute of Nutrition , Directorate of Fis heries , PO Box 185 , Bergen , N -5002 , Norway
Abstract : Four silage types were prepared from raw and cooked whole herring, whole mackerel, byproducts from the ülleting-line of cod and saithe, and from the viscera of cod. Each silage type was
stored at 4, 20 and 50ÄC for 48 days. The raw materials were analysed fresh and after 2, 4, 8, 16, 32 and
48 days of storage. Dry matter, crude protein and total fat were not aþ ected by the diþ erent storage
temperatures or the length of the storage period. Neither was there any change in amino acid contents. Chemical composition in diþ erent silage types reýected the amounts in the raw materials used
for silage production. Hydrolysis, on the other hand, varied with the type of raw material used for
silage production as well as with the temperature under which the silages were stored. Cooked raw
materials did not show any change in hydrolysis during storage.
( 1999 Society of Chemical Industry
Keywords : üsh silage ; storage temperature ; solubility ; quality changes
INTRODUCTION
Global catches of üsh and invertebrates is approximately 100 ] 106 tons annually, and about 70–85%
of this is composed of underutilised species as well as
üsh discards, shellüsh and by-products.1 In Norway,
about 0.55 ] 106 tons raw material representing at
least 6 ] 104 tons protein is discarded annually.
Moreover, as both the cost of üsh feed and the production rate of farmed üsh are increasing, there is a
demand for utilising more of the available protein
resources in producing high-quality üsh feeds. The
by-products from the üsh industry have low storage
stability if not frozen or preserved. In Norway, the
production of üsh silage has been carried out for
many years to preserve üsh and by-products from
the üsh processing industry. In general, the silage is
now produced by mixing minced üsh or by-products
from the üshery industry with formic acid to lower
the pH to values below 4.5. Previous work has shown
that no deterioration occurs if the raw material is of
good quality and the pH does not exceed 4.5 during
storage.2,3 Stored silage will solubilise due to
endogenous enzymes present in the minced üsh ; it
has long been known that the time of storage aþects
the solubility of the silages.2h6 The temperature
under which the silage is stored has been reported to
aþect the ünal solubility.7 Although much work has
been carried out concerning the üsh silage quality,
more work is needed to study the possibilities to
predict the quality of silages made from raw
materials traditionally used for silage production.
The available silage and its quality may vary
throughout the year. The following experiments
were run to test whether the changes occurring in
silage made from one type of raw material may be
used to predict changes in other types of silage raw
materials when preparation and storage conditions
are otherwise identical.
* Corres pondence to : Marit Es pe, Ins titute of Nutrition, Directorate of Fis heries , PO Box 185, N-5002, Bergen, Norway
Contract/grant s pons or : Norwegian Res earch Council, Felles kjÔpet
Havbruk AS, GC Rieber A/S
Contract/grant number : 106067/120
MATERIAL AND METHODS
Silage preparation
Silages from minced, whole herring (Clupea
harengus), whole mackerel (Scomber scombrus), mixtures of by-products from the cod (Gadus morhua)
and saithe (Pollachius virens) ülleting industry, and
from minced viscera of cod were prepared by adding
22 g kg~1 (v/w) formic acid (85%). The silages were
stored for 48 days at temperatures of 3 ^ 0.9¡C,
21 ^ 1.4¡C and 52 ^ 0.8¡C. Silages prepared from
each raw material, except from the viscera, were
heated for about 20 min, giving a core temperature of
about 90¡C, in a conventional micro wave oven
(eþect : 1.3 kW) prior to the addition of formic acid.
The cooked samples were then stored at about
21 ^ 1.4¡C, making a total of (4 ] 4) [ 1 silages to
be compared. Samples for chemical analyses were
collected from all raw materials prior to silage production. The four silage types were collected for
(Received 20 November 1997 ; revis ed vers ion received 17 April
1998 ; accepted 1 July 1998 )
( 1999 Society of Chemical Industry. J Sci Food Agric 0022–5142/99/$17.50
327
M Espe, E Lied
Raw
materials
Table 1. Effect of s torage
time and temperature on dry
matter, fat and protein content
in s ilages made from
different raw materials given
by the regres s ion lines
y \ a ] bx where y
repres ents dry matter, crude
protein or fat content (in
g 100 gÉ1) and x is s torage time
(in log days ). The contents
10
within the different raw
materials are given for
referencea
Regres s ion lines
Herring
Dry matter y \ 39.96 ] 0.64x aB
y
\ 48.36 ] 1.07x aA
cooked
Protein
y \ 16.14 [ 0.04x
Fat
y \ 22.72 ] 0.15x aB
y
\ 28.98 ] 1.08x aA
cooked
Mackerel
Dry matter y \ 45.17 ] 0.76x aB
y
\ 51.78 [ 0.48x aA
cooked
Protein
y \ 16.63 [ 0.74x
Fat
y \ 27.61 ] 0.27x bB
y
\ 26.36 ] 5.44x aB
50¡C
y
\ 31.48 ] 0.21x bA
cooked
Offal
Dry matter y \ 20.38 [ 0.53x aB
y
\ 21.56 [ 0.36x aA
cooked
Protein
y \ 13.99 [ 0.44x aB
y
\ 14.42 [ 0.33x aA
cooked
Fat
y \ 3.59 ] 0.22x aA
y
\ 3.65 ] 0.03x bA
cooked
Vis cera
Dry matter y \ 20.68 [ 0.35x
Protein
y \ 11.97 ] 0.11x
Fat
y \ 7.45 ] 0.03x
The content
within the
raw materials
r \ 0.27
r \ 0.14
r \ 0.04
r \ 0.10
r \ 0.14
P \ 0.23
P \ 0.77
P \ 0.94
P \ 0.68
P \ 0.77
40.7
r \ 0.26
r \ 0.27
r \ 0.38
r \ 0.10
r \ 0.89
r \ 0.09
P \ 0.25
P \ 0.56
P \ 0.05
P \ 0.73
P \ 0.01
P \ 0.85
44.5
r \ 0.43
r \ 0.64
r \ 0.47
r \ 0.50
r \ 0.63
r \ 0.47
P \ 0.05
P \ 0.12
P \ 0.03
P \ 0.25
P \ 0.02
P \ 0.29
19.0
r \ 0.46
r \ 0.11
r \ 0.06
P \ 0.05
P \ 0.63
P \ 0.75
20.0
11.7
8.0
14.8
22.7
16.9
27.6
14.3
3.3
a The content of dry matter, protein and fat in each s ilage type with coinciding s lopes and elevations are calculated as common regres s ion lines . Regres s ion lines for each s ilage type and
chemical analys is followed by the s ame upper and lower cas e letters have common s lopes
and/or elevations , res pectively.
chemical analyses when newly prepared and after 2,
4, 8, 16, 32 and 48 days of storage at each storage
temperature.
Chemical methods
The raw materials as well as the newly prepared and
stored silages were analysed for dry matter, crude
protein, total fat and ash content and pH were determined. Dry matter content was determined by
weighing after freeze drying. Crude protein was
determined colorimetrically after Kjeldahl digestion
using a method described previously.8 Total fat
content was determined by extraction with ethyl
acetate.9 Solubilities were estimated by determining
the free a-amino groups reacting with trinitrobenzenesulphonic acid (TNBS). The binding of
TNBS to free a-amino groups was determined
colorimetrically10 using L-leucine as standard. Total
TNBS-reactive amino groups were determined also
after hydrolysis in 6 M HCl for 20 h at 110¡C, and the
degree of hydrolysis in the silages were calculated as
TNBS-reactive a-amino groups as percent of the
total TNBS-reactive a-amino groups.
Raw materials and silages stored for 48 days were
hydrolysed in 6 M HCl for 22 h at 110¡C, and amino
acids were determined in the hydrolysate after prederivatisation with phenylisothiocyanate (PITCH})
using 0.25 mM norleucine as the internal standard.11
In the silages prepared from whole herring and from
328
Regres s ion Changes with
coefficient s torage time
whole mackerel, free fatty acids, anisidine and peroxide values were determined by the standard
methods used by the Central Laboratory, Directorate of Fisheries.12,13 Total oxidation was calculated as 2 ] peroxide value ] anisidine value.14
Statistical methods
The storage days were log transformed, thereafter
10
the regression lines : y \ a ] bx were calculated for
each silage-type at each storage temperature (15 lines
for each variable). Changes in slope, b, measures the
changes occurring when storage time increases 10
times, while the elevation, a, measures the amount of
each variable as time increases. Increasing the
storage time by 10, as given by log , is rather
10
inconvenient, therefore by multiplying the slope with
0.3 gives the percentage increase by doubling the
time of storage. (For the regression line y \ a ] bx,
content at day n \ a ] b log day n and content at
10
day 2n \ a ] b log day 2n \ a ] b log day n ] b
10
10
log
2; doubling the storage days gives [a ] b
10
log day n ] b log
2] [ [a ] b log
day n] \
10
10
10
blog 2 \ b ] 0.3.) Diþerences in regression lines
10
were evaluated by the covariance test.15 When no
diþerences were found in either slope or elevation,
common regression lines were calculated. When signiücant diþerences were obtained, Tukey’s test were
used for ranking the regression lines. Results were
considered statistically diþerent when P \ 0.05.
J Sci Food Agric 79 :327–332 (1999)
Chemical changes in stored silages aþ ects its quality
Figure 1. Development of the degree of hydrolys is in the s ilages
prepared from (L) whole herring, (K) whole mackerel, (=) fillet
offal of cod and s aithe and (@) in s ilage made from vis cera of cod
during s torage at (A) 4¡C, (B) 20¡C, (C) 50¡C or (D) being cooked
prior to s torage at 20¡C.
RESULTS AND DISCUSSION
No changes in pH were detected in any of the silages
when stored at diþerent temperatures ; the pH
remained at 3.7 ^ 0.1 during the 48 days of storage.
This showed that the amount of acid added was sufücient to secure a pH less than 4 even if the raw
J Sci Food Agric 79 :327–332 (1999)
material was rich in bones such as in the oþal from
the ülleting line of cod and saithe. The content of
dry matter, crude protein and total fat in the silages
are given in Table 1, and reýects the contents of the
respective raw materials from which the four silage
types were produced. No eþect of storage temperature on the contents of dry matter, protein or fat
was detected, with the exception of the fat content in
silage made from minced whole mackerel stored at
50¡C. In this silage, the fat content increased signiücantly (P \ 0.01) with increasing storage time. This
was probably due to sampling errors that were due to
the very high fat content of the mackerel and the difüculties involved in preparing homogenous samples
at this high storage temperature as the separated fat
was difficult to mix with the rest of the material.
Further, the fat content increased and protein
content decreased with storage time in the silage
made from the oþal of cod plus saithe. The reasons
for this are not known, but may be a result of inhomogeneous sampling. Independent of the raw
materials used for silage production, the cooked
silages showed higher elevations as compared to
those made from the respective uncooked raw
materials. This was most probably due to evaporation of water during cooking in the microwave
oven.
Knowledge of the eþects of silage storage when
raw materials and temperature diþer are important
in testing the possibility of predicting chemical
changes or composition in a silage type or of üsh
protein concentrates produced from the silage in
question. The dry matter, crude protein and total fat
content did not increase with increased storage time,
with the exception of mackerel silage stored at 50¡C
(Table 1). Analysis of dry matter, fat and protein
reýected the contents in the raw materials.
Raw material containing enzyme-rich fractions as
the gills and viscera were more solubilised at the
start of the silage production compared to those produced from raw materials less rich in enzymes (Fig
1). In all silages, except the one prepared from
cooked üsh, the solubility increased with storage
time due to the endogenous enzymes present in the
minced üsh. The increase in solubilities in uncooked
with no increase in cooked raw material for increased
storage time are in agreement with results reported
previously.16,17 The degree of hydrolysis was highly
dependent on the storage temperature (Table 2).
Silages stored at room temperature hydrolysed on
average by 6–11% by doubling the storage time, followed by silages stored at about 4¡C which showed
an increase of 4–7%. All silage types stored at these
two temperatures showed signiücantly increased
hydrolysis throughout the storage period of 48 days.
This dependency upon storage temperature of the
degree of hydrolysis has been reported earlier when
capelin silages were stored at 20 or 37¡C,7 in fermented silage prepared from tilapia stored at 30¡C,18
and in silage made from salmon farm mortalities
329
M Espe, E Lied
Table 2. Effect of s torage
temperature on hydrolys is in
s ilages made from different
raw materials . y repres ents
free a-amino N as percent
of total a-amino N and x
s torage time (log days ).
10
Regres s ion lines for each
raw material followed by
the s ame upper cas e s mall
and big letters indicate no
s ignificant difference
(P \ 0.05) in s lopes and
elevation, res pectively.
Raw
materials
Regres s ion lines
Regres s ion
coefficient
Changes with
s torage time
Herring
y
\ 26.83 ] 18.69x aB
4¡C
y
\ 35.57 ] 23.82x aB
20¡C
\ 42.23 ] 7.05x bC
y
50¡C
y
\ 10.18 [ 0.39x bD
cooked
y
\ 22.03 ] 22.56x bB
4¡C
\ 30.58 ] 36.14x aA
y
20¡C
y
\ 38.55 ] 10.79x cC
50¡C
y
\ 14.61 ] 0.78x cD
cooked
\ 11.96 ] 20.04x bB
y
4¡C
y
\ 15.03 ] 35.83x aA
20¡C
y
\ 25.64 ] 11.58x cC
50¡C
\ 9.04 ] 1.03x dD
y
cooked
y
\ 49.93 ] 13.84x aAB
4¡C
y
\ 55.14 ] 18.99x aA
20¡C
y
\ 53.06 ] 3.06x aB
50¡C
r \ 0.99
r \ 0.92
r \ 0.66
r \ 0.24
r \ 0.99
r \ 0.98
r \ 0.75
r \ 0.40
r \ 0.98
r \ 0.98
r \ 0.69
r \ 0.73
r \ 0.91
r \ 0.87
r \ 0.21
P \ 6 ] 10É6
P \ 4 ] 10É3
P \ 0.11
P \ 0.61
P \ 9 ] 10É6
P \ 2 ] 10É4
P \ 0.05
P \ 0.37
P \ 9 ] 10É5
P \ 9 ] 10É5
P \ 0.09
P \ 0.06
P \ 0.04
P \ 0.01
P \ 0.65
Mackerel
Offal
Vis cera
stored at 5 or 22¡C.19 The silages stored at about
50¡C autolysed less than silages stored at lower temperatures, showing an average increase of 0.2–4% by
doubling the storage times (Table 2). Lower
amounts of hydrolysed protein has also been reported in a üsh silage produced from mackerel by preheating to 60¡C prior to addition of acid and
storing20 and in a dogüsh silage stored at 45¡C.21
The low hydrolysis at the higher temperatures is
probably due to denaturation of the proteolytic
enzymes. As expected the cooked silage did not
hydrolyse. Diþerent raw materials used for production of silage resulted in diþerent developments
and total amount of hydrolysis when stored at similar
temperatures.
The highest degree of hydrolysis was found in the
raw materials riches in endogenous enzymes and the
most rapid hydrolysis was detected in the mackerel
and viscera silages (Table 3). Thus, silages do not
reach the same degree of hydrolysis when produced
from diþerent raw materials. The raw materials used
in silage production will determine the amount of
hydrolysed protein at the temperature chosen for
storage. The degree of hydrolysis is reported to have
great eþects upon the utilisation of silage protein by
salmonid üsh.22h24 These facts underscore the
importance of analysing the degree of hydrolysis in
silages when used in feed mixtures.
It has been claimed that the concentration of tryptophan, lysine and methionine are reduced during
the silage process, thereby producing silages low in
these essential amino acids.4,18,22,25 In the present
experiments, the contents of both the essential and
the non essential amino acids were found to be rather
constant during silage production and storage (Table
4). In previous experiments on silage production, no
reduction in tryptophan was reported during
increased storage.2,17 Decreased levels of amino acids
may be due either to decomposition following acid
addition or deterioration during storage of the silage.
The use of high temperatures has, however, been
found to reduce the content of tryptophan when
producing üsh protein concentrate from silages by
evaporation of water at approximately 70¡C
(unpublished results). By using raw material which is
not fresh or adding concentration of acid too low to
achieve a pH below 4.5, development of biogenic
amines and thereby destruction of the corresponding
amino acids have been reported.17 In the papers
reporting a reduction in amino acids, the quality of
the raw materials used might have contained biogenic amines or the silage may have been produced
at too high a pH value. As the raw materials used in
the present experiment were of high quality, fresh or
frozen at [ 30¡C until used for silage production,
and pH did not change during storage time, the pro-
Table 3. Effect of different raw materials on the proteolys is given by the regres s ion lines y \ a ] bx , in which y
repres ents the degree of hydrolys is and x s torage time. Equal s lopes (b) and elevations (a) for the regres s ion lines mean
no differences in development and amount of hydrolys is with increas ed s torage time, res pectively
Storage
temperature (¡C )
4
20
50
cooked
330
Slopes
Mackerel P offal P herring [ vis cera
Mackerel \ offal [ herring \ vis cera
Mackerel \ offal \ herring \ vis cera
Mackerel \ offal \ herring
Elevations
Vis cera [ mackerel \ herring [ offal
Vis cera P mackerel P herring P offal
Vis cera \ mackerel \ herring [ offal
Mackerel [ herring \ offal
J Sci Food Agric 79 :327–332 (1999)
Chemical changes in stored silages aþ ects its quality
Table 4. Amino acids (mg gÉ1 protein) in the raw materials (RM) us ed for s ilage production and in the res pective s ilages s tored for 48
days (S) (Each value is the mean of 4 ^ SEM)
Herring
As p
Glu
OH-Pro
Ser
Gly
His
Tau
Arg
Thr
Ala
Pro
Tyr
Val
Met
Ile
Leu
Phe
Lys
Trp
NEAAa
EAAa
Offal of cod ] s aithe
RM
S
Mackerel
RM
S
RM
S
101 ^ 0.8
131 ^ 5.4
9 ^ 1.1
46 ^ 0.8
64 ^ 0.4
23 ^ 0.6
10 ^ 0.6
65 ^ 0.6
49 ^ 0.9
65 ^ 0.3
43 ^ 0.3
32 ^ 1.0
51 ^ 0.6
27 ^ 0.8
43 ^ 0.4
82 ^ 0.3
42 ^ 0.3
81 ^ 0.2
9 ^ 0.1
500 ^ 5.8
463 ^ 2.2
92 ^ 0.2
122 ^ 2.3
10 ^ 0.3
43 ^ 0.3
60 ^ 1.6
20 ^ 0.1
8 ^ 0.8
58 ^ 1.0
44 ^ 0.9
60 ^ 2.3
40 ^ 0.9
31 ^ 2.3
47 ^ 3.0
29 ^ 2.5
38 ^ 2.0
77 ^ 2.9
39 ^ 1.5
80 ^ 12.1
14 ^ 0.1
465 ^ 6.3
430 ^ 25.8
86 ^ 4.0
117 ^ 27.8
9 ^ 0.9
38 ^ 2.0
55 ^ 4.4
28 ^ 1.9
8 ^ 0.4
56 ^ 5.8
42 ^ 4.6
56 ^ 5.0
38 ^ 3.0
30 ^ 2.9
43 ^ 4.7
23 ^ 0.9
36 ^ 4.5
72 ^ 7.7
37 ^ 4.8
67 ^ 8.9
10 ^ 0.1
436 ^ 14.6
403 ^ 49.3
88 ^ 3.7
139 ^ 13.3
8 ^ 0.3
40 ^ 0.8
55 ^ 0.4
30 ^ 2.4
9 ^ 0.1
58 ^ 2.4
42 ^ 1.3
56 ^ 2.0
37 ^ 1.5
31 ^ 1.9
47 ^ 2.8
28 ^ 1.3
38 ^ 1.8
75 ^ 3.5
35 ^ 1.1
72 ^ 2.1
10 ^ 0.1
461 ^ 24.2
424 ^ 13.8
80 ^ 0.8
154 ^ 1.0
18 ^ 1.1
42 ^ 1.7
89 ^ 3.5
16 ^ 3.9
13 ^ 0.1
66 ^ 1.3
39 ^ 1.0
59 ^ 3.0
47 ^ 2.9
24 ^ 0.6
40 ^ 0.5
26 ^ 0.6
34 ^ 0.4
66 ^ 1.1
33 ^ 1.0
61 ^ 1.9
8 ^ 0.1
521 ^ 16.0
380 ^ 11.1
Vis cera of cod
RM
S
86 ^ 1.9
137 ^ 7.9
21 ^ 3.3
51 ^ 3.8
94 ^ 13.1
16 ^ 3.4
13 ^ 4.5
69 ^ 5.4
45 ^ 2.3
64 ^ 4.2
53 ^ 5.0
24 ^ 1.6
40 ^ 2.7
24 ^ 1.0
32 ^ 1.1
68 ^ 2.6
35 ^ 1.6
67 ^ 2.9
10 ^ 0.1
543 ^ 41.3
395 ^ 22.9
81 ^ 1.2
127 ^ 0.4
19 ^ 4.0
54 ^ 2.0
71 ^ 6.0
11 ^ 0.1
86 ^ 2.8
73 ^ 0.5
53 ^ 0.3
62 ^ 1.7
52 ^ 1.2
26 ^ 1.8
44 ^ 0.5
26 ^ 2.2
35 ^ 0.3
73 ^ 0.2
34 ^ 0.1
58 ^ 1.9
8 ^ 0.1
589 ^ 15.3
396 ^ 1.3
101 ^ 0.2
145 ^ 2.7
16 ^ 0.5
59 ^ 1.4
72 ^ 1.3
14 ^ 1.9
59 ^ 0.8
80 ^ 3.1
60 ^ 1.7
68 ^ 1.6
52 ^ 0.2
37 ^ 3.6
50 ^ 2.6
29 ^ 1.3
40 ^ 3.2
89 ^ 2.7
43 ^ 2.5
69 ^ 4.5
9 ^ 0.1
608 ^ 10.7
472 ^ 19.4
a NEAA and EAA is s um of non-es s ential and es s ential amino acids , res pectively.
duction of biogenic amines should not be
expected.3,17 Based upon previous knowledge, biogenic amines were therefore not analysed in the
present experiments.
Herring s ilage
50¡C
2 days
48 days
4¡C
2 days
48 days
20¡C
2 days
48 days
Cooked
2 days
48 days
Table 5. Free fatty acids
(ffa, mg gÉ1 total fatty
acids ) and total oxidation
(tot ox is 2 ] peroxide value
] anicidin value) in fis h
s ilages produced from both
herring and mackerel s tored
for 2 and 48 days at
different temperatures
J Sci Food Agric 79 :327–332 (1999)
Mackerel s ilage
50¡C
2 days
48 days
4¡C
2 days
48 days
20¡C
2 days
48 days
Cooked
2 days
48 days
The raw materials used in making the mackerel
and the herring silages contained high concentrations
of fat. The autolysis of fat, measured as free fatty
acids, and the oxidation of the fat were determined
ffa
*ffa (%)
tot ox
* tot ox (%)
2.0 ^ 0.1
3.9 ^ 0.1
]48.7
17.1 ^ 0.2
15.5 ^ 0.7
[10.3
2.2 ^ 0.1
2.8 ^ 0.1
]21.4
16.0 ^ 0.8
16.7 ^ 0.1
]4.2
2.2 ^ 0.1
2.9 ^ 0.1
]24.1
17.2 ^ 0.1
18.7 ^ 0.1
]8.0
0.9 ^ 0.1
1.1 ^ 0.1
]18.2
18.3 ^ 0.8
23.5 ^ 1.5
]22.1
0.5 ^ 0.1
2.1 ^ 0.1
]76.2
11.3 ^ 0.2
17.9 ^ 0.2
]36.9
0.6 ^ 0.1
0.9 ^ 0.1
]33.3
11.7 ^ 2.6
13.2 ^ 1.2
]11.4
0.7 ^ 0.1
1.2 ^ 0.1
]41.7
13.2 ^ 0.3
12.5 ^ 0.4
[5.6
0.5 ^ 0.1
0.7 ^ 0.1
]28.6
12.0 ^ 1.8
20.4 ^ 2.2
]41.2
331
M Espe, E Lied
both in the raw materials and in silages stored at 4,
20 50¡C and in cooked raw materials, ensiled and
stored at 20¡C after 2 and 48 days of storage (Table
5). The content of free fatty acids increased relative
to both the temperature and time of storage. In contrast to proteolysis, the silages stored at 50¡C showed
more lipolysis than those stored at 20¡C and 4¡C but,
as for proteolysis, silages produced from mackerel
showed higher total lipolysis as compared to the
herring silage. The oxidation of fat varied with no
systematic changes. This was also reported in a previous paper using capelin (Mallotus villosus) as the
raw material17 but, as long as the total oxidation does
not exceed 20, the fat quality is regarded to be
acceptable for feed production.
CONCLUSION
The composition of üsh silage reýects the composition of the raw materials used for silage production.
The hydrolysis is dependent both on storage time
and storage temperature as well as the raw material
used for silage production. Silages should be
analysed for the content of dry matter, total fat and
crude protein unless the raw material from which it
is produced is known. The degree of hydrolysis
should always be taken into consideration before
production of protein concentrate to be used for üsh
feed production.
ACKNOWLEDGEMENTS
The analytical assistance by Anita Birkenes and Edel
Erdal is highly appreciated. The Norwegian
Research Council (NFR), FelleskjÔpet Havbruk AS
and GC Rieber A/S are thanked for ünancial support
through the project no 106067/120.
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