Oil migration from whole almonds to surrounding chocolate

Oil migration from whole almonds to surrounding chocolate
K.L. McCarthy a , A. Altan b , M.J. McCarthy a ,
a Dept. of Food Science and Technology, University of California, Davis, Davis, CA, USA
(klmccarthy@ucdavis.edu; mjmccarthy@ucdavis.edu)
b Dept. of Food Engineering, University of Mersin, Ciftlikkoy, Mersin, Turkey (aaltan@mersin.edu.tr)
ABSTRACT
Oil migration from high oil content whole almonds into enrobing chocolate can cause changes in product
quality. The objective of this study was to evaluate oil migration from whole almonds into surrounding
chocolate as a function of almond roasting conditions, chocolate formulation, and storage conditions. Plastic
sample containers were designed to have 8 compartments and a protective lid. Each compartment contained
an almond embedded in either dark or milk chocolate. Roasting conditions for the almonds were air roasted
or oil roasted. Sample containers were stored at 20, 25, and 30ºC for 400 days. Magnetic resonance imaging
(MRI) was used to monitor spatial and temporal changes of liquid lipid content over the experimental time
frame. A multi-slice spin echo pulse (MSSE) sequence was used to acquire images with a 6.9 ms echo time
and a 1000 ms repetition time using 1.03T Aspect AI MRI spectrometer. Data analysis was performed by
identifying the almond region and chocolate region in each image and quantifying liquid oil content in those
regions. No detectable oil migration occurred in any of the sample trays. This assessment was based on the
average signal intensity in each region and its coefficient of variation. The goal of this work was to provide
the California almond industry with processing and storage guidelines that ensure the maintenance of high
quality almond confectionary products.
Keywords: almonds; chocolate; magnetic resonance imaging (MRI); oil migration
INTRODUCTION
The intent of this research project was to improve knowledge about the impact of processing and storage on
oil migration from whole almonds to surrounding media. This knowledge will be helpful in designing
appropriate processing, storage and formulations for whole almonds in confectionary products.
Specific objectives:
1. Quantify oil migration from almonds as a function of roasting conditions,
2. Quantify oil migration from almonds as a function of storage temperature (e.g., 20, 25, 30ºC), and
3. Quantify the influence of surrounding media on the migration of oil from almonds.
MATERIALS & METHODS
Experimental design
To evaluate oil migration, almonds were embedded in dark chocolate or milk chocolate. The experimental
factors were: type of roast (air and oil), level of roast (light, medium and heavy), and storage temperature (20,
25, and 30ºC). Trays which contained 6 embedded almonds and a raw almond as reference were prepared in
late summer 2009. Data were acquired through December 2010 to assess storage stability.
Sample preparation
Almonds (Nonpareil variety) were provided by the Almond Board of California (Modesto, CA). Two types
of almonds were provided: raw and commercially air roasted almonds at different roasting levels. The
roasting conditions of air roasted almonds were as follows: 137.8C for 22 min; 148.9C for 19 min; 160C
for 16 min. Roasting ranged from qualitatively light roast to heavy roast. The almonds were sorted to obtain
uniformly shaped, unbroken, and minimally scuffed pieces for use in the study. In addition, the mass of each
almond was between 1.10 and 1.25 g. The sorted raw almonds were divided into two groups: one group was
oil roasted and the other group was used as the control. The oil roasting was performed in cocoa butter to
ensure that only two types of fat were present: cocoa butter and almond oil. For oil roasting in cocoa butter,
40 g of almonds were placed in an enclosed wire mesh basket and submerged in a Fisher HiTemp Bath
(Model 160A, Fisher Scientific) containing 2 L of cocoa butter maintained at the roasting temperature. The
oil temperature was monitored using a digital thermometer connected with 2 type K thermocouples. The

oasting time was 10 min at 130ºC for light roast, 135ºC for medium roast, and 140ºC for heavy roast.
Commercial dark chocolate (Solitaire) and milk chocolate (Heritage) were purchased from Guittard
Chocolate Company (Burlingame, CA). Dark chocolate was semi-sweet chocolate; ingredients included
sugar, chocolate liquor, cocoa butter, soya lecithin and pure vanilla. Milk chocolate consisted of sugar, cocoa
butter, milk, chocolate liquor, soya lecithin and pure vanilla. Cocoa solid contents of dark chocolate and milk
chocolate were 63% and 32.5 %, respectively. Complete proximate analyses for the almonds and the
chocolates are given in Table 1. The chocolates were tempered using a tempering machine (Revolation2,
ChocoVision Corp., Poughkeepsie, NY).
Each tray had eight compartments, each with dimensions of 14.5 mm wide, 26 mm long and 25.5 mm depth,
and a protective lid (Wilmad-Labglass, Buena, NJ). Six treated almonds (embedded) and one raw almond
(not embedded) were included in the tray. Compartments were numbered as given in Fig. 1. Detailed
designation of batches and samples is given in Table 2.
Table 1. Proximate analyses for almonds and chocolate, given in percent (wb). Analyses performed by Covance
Laboratories Inc. (Madison, WI).
Substrate type Fat Moisture Carbohydrates Protein Ash Sum
(as measured)
Raw Almonds 52.1 5.1 21.5 19.4 3.1 101.2 54.9
Air Roasted - Light 54.7 2.8 20.8 20.3 3.1 101.7 56.2
Air Roasted - Medium 53.3 2.9 22.3 19.7 3.1 101.3 54.9
Air Roasted - Heavy 53.7 3.0 21.3 19.9 3.0 100.9 55.3
Cocoa Butter Roasted - Light 55.5 3.9 19.1 19.6 3.0 101.1 57.7
Cocoa Butter Roasted - Medium 55.7 3.6 19.1 19.7 3.1 101.2 57.8
Cocoa Butter Roasted - Heavy 55.2 3.8 20.2 18.8 2.9 100.9 57.4
Dark Choc - Guittard 33.1 0.4 61.8 4.82 2.1 102.2 33.2
Milk Choc - Guittard 32.5 0.6 62.8 5.26 1.6 102.8 32.7
Fat
(db)
Table 2. Detailed designation of Batches and Samples (compartment numbers are given). Two trays each were stored at
20, 25, and 30ºC. The temperature was constant to ±0.5ºC in each of the incubators.
Batch Type of almond Surrounding medium
A Air roast Cocoa butter roast
1-heavy 5-heavy Dark chocolate Dark chocolate
2-medium 6-medium Dark chocolate Dark chocolate
3-light 7-light Dark chocolate Dark chocolate
4-no sample
8- Raw almond
B Air roast Air roast
1-heavy 5-heavy Dark chocolate Milk chocolate
2-medium 6-medium Dark chocolate Milk chocolate
3-light 7-light Dark chocolate Milk chocolate
4-no sample
8- Raw almond
MRI Measurements
Magnetic resonance imaging (MRI) measurements were performed using the 1T Aspect AI
magnet/spectrometer, equipped with a solenoid coil with 44 MHz for 1 H-resonance frequency. A multi-slice
spin echo pulse (MSSE) sequence provided 8 images (slices) with a 6.9 ms echo time and a 1000 ms
repetition time. Each image was 128x128 pixels and represented signal averaged over a slice thickness of 5
mm. The field of view was 68 mm. Two scans were acquired for each measurement. The acquisition time
was approximately 256 s. Of the eight images (slices) acquired in the MR imaging protocol, slice numbers 3
and 7 were analyzed for oil migration. These images were chosen due to their central positioning over the
almonds.

5
6
7
8
1
2
3
Figure 1. A tray is illustrated in the holder prior to be inserted into the MR magnet.
Compartments 1, 2, 3, 5, 6, 7, and 8 are referred to as “samples”.
x 10 5
4.5
20
4
3.5
40
60
80
100
120
3
2.5
2
1.5
1
0.5
20 40 60 80 100 120
Figure 2. Representative image of air roasted almonds embedded in milk chocolate (Day 301).
The image includes Compartments 5, 6, 7 and 8.
RESULTS & DISCUSSION
Figure 2 illustrates a representative MR image of air roasted almonds in milk chocolate taken at slice 7
(Batch B). MR signal intensity (SI) reflects the hydrogen content of the sample region. With both almonds
and chocolate, the hydrogen signal is due to the water and liquid oil content of a voxel (volume element).
Due to the low moisture nature of both chocolate (especially) and almonds, the signal is due primarily to the
liquid oil content. There are a number of general observations in this study consistent with oil and/or
moisture content, as given in Table 1:
1. SI(almond) > SI(chocolate) due to the greater amount of liquid fat per unit volume,
2. SI (chocolate at 30ºC) > SI (chocolate at 25ºC) > SI (chocolate at 20ºC) due to the
temperature dependence of the solid to liquid fat ratio in chocolate,
3. SI (milk chocolate) > SI (dark chocolate) due to the higher liquid fat content of milk
chocolate compared to dark chocolate,
4. SI (oil roasted almonds) > SI (air roasted almonds) due to the higher moisture and fat
content of the oil roasted almonds compared to the air roasted almonds, and
5. SI (raw almonds) > SI (air roasted almonds) due to the higher moisture content.

For data analysis, the signal intensity data at the initial time (Day 1) were used to identify three different
regions of each sample. The three regions were designated as: almond, boundary, and surrounding medium
(dark or milk chocolate). In this masking process, the almond region was entirely almond, the chocolate
region was entirely surrounding medium. The boundary region was primarily surrounding medium, but could
be influenced by signal intensity from the almond due to partial volume averaging. This masking procedure
was successfully used throughout the experimental time frame because the sample tray could be precisely
positioned within the magnet using the sample tray holder shown in Fig. 1.
Day-to-day signal variations of the spectrometer were addressed by summing signal intensity from the three
regions (almond, boundary, and surrounding medium) to 1.0. Oil migration could then be detected over time
using changes in the relative fraction of signal from each of the three regions. In other words, the fraction of
signal intensity from the almond region in a single sample was:
SI
almond
SI
SI
boundary
almond
SI
surrounding medium
The fractions of the signal intensity from the boundary region and the surrounding medium region were
calculated in a similar fashion. The disadvantage of this approach was that if there were phase changes (e.g.,
solid fat liquefies), the changes would not be observed by a net increase or decrease in signal intensity over
time. However, based on previous experience with Guittard chocolate at similar temperatures reported in [1],
phase changes were not expected in the chocolate.
The statistics calculated for each region were mean signal intensity from that region and the coefficient of
variation over time, calculated as the ratio of the standard deviation (stdev) to the mean of the signal
intensity,
stdev
COV
(2)
SI
Figure 3 illustrates the fraction signal intensity over time for Batch A at T=30ºC for the almond and
chocolate regions. Each vertical dotted line represents a data acquisition day. Sample 8 in the top figure is
the signal intensity from the raw almond, which was considered time invariant and gives the expected noise
in the other samples. At Day 270, the spectrometer was upgraded and data were more consistent after the
upgrade. However, no trend in oil migration is evident for either the almond region or the chocolate region
for Samples 1-3 (air roasted) or Samples 5-7 (cocoa butter roasted) (Samples 1 and 5 are not shown in the
Fig. 5). No significant changes in SI were observed in the trays stored at 20 or 25ºC either.
The statistics for Batch A samples stored at T=30ºC are given in Table 3. Although statistics were calculated
for all storage temperatures, the focus of the discussion will be on samples stored at T=30ºC since the
greatest changes were expected at the higher storage temperature. To illustrate the normalization procedure,
the first row of Table 3 gives the fraction of signal intensity in Almond, Boundary and Chocolate as 0.69,
0.08, and 0.22 respectively. These fractions sum to 0.99; they are short of 1.00 due to round off error.
Consider the next row, 0.74+0.08+0.18=1.00. Two cells and 2 columns are highlighted. The 2 cells are
fraction of signal due to almond in Sample 8 (raw almond): 0.96 and 0.94. These values are typical of the
signal intensity obtained with the mask for the almond region. The samples in Batch A have coefficient of
variation for the almond region between 0.01 – 0.04. This small value indicates signal intensity in the
almond region was not changing over time. The chocolate column is highlighted to illustrate the relative
contribution of that region at around 20% to the total signal intensity. Although the COV is higher (at around
0.10), there was no evidence of almond oil migrating into the chocolate region.
Figure 4 summarizes the results for Batch B at T=30ºC. Batch B was developed to provide information
regarding oil migration in milk chocolate versus dark chocolate. Sample 8 (top figure, top curve) again gives
the expected noise in the other samples. No trends were evident in either the almond region or the chocolate
region, although there is greater contribution of signal intensity of the milk chocolate region (Samples 5-7)
compared to dark chocolate region (Samples 1-3) in the chocolate region due to the higher liquid fat content.
(1)

The statistics for Batch B samples stored at T=30ºC are given in Table 4. Again, two cells and 2 columns are
highlighted. The 2 cells are fraction of signal due to almond in Sample 8 (raw almond): 0.95 and 0.93, which
was indicative of a successful masking procedure. The samples in Batch B have coefficient of variation for
the almond region between 0.01 – 0.04. This small value indicates signal intensity in the almond region was
not changing over time. The chocolate column in Table 4 is highlighted to illustrate the greater contribution
of signal intensity of the milk chocolate region (Samples 5-7, dark shading) compared to dark chocolate
region (Samples 1-3, light shading).
1
Almond region Intensity
0.95
0.9
0.85
0.8
0.75
0.7
0.65
2345678910 11 12 1314 15 1617 18 192 2528313437 46 47 53 60 7074 81 88 98102 109 123 142 158 172 186 200 214 228 242 256 270 284 298 327 353 387 428
0.4
0.35
Chocolate region intensity
2
3
6
7
8
0.3
0.25
0.2
0.15
0.1
0.05
0
2345678910 11 12 1314 15 1617 18 192 2528313437 46 47 53 60 7074 81 88 98102 109 123 142 158 172 186 200 214 228 242 256 270 284 298 327 353 387 428
Figure 3. Signal intensity over 428 days for the almond region and the chocolate region for Batch A, storage temperature
of 30ºC, replication 1. Each vertical dotted line represents a data acquisition day. Sample 8 in the top figure (top curve)
is the signal intensity from the raw almond, which was considered time invariant.
Table 3. Statistics for Batch A: mean signal intensity and coefficient of variation.
Temp Rep Sample Almond COV_almond Boundary COV_boundary Chocolate COV_chocolate
30 1 1 0.69 0.04 0.08 0.10 0.22 0.10
30 1 2 0.74 0.03 0.08 0.09 0.18 0.10
30 1 3 0.74 0.04 0.08 0.09 0.18 0.12
30 1 5 0.68 0.04 0.10 0.08 0.23 0.10
30 1 6 0.76 0.03 0.08 0.07 0.16 0.10
30 1 7 0.74 0.03 0.08 0.09 0.18 0.11
30 1 8 0.96 0.01 0.04 0.18
30 2 1 0.70 0.03 0.09 0.08 0.21 0.09
30 2 2 0.77 0.03 0.07 0.07 0.16 0.10
30 2 3 0.73 0.03 0.08 0.07 0.19 0.10
30 2 5 0.67 0.04 0.09 0.06 0.24 0.10
30 2 6 0.72 0.03 0.08 0.07 0.20 0.10
30 2 7 0.73 0.03 0.09 0.07 0.19 0.10
30 2 8 0.94 0.01 0.06 0.14

1
Almond region Intensity
0.95
0.9
0.85
0.8
0.75
0.7
0.65
12345678910 11 12 13 20 27 34 4448 55 62 7276 83 97 11 116 132 147 160 174 188 202 217 231 245 259 273 301 328 362 404
0.4
0.35
Chocolate region intensity
2
3
6
7
8
0.3
0.25
0.2
0.15
0.1
0.05
0
12345678910 11 12 13 20 27 34 4448 55 62 7276 83 97 11 116 132 147 160 174 188 202 217 231 245 259 273 301 328 362 404
Figure 4. Signal intensity over 404 days for the almond region and the chocolate region for Batch B, storage temperature
of 30ºC, replication 1. Each vertical dotted line represents a data acquisition day. Sample 8 in the top figure (top curve) is
the signal intensity from the raw almond, which was considered time invariant.
Table 4. Statistics for Batch B: mean signal intensity and coefficient of variation.
Temp Rep Sample Almond COV_almond Boundary COV_boundary Chocolate COV_chocolate
30 1 1 0.70 0.03 0.09 0.06 0.21 0.08
30 1 2 0.76 0.02 0.08 0.05 0.17 0.07
30 1 3 0.75 0.02 0.07 0.06 0.18 0.06
30 1 5 0.63 0.03 0.11 0.05 0.27 0.07
30 1 6 0.64 0.02 0.10 0.04 0.27 0.04
30 1 7 0.65 0.02 0.11 0.04 0.24 0.05
30 1 8 0.95 0.01 0.05 0.18
30 2 1 0.71 0.02 0.08 0.08 0.21 0.08
30 2 2 0.76 0.02 0.07 0.05 0.16 0.08
30 2 3 0.73 0.03 0.08 0.09 0.19 0.09
30 2 5 0.56 0.04 0.12 0.04 0.32 0.06
30 2 6 0.62 0.02 0.11 0.04 0.27 0.04
30 2 7 0.68 0.02 0.11 0.04 0.22 0.05
30 2 8 0.93 0.02 0.07 0.26
CONCLUSION
Sample trays were prepared to evaluate oil migration from almonds to two types of chocolate: dark chocolate
and milk chocolate. Different roast conditions and levels were incorporated into the experimental design.
Samples were held at different, but constant, storage temperatures. No oil migration was detected from whole
almond to its surrounding (or vice versa) by magnetic resonance imaging in any of the sample trays over a
time frame of 14 months. This assessment was based on the mean signal intensity and COV in each of the
three designated regions: almond, boundary, and surrounding medium.
REFERENCES
[1] Lee W.L., McCarthy M.J. & McCarthy K.L. 2010. Oil migration in 2-component confectionery systems. Journal of
Food Science, 75(1), E83-E89.