A comparison of soy milk and rice milk
as a substitute for cow's milk
by testing the carbohydrate, protein,
amino acid, and lipid content.
By:
Liz Vanderlaan
Stephen
Peretin
Lindsay
Wieber
Kyle
Kutsche
Abstract
By:
Stephen Peretin, Revised by Lindsay Wieber and
Liz Vanderlaan
Many medical altercations such as
lactose intolerance and other milk allergies call for the use of a
nutritionally sufficient milk substitute. In order to determine the best
substitute for natural cowâs milk an array of tests were used to compare the
carbohydrate, protein, amino acid, and lipid content of cowâs milk, rice milk,
and soy milk. It was hypothesized that soy milk will have the closest results
to that of cowâs milk, making it the best milk substitute.ÊÊ
The first portion of these tests to
determine whether rice or soy milk is the best alternative involved examining
the carbohydrate content. Barfoedâs test was used to
distinguish monosaccharides from disaccharides and
polysaccharides. Selivanoffâs test was used to differentiate between ketoses
and aldoses. The Iodine test was used to distinguish starch from other sugars.
A
Discussion
By
Lindsay Wieber, Liz Vanderlaan,
and Kyle Kutchse, Revised by Steve Peretin
Lactose intolerance is a widespread
issue as it affects 30 to 50 million people nationwide.Ê A deficiency in the lactase enzyme causes an
inability to break down lactose into its simpler forms of glucose and galactose (Vonk and Priebe, 2003).Ê This
lactase shortage creates numerous digestive problems, including cramps,
bloating, nausea, and diarrhea about 1-2 hours after consuming significant
amounts of lactose.Ê Lactose is the most
prominent sugar in milk, posing a dilemma for millions of people.Ê Milk is one of the main components of the
dairy group, which the food guide pyramid recommends 2-3 cups per day (Horvath,
2005).Ê It provides essential nutrients
to the body, and is particularly known for its calcium, vitamin A and D, and
protein.Ê Simply eliminating milk from
lactose intolerant individualsâ diet would sacrifice a healthy, balanced
diet.Ê To avoid this consequence many
people are led to alternative milk sources which may have questionable
nutritional values.Ê Milk allergies, like
lactose intolerance are also prevalent in infants, forcing them to consume an
alternative formula.Ê Seeing that infants
donât consume solid foods until they are 4-7 months old it is vital that their
milk formula has adequate amounts of nutrients (Dowshen
and Rutherford, 2001).Ê
ÊÊÊÊÊÊÊÊÊÊÊ This experiment focused on two
popular milk alternatives; soy milk and rice milk.Ê These milks were analyzed based on their
carbohydrate, protein, amino acid, and lipid content and compared to 2% cowâs
milk.Ê The best milk substitute was
determined by the milk that compared the closest to the cowâs milk.Ê It was hypothesized that soy milk would
produce the most similar results to those of cowâs milk.Ê More specifically, the sugar tests were
predicted to confirm that cowâs milk contains lactose, a disaccharide composed
of glucose and galactose.Ê Soy milk was predicted to contain more
complex sugars while rice milk was expected to contain simple sugars, but
neither were thought to have lactose.Ê In particular soy milk was expected to test
positively for sucrose, as it is a sugar in certain soy milks that has a
negative effect on teeth (Davison and Simmer, 1997).Ê It was also expected that soy milk would
contain the highest amount of protein because the soy bean is 40 % protein (Murano, 2004).Ê
Typically, foods that are quality sources of protein can attribute this
benefit their amino acid content, leading to the prediction that soy milk would
have more essential amino acids than rice milk (Anonymous-4, 2004).Ê For the lipid test, it was predicted that soy
milk would be much closer to the fat content of cows milk because the soy bean
is 20 % fat (Murano, 2004).Ê One of the popular aspects of rice milk is
its low fat content, leading to the prediction that it will not even compare to
the amount of fat in cows milk.Ê The four
aspects; carbohydrate, protein, amino acids, and lipids, gave a general idea of
which alternative source of milk is the best alternative. It was predicted that
soy milk would produce the most similar results to cowâs milk, therefore making
it the best option.Ê After many
experiments this hypothesis was confirmed as soy milk had very comparable
results to cowâs milk and rice milk was distinctive and not similar to cowâs
milk.Ê
The main sugar in cowâs milk is
lactose, also known as the ãmilk sugarä.Ê
It is a disaccharide characterized by the linkage of two aldoses,
glucose and galactose (Freeman 2005).Ê The carbohydrate portion of this research
confirmed the presence of lactose in cowâs milk.Ê Barfoedâs test
determined that it was a di- or polysaccharide and Selivanoffs test determined that it contained aldoses.Ê This is agreeable with the structure of
lactose, as both glucose and galactose are aldoses.
It was predicted that soy milk would
contain complex sugars, especially sucrose, and that there would be an absence
of lactose.Ê Previous research suggests
that certain soy milk formulas consist of maltodextrins,
corn starch (isomaltose) or sucrose (Davison and
Simmer, 1998).Ê The results of this
experiment, however, did not support this prediction.Ê Sucrose is a disaccharide characterized by the
glycosidic linkage of one molecule of glucose and one
molecule of fructose.Ê Disaccharides
containing fructose should react intermediately between that of fructose alone
and one of the aldoses in Selivanoffâs test (Khra
2006).Ê The soy milk samples in this
experiment showed evidence of di- or polysaccharides,
the presence of an aldose and the presence of a starch.Ê The existence of a di-
or polysaccharide was consistent with the sugars described in previous
experiments, and consistent with the indication of starch, a coiled polymer of
glucose.Ê
It was predicted that rice milk
would contain simple sugars which provide the body with a short surge of energy
but then the sugar turns to fat (Hart, unknown).Ê Simple sugars, or monosaccharides,
include glucose, fructose, and galactose.Ê The series of sugar tests performed on rice
milk generated results that indicated the presence of a monosaccharide as well
as an aldose.Ê It did not, however, show
evidence of starch.Ê These results lead
to the fact that soy milk and cowâs milk provide the body with complex
carbohydrates which take longer to digest.Ê
Complex carbohydrates are useful in supplying the body with long term
energy, while simple carbohydrates will not maintain sufficient energy
levels.Ê It is advised to avoid getting
the bulk of your carbohydrates from refined foods, which contain simple
carbohydrates because they are typically low in nutrients and energy (Grieger, unknown).Ê
This leads to the fact that soy milk is not only the closest to cowâs
milk in terms of sugars, but it is providing the body with the most efficient
and more recommended sugar.Ê
Another crucial aspect of nutrition
to consider is the protein content.Ê
Proteins are vital in maintaining the structure of red blood cells,
enzyme regulation, proper function of antibodies, and most commonly known for
their aiding in growth and repair of body tissues (Anonymous-8, unknown).Ê It was predicted that soy milk would contain
more protein than cowâs milk or rice milk.Ê
Soy milkâs main economic and nutritional advantage is the soy beanâs
rich protein content of 40 percent (Murano,
2004).Ê It is important to understand,
however, that soy protein contains only one-third of available nitrogen as
essential or semi-essential amino acids, and therefore has a lower biological
value than milk protein (Davison and Simmer, 1997).Ê In general, animal proteins are considered
higher quality than plant proteins.Ê The
results of this experiment confirmed the predictions, indicating that soy milk
does in fact contain the highest amount of protein at 124.218 mg/ml.Ê It was followed closely by 2% milk with
109.731 mg/ml.Ê Rice milk was found to
contain dramatically lower amounts of protein than both soy and cowâs milk with
a value of 6.059 mg/ml.Ê It should be
noted, however, that previous research describes rice as relatively rich in
energy and a good source of protein, with the highest protein digestibility
among staple grains (Fiocchi 2003).Ê Though soy milk was found to have a greater
amount of proteins than both cowâs milk and rice milk, the protein content of
cowâs milk is considered to be ideal due to the essential amino acids
responsible for the composition of proteins.Ê
A third key aspect that was analyzed
directly relates to the protein content.Ê
Amino acids are the building blocks of protein, meaning that a milk
source high in protein should contain many amino acids.Ê After calculating the Rf values for each of the amino acid bands, the
values were compared to the corresponding amino acid.Ê It was difficult to determine the amino acids
present because each trial for each sample varied slightly.Ê It was determined that 2% milk contained cystine, lysine, and histidine.Ê Soy milk contained arginine,
alanine, valine, and
tyrosine.Ê Rice milk was found to contain
alanine and tyrosine.ÊÊ There were other amino acids found that
occurred in only a single trial that were not included due inconsistency
between the trials.Ê This could mean that
the amino acid was in fact present in the milk, but in very low concentrations,
making its presence on the TLC vary.Ê It was also difficult to distinguish between
the many bands on the TLC strip and it could have been very close to another
band, causing them to blend together.Ê
ÊÊÊÊÊÊÊÊÊÊÊ Despite the problems with the results
of this test, they did support the hypothesis that soy milk is most similar to
cowâs milk.Ê This was determined from
several conclusions.Ê First, soy and 2%
milks each showed signs of essential amino acids.Ê Essential amino acids were those that were
needed in the body but could not be synthesized by the body.Ê Therefore, essential amino acids must be
included in the diet.Ê There are eight
essential amino acids for humans (Anonymous-8, 2006).Ê According to this test, soy milk had one of
these, valine.Ê
Two percent milk also had one essential amino acid, lysine.Ê Histidine, another
amino acid in 2% milk, is essential for children only.Ê Rice milk produced no essential amino acids,
meaning that all 8 essential amino acids would have to be obtained from another
food source because the body cannot produce them.Ê
ÊÊÊÊÊÊÊÊÊÊÊ More evidence that soy milk was
closer nutritionally to cowâs milk came from previous research.Ê The results collected through this lab were
not complete, yet they compared well to the research of others.Ê In a 2002 article, Tom Overton discussed
research that compared the effects of amino acids on protein content in dairy
milk.Ê This study concluded that a
balance of essential amino acids in a cowâs diet increased protein content in
cow milk (Overton, 2002).Ê This linkage
of amino acids and proteins made sense in that proteins are composed of amino
acids.Ê When comparing the amino acid
content from the TLC to the protein content from the
The lipid content of
each milk was analyzed in a similar fashion to the protein assay.Ê A standard curve was constructed, but the absorbances of the milk samples were too low to fit the
curve, therefore the lipid content of each milk sample could not be
quantified.Ê Instead, the absorbance
values were compared to each other to give a comparative analysis of the lipid
content.Ê This was a relatively good
indicator of the lipid content because the standard curve followed a linear
trend, indicating that the amount of fat is directly proportional to absorbance.ÊÊ Soy milk was found to have the highest
average fat content, with an absorbance of 1.021.Ê The average absorbance for cowâs 2 percent
milk was reported as 0.377, which is significantly lower than soy milk.Ê As expected, the skim milk registered the
lowest at 0.188.ÊÊ Rice milk reported an
average absorbance of 0.279, which is concordant with the suggestion that rice
milk is low in fat compared to soy and cowâs milk.Ê Previous experiments report that rice milk
contains only one fourth as much fat as cowâs milk (Gilbert, unknown).Ê The results found in this experiment do not
indicate it to be one fourth the absorbance of cowâs milk; however, they do
show that a much lower absorbance value was found.Ê Another recent study in which the fatty acid
composition of cowâs milk substitutes were compared to breast milk reports
that, although the substitute formulas were found to be highly variable, the
lipid values acquired for soy milk were much lower than that of human milk (Kaila 1999).Ê The
absorbance for soy milk was significantly higher than that of cowâs milk, when
in reality they should have had nearly the same absorbance.Ê This is likely attributed to some reaction
that occurred with the soy milk and ethanol to produce a slightly darker, less
transparent solution.Ê
ÊÊÊÊÊÊÊÊÊÊÊ The four experimental processes all
involved a slight bit of error, which was mostly accounted for when analyzing
the results.Ê The sugar tests ran
smoothly with the exception of the difficulty of distinguishing between an
intermediate, positive, or negative result.Ê
This was mostly due to the translucent milk, which made the solutions a
slightly different color than the controls which were transparent.Ê The
ÊÊÊÊÊÊÊÊÊÊÊ ÊThe experiments became more complicated with
the thin-layer chromatography amino acid test.Ê
It took numerous attempts to get just three amino acid bands for the 2%
milk.Ê This could be attributed to many
factors.Ê It took a long period of time
(about two and a half hours) for the solvent front to rise to a point on the
strip in which the amino acids could separate enough to distinguish.Ê Had the strips developed for a longer period
of time, the results may have been more accurate.Ê Another problem with this test was in the
dilutions of the milk samples.Ê The 2%
milk inhibited the solvent front from moving far enough up the strip.Ê Multiple dilutions were performed with little
success.Ê If this part of the experiment
were to be repeated the appropriate dilution for 2% milk would need to be
determined in order to obtain working results.Ê
The lipid test also had much room
for error as the soy milk produced values that didnât correspond with the
label.Ê The soy milk concentration was
significantly higher than the 2 % cowâs milk, when in reality they should have
been relatively close values.Ê This is
likely due to the fact that the soy milk reacted with the ethanol, causing the
absorption to increase.Ê Again, with the
use of the spectrophotometer, fingerprints on the cuvette or dirty cuvettes can
cause skewed absorbance data.Ê A second
dilemma with the lipid test occurred when analyzing the results.Ê A poor experimental design when creating the
standard curve caused the dilutions of the standard curve to be too different
from the dilution of the ethanol and milk samples to compare.Ê This made it impossible to quantify the
amount of fat in the milks; instead the absorbance values were compared to give
a general idea of which milk contains the most fat.Ê
Based on the results of the
experiments performed, soy milk can be considered the best alternative to cowâs
milk.Ê This conclusion isnât an indicator
of how healthy soy milk, but an indicator of how it compares to cowâs milk by
composition.Ê The results verify the
hypothesis that soy milk is the best lactose-free alternative to cowâs milk
based on its similarities in carbohydrates, proteins, amino acids and
lipids.Ê It should be taken into
consideration, however, that previous research has found that up to 40 percent
of infants allergic to cowâs milk may also develop soy protein allergy (Davison
and Simmer 1997).Ê Although soy proteins
appear to be less sensitizing than cowâs milk proteins there is still a risk
that an individual experiencing an allergic reaction to the proteins found in
milk might experience a similar situation with soy milk as a dairy alternative
(Murano 2001).Ê
This research gives a microscopic
view of the components of cow milk, rice milk, and soy milk, leaving the
opportunity for further studies.Ê Many
other aspects of the milks could be explored to further compare them.Ê Relating to the soy protein allergy, it could
be determined what quantity of protein is causing the allergic reaction in many
infants, and how their substitute formulas compare to cow and soy milk.Ê A quantitative vitamin analysis could be
performed to confirm the vitamin content listed on the labels.Ê Another interesting aspect pertaining to the
benefits of soy milk could involve testing for isoflavones.Ê Isoflavones are in
soy beans and are thought to effect estrogen levels and act as an anti-oxidant
(
Figures
Figure 1: Bar graph of average protein
amounts found in three different types of milk using the Bradford Assay. Fifty μl of each milk sample was aliquoted
into glass tubes.Ê Fifty μl of NaOH was then added to
each and vortexed.Ê
Three ml
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