Chapter 4. Minerals in Foods
From a chemical perspective the foods we eat consists of proteins, fats, carbohydrates,
minerals and water; the so-called edible biochemicals. No two foods provide the same
quantity and quality of nutrients, which also applies to minerals. Dairy products that are rich in
calcium are poor sources of iron, zinc and copper. With an increased emphasis on plant-based
as opposed to animal based foods, concern has been raised as to the bioavailability of plant
minerals compared to minerals from animal sources. Nutrition and Food Science come
together in evaluating this threat to the health-promoting properties of plant foods. Many of
the technical advances in food science have been devoted to preserving mineral elements
during food processing, storage, and cooking. Thus, a discussion of minerals in food must first
recognize that minerals in raw agricultural products although present in high quantity have
limitations in meeting health needs. In this chapter we will divide food into its basic
biochemical components because this is how food items are categorized on the labels of
products sold in the store. Specific objective are:
1. To examine the quantity and quality of biominerals in foods,
2. To identify changes that occur to minerals during processing,
3. To investigate the effects of food processing in general on mineral bioavailability
4. To view minerals in foods from the standpoint of food safety
I. Historical Perspective
Food processing was initially intended to address issues of food safety. The conditions
of slaughter houses when brought to light threaten the acceptance of American food exports.
During Theodore Roosevelt’s administration, Congress passed legislation under the auspices of
the Pure Food and Drug Act and the Beef to address issue of food safety and in 1927 passed
legislation giving rise to the U.S. Food, Drug, and Insecticide Administration, shortened to the
Food and Drug Administration or FDA to enforce the Pure Food and Drug Act. The new agency
had powers to regulate food supplements and insecticide levels in foods. Minerals in foods
was not an important issue, however. At the level of the diet, sheep, cows, pigs and fowl
feeding on plants or plant products grown in soils deplete of essential minerals or overly
balanced in one or more were shown to develop impairments in growth, health and commodity
products (wool, meat, etc). Changing the grazing area effectively resolved the problem and
even reversed the deficiency signs of afflicted animals. Elements in the soil thus became a
focus. In time it was found that a cobalt or selenium deficient soil could be linked to specific
symptoms. The swayback in lambs, the white muscle disease in sheep, and fallings disease in
cattle were all shown to arise from mineral deficiencies. Seminal to humans was the discovery
that higher incidences of goiter could be traced to lower levels of iodine in the soil. These
observations promoted a strong awareness of the value of minerals to growth and
development and stimulated interest in internal functions dependent on minerals that could
explain the overt symptoms. More important, they shifted the emphasis to plant minerals in
foods that were consumed.
I. The fundamentals
The minerals in a food source contribute to its flavor, texture, and when digested can
provide the cofactors for enzymes that influence nutrition. Their content in foods will vary
depending on genetics, climatic changes, and agricultural practices. Minerals are also
responsible for food spoilage on storage and must be dealt with in the context of shelf life and
food safety. Their nutrition relates to the ease with which minerals are rendered bioavailable
to the organism, a major concern considering losses during food processing and cooking that
render the processed food source less mineral rich than its unprocessed predecessor. Minerals
that survive processing are in digests presented to the intestine for absorption. Digestion,
however, has limitations. Fibers in food that engage iron, zinc, copper, calcium and magnesium
ions are never fully digested. In addition, plant foods, particularly legumes, are rich in
organophosphate compounds that prevent mineral absorption. Food processing procedures
use biofortification, enzymes and genetic manipulation to confront these problems, the
outcome of which is to give some assurance that the content of important minerals in foods
remain at their health-promoting levels.
2. Bioavailability of Food Minerals
Not all minerals in natural food products become functional biochemical components in
the system. The scheme shown in Figure 4.1 is testament to processes involved in delivering a
mineral to a target in a cell. As noted, losses occur along the way. Minerals that are retained in
a food source after processing are referred to as “chemically available”, which is always higher
than “biologically available” minerals. The latter are in reference to minerals which upon
consumption become biologically functional to the organism; the former to minerals in the
Before Processing (100%)
After Processing
Chemically Available
Digestion
Intestinal Absorption
Cellular Absorption
Biologically Available
Functional Mineral
Figure 4.1. Steps in the Processing of Food from Plants that can Impact on Mineral Content
processed food that remains in the ash after all organic matter has been removed by
calcinations or combustion. Ash minerals generally take the form of metal oxides, phosphates,
sulfates, nitrates, and halide. Their amount can be quantified by various colorimetirc and
spectrophotometric procedures. Metal ions in the ash are usually measured by atomic
absorption analysis which utilizes emission spectrometry combined with absorption at specific
wave lengths to quantify individual metal ions. The procedure can accurately quantify metal
ions in the nanomolar (10-9M) range. More advanced technology uses inductively coupled
plasma spectrometry to quantify a number of elements simultaneously. The outcome of the
analysis is to know precisely how much metal ion on a per gram wet or dry basis is in the food.
Comparing that figure with the post-processing value allows an estimate of the amount that is
lost as a result of processing.
3. Food Processing Strategies and Mineral Bioavailability
Food Processing is designed to make foods healthier, taste better, and increase shelf
life. Processing also increases accessibility of micronutrients and decreases anti-nutrients.
Because processing of plant foods uses procedures such as heating, grinding, soaking,
fermentation, germination/malting etc. to achieve these goals, processing can have a telling
effect on mineral bioavailability. Mineral losses during processing, particularly microminerals,
are irretrievable. Watzke (1998) has noted, however, that depending on the particular plant
and mineral, processing can have a beneficial effect on mineral nutrition. In benefiting
minerals, processing effects relate to rendering minerals in foods matrix more diffusible and
absorbable. For example, blanching and cooking of spinach leaves has been shown to improve
HCl-extractable iron, calcium and zinc (Yadav and Sehgal, 1995; 2002), thus mimicking
happenings that could occur in the acidic environment of the stomach. Table 4.1 illustrates the
more negative aspects to processing. Minerals such as calcium, iron, zinc, etc. are lost by
milling, and heat treatments; the higher the degree of milling or the longer the exposure time,
the greater the proportion of minerals lost. Likewise, packaging can alter food composition
and thus influence mineral bioavailability (Johnson, 1991). Although baking destroys phytates
and removes a major anti-nutrient (see below), baking, also destroys vitamin C, an important
stimulator of iron absorption (Hallberg, 1981). These duel effects that impinge on mineral
bioavailability illustrate the scope of problems one encounters when attempting to assure food
products retain acceptable bioavailable levels.
Perhaps the greatest loss of minerals from food occurs in the kitchen. Minerals leached
into hot cooking fluids is a particular concern. Other losses occur by heat-induced chemical
reactions between reducing sugars and amino acids or proteins to form compounds that bind
minerals. These browning reaction products are more resistant to digestion and hence capable
of hence have their mineral-binding properties remain intact. Heating also destroys ascorbic
acid and in so doing eliminates a key component that assists iron absorption (Hallberg, 1981).
4. Anti-Nutrients in Plant Foods
Phytic Acid and Phytase
Key minerals such as Ca2+, Fe2+, Zn2+ have the capacity to form complexes with phytic
acid (pytate), fiber, and tanning and lectin compounds. The phytates in particular are common
to cereal grains and because they target divalent metal ions, phytates compete with absorbing
cells for Ca2+, Mg2+, Zn2+, Fe2+ and Cu2+, which could result in a deficit of these essential
minerals to the organism. Consequently negating the action of phytates has received
considerable attention in food processing. Treating the grain of cereal with phytases, enzymes
that destroy the phytates, in a process known as dephytinzation is one approach. When
applied to infant cereals phytases treatment resulted in an increase in the uptake, transport
and retention efficiency of iron and zinc (Frontela et al, 2009). Calcium uptake was also
enhanced by this treatment.
Table 4.1 Processing Procedures that Affect the Mineral Content
of Food
Thermal treatments
Detrimental
Beneficial
Sterilization
Baking
Pasteurization
Blanching
Boiling
Steaming
Frying
Blanching
Baking
Mechanical treatments
Detrimental
Milling
Extrusion
Soaking
Drying
Freezing
Storage
Packaging
Beneficial
Canning
Fermentation
Oxalic Acid and Oxalates
Partially or completely ionized salts of oxalic acid (oxalates) form water soluble
complexes with Na+, K+ and NH4+, but insoluble complexes with Ca2+, Fe2+ and Mg2+. With
the exception of Zn2+ which is relatively unaffected, oxalates are a par with phytates as antinutrients by virtue of their disruption of mineral absorption. Foods rich in oxalates potentially
can decrease important minerals absorbed into the system. At pH of 2, the oxalates exists as
monovalent ions primarily in association with potassium as the counter ion. Above pH 6, the
oxalic acid parent structure is completely ionized and in this form will form insoluble complexes
with Ca2+ and Mg2+ . Thus acid sensitivity is an important consideration for stability of the
complex. Soaking or cooking foodstuff removes oxalates and lessens the threat of diet rich in
extracts of tea leaves, spinach or cocoa, which can contain as high as 300 to 2000 mg of oxalic
acid (Noonan et al, 1999).
5. Minimizing Minerals loss in processing
Losses of minerals incurred during processing can be amended by replenishing or
enriching the raw or unprocessed food product with the minerals. This can be accomplished by
direct addition or genetic manipulation. As an example of the former, enriching rice with a
mineral mix in the form of a powder followed by heating to coat the rice with an edible film has
been a good strategy for replenishing minerals that were lost from the rice hull during milling.
Akin to this is a procedure is the use of paraboiling to drive the mineral from the hull into the
endosperm of the rice grain before removing the hull or coat by milling.
Biofortification
Biofortification refers to procedures for enhancing nutrient quality of a plant.
Application is generally directed at the growing phase. This can be achieved through
fertilization, plant breeding, or biotechnology. In one of the earliest applications to minerals,
rice seeds were iron-fortified by overexpressing a soybean ferritin gene (Goto et al, 1999). The
technique raised the iron retained in the endosperm by as much as three-old over non-fortified
rice. Bioforitification has also been applied to applied to enriching the calcium content of
carrots by overexpressing the CAX1 gene. The gene codes for a vacuolar Ca/H+ antiporter
(Morris et al, 2008). Since the higher amounts of calcium were retained in the edible portion of
the carrot, when consumed, the modified carrot delivered twice the amount of calcium into the
bones of experimental animals and raised the amount of the calcium absorbed in humans by
over 4o percent. A similar enrichment of calcium in tomatoes and potato tubes was obtained
when the CAX1 gene from Arabidopsis was over-expressed in these plants (Parker et al, 2005 ;
Kim et al, 2006). Calcium enrichment did not harm the tomato, but on the contrary, gave the
tomato a longer shelf life. Since the Ca/H+ anti-porter transports other minerals besides
calcium, CAX1 gene over-expression has the potential to increase the bioavailability of other
minerals. Experiments with the carrot, however, showed that only calcium was enriched. An
Important concern for using this approach is the indiscriminant uptake and accumulation of
toxic metals such as cadmium and lead. Such unwanted accumulation can be minimized,
however, by genetically modifying critical residues at the metal binding site, tailoring them to
bind only calcium (Shigaki et al, 2005)
6. Issues of Food Safety and Minerals
Toxic Mineral Absorption by Plants
Recognizing that biological minerals cannot be produced in the system, one must look
to the diet as the immediate source and the earth as the ultimate mineral source. Because
plants grown in soils are unable to screen harmful minerals from beneficial ones, toxic minerals
in the soil can be a serious health risk particularly if the toxic minerals are absorbed into edible
parts of the plant. The same applies indirectly to meat from animals grazing on plants grown in
such soils. For them, there is also the risk of insufficiencies of specific minerals. An
insufficiency can have the same impact as a toxicity, although the former cannot affect meat
products derived from the animal. Toxic minerals may arise from deposits of sludge, spillage,
or mining operations conducted in the area or even the fertilizer applied to the field. Minerals
seemingly of greater concern are arsenic, cadmium, mercury, lead, and selenium (Mclauchlin et
al, 1999). Cadmium in particular tends to build up in body tissue over time and is not readily
removed from the system. Drinking water is a primary threat posed by arsenic. Arsenic as well
as chromium are known to be carcinogenic, which places a special concern for arseniccontaminated rice.
Redox-Active Metal Ions
A liberated mineral is not necessarily a safe mineral. This is especially true for transition
metal ions such as Fe2+ and Cu2+ that have the capacity to act as prooxidants and catalyze
formation of free radicals that destroy organic molecules. In their free state redox-active metal
ions can lead to spoilage and short shelf lives. Fatty acids, fat soluble and water soluble
vitamins are particularly vulnerable to their oxidation. Their quantity in foods can be lowered
significantly. Despite this apparent hazard, supplementing food products with iron is still in
vogue. To minimize exposure, the Iron is generally added at a later stage in the processing
procedure. Ferrous sulfate is particularly dangerous; iron compound less prone to food
spoilage tend to have less bioavailability (Hurrell, 1989).
A second concern of redox active metals is their effects on food storage and shelf life.
Oxidants can be particularly important in food spoilage and packaging. In addition to
destroying essential vitamins and fat, redox active minerals can catalyze peroxidations that
tend to lead to rancidity.
4. A Comparison of Minerals in Foods from Animals and Plants
Although most of the emphasis of food minerals has been on plants as opposed to
animals, there remains to be decided which of the two is the better mineral source. Table 4.4
shows that foods from plants tend to be richer in key macrominerals such as calcium,
potassium and phosphorus. Dairy products however, exceed plants as a calcium source.
Table 4.4. Comparison of Calcium, Potassium and Phosphorus Content of Foods from Plants
and Animals.
Calcium
Potassium
Phosphorus
Plant
Brown Rice
10
250
310
Peanuts
16
183
100
Spaghetti
15
146
110
Lettils
15
287
40
Peas
21
330
118
Broccoli
89
588
138
Potato
23
119
190
Animal
Hard Cheese
202
27
143
Beef
Egg White
Milk
3
4
252
85
94
306
134
8
209
Quantity alone does always translate into better bioavailability. Below is a list of advantages
animal have over plants as sources of dietary minerals. First and foremost, animals foods such
as meats, eggs, and dairy products undergo very little processing before being submitted to
consumers, which basically says bioavailability is determined after, not before consumption.
Second, animal products regardless of tissue source are void of phytates and oxalates, thereby
1. Animal products are subject to less processing and thereby retain more of their mineral
content,
2. Animal products do not contain phytates and oxalates
3. Phosphate in animal products is mostly protein bound; in plants most of the phosphate is
associated with phytates,
4. Iron from animal products is richer, more bioavailable and absorbable than iron from plants.
This can be said for most of animal microminerals.
avoiding two major anti-nutrients that interfere with mineral absorption. Muscle protein itself
is a rich source of both heme iron and non-heme iron, the latter generally considered a more
absorbable and bioavailable form of iron. Weighing the two can lead to no hard fast conclusion
as to which is better, although the lack of processing and the known losses that occur in
processing tend to put animal products ahead of plants as bioavailable sources of minerals.
SUMMARY
There is ample evidence that food processing alters the mineral content of food and can
be both detrimental and beneficial to mineral bioavailability. As a benefit, processing can
increase absorptivity by enhancing the diffusability of food-locked minerals. The negative
factors, however, are a greater concern to food science. The harm that occurs through milling,
extrusion cooking, thermal treatments etc. can result in a food product that lacks essential
minerals or falls below amounts necessary to retains properties. Losses incumbent on
processing can be addressed by enriching a food source with the minerals prior to processing.
Another approach is biofortification which employs genetic modifications to the plant as a
means to enrich the mineral content while not harming the plant. Unfortunately, this
technique to date has only been applied to a few important minerals and may not be applicable
to all. Lingering in the dark side of minerals is the need to be constantly aware of the potential
toxic properties of minerals that are manifested in crops grown in contaminated fields as well
as a processing plant. It is still undecided if minerals from plants are less bioavailable than
minerals from animals food sources, although present understanding leans in favor of animal
products as the better source of minerals.