BLBS102-c01 BLBS102-Simpson March 21, 2012 11:8 Trim: 276mm X 219mm Printer Name: Yet to Come
1 Introduction to Food Biochemistry 21(e.g. citrus fruits are high in terpenoids as is mango; Aharoni
et al. 2004).NUCLEIC ACIDS AND FOOD SCIENCE
DNA StructureAlthough nucleic acids are not generally important components
of foods, they are perhaps (and ironically) the most important
biomolecule class of all for the simple reason that everything
we eat was at some point alive, and every process and struc-
ture in those organisms were determined via enzymes encoded
for by DNA. Indeed, the entire purpose of breeding programs
is to control the propagation of DNA between successive gen-
erations. Understanding the basics of DNA (deoxyribonucleic
acid) and its biochemical forms is important for food scientists
to gain an appreciation of the basis for genetic manipulations of
food-related proteins as well as DNA-based food authentication
techniques.
DNA is composed of three main chemical components: a ni-
trogenous base, a sugar and a phosphate. There are four bases:
Adenine (A) and guanine (G) are purines, while thymine (T)
and cytosine (C) are pyrimidines. The bases bound to both the
sugar and phosphate moieties make up nucleotides and the four
building blocks of DNA are deoxyadenosine triphosphate, de-
oxyguanosine triphosphate, deoxythymidine triphosphate and
deoxycytidine triphosphate (dATP, dGTP, dTTP and dCTP, re-
spectively). These building blocks are typically referred to as
‘bases’ or simply A, T, G and C; however, common usage of
these terms are meant to imply nucleotides as opposed to bases
alone. The nucleotides covalently bond together forming a DNA
strand that is synthesised by DNA polymerase. Additionally,
each nucleotide’s base moiety can bond via hydrogen bonds to
other bases; A–T and G–C, termed base pairs and are said to
be complementary. In fact, DNA exists in two-stranded form,
consisting of two complementary strands. For example, a strand
ATCG would be paired to its complement TAGC.5’-ATCG-3’
3’-TAGC-5’Two-stranded DNA spontaneously forms a helix, hence the
term double helix, which can contain many thousands of base
pairs with molecular weights in the millions, or billions, of
Daltons. By comparison, the largest known protein is a mere 3
million Daltons.Genesare stretches of DNA that encode for the
synthesis of proteins; DNA is transcribed, yielding messenger
RNA (mRNA), which is then translated at ribosomes, yielding
specific sequences of amino acids (proteins).Genetic ModificationThe advances in how to copy DNA, modify its sequence, and
transfer genes between organisms efficiently and at low cost
has produced a tremendous ability to study the roles of spe-
cific proteins in organisms as well as the roles of specific amino
acids within proteins. This ability has produced an alternateto traditional breeding programs in the search for food plants
and animals that have desired traits, such as increased yield, in-
creased pesticide tolerance, lower pesticide requirements/higher
pest resistance, longer shelf life post-harvest, etc. This alternate
strategy is the basis of genetic modification (GM).
Critical to the study, transfer and manipulation of genes was
the advent of the polymerase chain reaction (PCR), which al-
lows for the easy and accurate copying and, equally important,
the amplification of DNA sequences. Briefly, PCR works by
inducing repeated copying of a given DNA sequence by the en-
zyme DNA polymerase via repeated temperature cycles such
that exponential amplification results, i.e. the first round yields
only a doubling, but the second round then makes new copies of
each of the first round’s copies and the originals; 2, 4, 8, 16, 32,
64, 128, etc. After 25–30 PCR rounds, millions of copies result.
Thus, a gene encoding for a useful gene in organism A (e.g. an
anti-freeze protein) can be copied, amplified and subsequently
transferred to organism B (e.g. a fruit).
An example of GM is that of the Flavr savrTMtomato, orig-
inally available for consumption in 1994 (Martineau 2001). A
non-sense gene is a DNA sequence that encodes for complemen-
tary mRNA, which base-pair matches and binds to a natural gene
transcript, thereby suppressing its translation. A ‘non-sense’,
gene acting against the polygalacturonase gene, an enzyme re-
sponsible for the breakdown of a cell-wall component during
ripening, was incorporated into a strain of tomato. The result
was slowed softening of the texture of the engineered tomato
compared to normal tomatoes, thus allowing producers to vine-
ripen the Flavr savrTM, reducing losses (e.g. bruising) during
subsequent transport to market. Superior flavour and appearance
relative to natural tomatoes picked green, as well as equivalent
micro- and macronutrient content, pH, acidity and sugar content
relative to non-transgenic tomatoes resulted.
In terms of food processing, lactic acid bacteria and yeast have
been developed to solve problems in the dairy, baking and brew-
ing industries (Tables 1.20 and 1.21). As with biotechnology-
derived food enzymes, the use of genetically modified organisms
is governed by laws of nations or regions (e.g. the European
Union).
Safety assessments of GM foods before being released to
market are done. These comparisons to non-engineered, conven-
tional counterparts include proximate analysis as well as anal-
yses of nutritional components, toxins, toxicants, anti-nutrients
and other components relevant to given cases. As well, ani-
mal feeding trials are conducted to determine if any adverse
health effects are observable (Institute of Medicine and National
Research Council of the National Academies 2004). Ideally,
the reference food for the above comparisons is the isogenic
food (i.e. non-transformed) from which the GM version was
derived.Food Authentication and the Role of DNA
TechnologiesAnother area utilising DNA technology isfood authentication.
Analysing processed food and ingredients for the presence of
fraudulent or foreign components by DNA technologies can be