Food Biochemistry and Food Processing (2 edition)

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4 Browning Reactions 71

degradation. Contribution from sugar-amine reactions is negligi-

ble, as evident from the constant total sugar content of degraded

samples. The presence of amino acids and possibly other amino

compounds enhance browning (Roig et al. 1999).

Both oxidative and nonoxidative degradation pathways are

operative during storage of citrus juices. Since large quantities

of DHAA are present in citrus juices, it can be speculated that the

oxidative pathway must be dominant (Lee and Nagy 1996, Ro-

jas and Gerschenson 1997a). A significant relationship between

DHAA and browning of citrus juice has been found (Kurata

et al. 1973, Sawamura et al. 1991, Sawamura et al. 1994). The

rate of non-oxidative loss of ascorbic acid is often one-tenth or

up to one-thousandth the rate of loss under aerobic conditions

(Lee and Nagy 1996). In aseptically packed orange juice, the

aerobic reaction dominates first and it is fairly rapid, while the

anaerobic reaction dominates later and it is quite slow (Nagy and

Smoot 1977, Tannenbaum 1976). A good prediction of ascor-

bic acid degradation and the evolution of the browning index of

orange juice stored under anaerobic conditions at 20–45◦Cmay

be performed by employing the Weibull model (Manso et al.

2001).

Furfural, which is formed during anaerobic degradation of

ascorbic acid, has a significant relationship to browning (Lee

and Nagy 1988); its formation has been suggested as an ade-

quate index for predicting storage temperature abuse in orange

juice concentrates and as a quality deterioration indicator in

single-strength juice (Lee and Nagy 1996). However, furfural is

a very reactive aldehyde that forms and decomposes simultane-

ously; therefore, it would be more difficult to use as an index for

predicting quality changes in citrus products (Fennema 1976).

In general, ascorbic acid would be a better early indicator of

quality.

Control of Ascorbic Acid Browning

Sulfites (Wedzicha and Mcweeny 1974, Wedzicha and Imeson

1977), thiols compounds (Naim et al. 1997), maltilol (Koseki

et al. 2001), sugars, and sorbitol (Rojas and Gerschenson 1997b)

may be effective in suppressing ascorbic acid browning. Doses

to apply these compounds highly depend on factors such as con-

centration of inhibitors and temperatures.l-Cysteine and sodium

sulfite may suppress or accelerate ascorbic acid browning as a

function of their concentration (Sawamura et al. 2000). Glucose,

sucrose, and sorbitol protectl-ascorbic acid from destruction

at low temperatures (23◦C, 33◦C, and 45◦C), while at higher

temperatures (70◦C, 80◦C, and 90◦C), compounds with active

carbonyls promoted ascorbic acid destruction. Sodium bisulfite

was only significant in producing inhibition at lower temper-

ature ranges (23◦C, 33◦C, and 45◦C; Rojas and Gerschenson

1997b).

Although the stability of ascorbic acid generally increases as

the temperature of the food is lowered, certain investigations

have indicated that there may be an accelerated loss on freezing

or frozen storage. However, in general, the largest losses for

noncitrus foods will occur during heating (Fennema 1976).

The rapid removal of oxygen from the packages is an im-

portant factor in sustaining a higher concentration of ascorbic

acid and lower browning of citrus juices over long-term storage.

The extent of browning may be reduced by packing in oxygen

scavenging film (Zerdin et al. 2003).

Modified-atmosphere packages (Howard and Hernandez-

Brenes 1998), microwave heating (Villamiel et al. 1998, Howard

et al. 1999), ultrasound-assisted thermal processing (Zenker et al.

2003), pulsed electric field processing (Min et al. 2003), and

carbon dioxide-assisted high-pressure processing (Boff et al.

2003) are some examples of technological processes that allow

ascorbic acid retention and consequently prevent undesirable

browning.

Lipid Browning

Protein-Oxidized Fatty Acid Reactions

The organoleptic and nutritional characteristics of several foods

are affected by lipids, which can participate in chemical modifi-

cations during processing and storage. Lipid oxidation occurs in

oils and lard, and also in foods with low amounts of lipids, such

as products of vegetable origin. This reaction occurs in both un-

processed and processed foods, and although in some cases it is

desirable, such as in the production of typical cheeses or fried-

food aromas (Nawar 1985), in general, it can lead to undesirable

odors and flavors (Nawar 1996). Quality properties such as ap-

pearance, texture, consistency, taste, flavor, and aroma can be

adversely affected (Erikson 1987). Moreover, toxic compound

formation and loss of nutritional quality can also be observed

(Frankel 1980, Gardner 1989, Kubow 1990, 1992).

Although the lipids can be oxidized by both enzymatic and

nonenzymatic reactions, the latter is the main involved reaction.

This reaction arises from free radical or reactive oxygen species

(ROS) generated during food processing and storage (Stadtman

and Levine 2003), hydroperoxides being the initial products. As

these compounds are quite unstable, a network of dendritic reac-

tions, with different reaction pathways and a diversity of prod-

ucts, can take place (Gardner 1989). The enzymatic oxidation of

lipids occurs sequentially. Lipolytic enzymes can act on lipids

to produce polyunsaturated fatty acids that are then oxidized by

either lipoxygenase or cyclooxygenase to form hydroperoxides

or endoperoxides, respectively. Later, these compounds suffer a

series of reactions to produce, among other compounds, long-

chain fatty acids responsible for important characteristics and

functions (Gardner 1995).

Via polymerization, brown-colored oxypolymers can be pro-

duced subsequently from the lipid oxidation derivatives (Khayat

and Schwall 1983). However, interaction with nucleophiles such

as the free amino group of amino acids, peptides, or proteins can

also take place, because of the electrophilic character of free

radicals produced during lipid oxidation, including hydroper-

oxides, peroxyl and alkoxyl radicals, carbonyl compounds and

epoxides. As a result of this, end products different from those

formed during oxidation of pure lipids can be also produced

(Gillat and Rossell 1992, Schaich 2008). Both lipid oxidation