FCH - BROWNING Flashcards
One of the most important reactions taking place during food processing and storage.
Browning
Two types of browning reaction in food:
- Enyzmatic
- Non-enzymatic
Ubiquitous in nature, seen not only plants but also in animal food products.
Browning reactions
Occur due to wounding, mechanical damage and cutting common to fruits and vegetables.
Browning reactions
Also occurs due to physiological changes such as senescence, which is a condition or process of deterioration with age. During senescence cell and tissue breakdown occurs allowing browning to occur rapidly
Browning reactions
Storage and food process conditions also play a major role in the onset of _________________.
Browning reactions
Elevated temperature such as frying and baking leads to favorable browning qualities of baked and fried products.
Browning reactions
Low temperature storage may also cause browning due to chill injury which affects frozen products negatively.
Browning reactions
Plant diseases also cause ___________________ as a result of cellular leak that favors browning reactions.
Browning reactions
Other horticultural crops such as apples are prone to C02 injury which also causes browning to this fruit.
Browning reactions
Enzymatic browning or also known as ______________________.
Phenolase browning
One of the most important color reactions that affect fruits, vegetables, and seafood.
Enzymatic browning
Enzymatic browning is catalyzed by the enzyme _________________.
Polyphenol oxidase
Polyphenol oxidase is also preferred to as:
- phenol oxidase
- phenolase
- monophenol oxidase
- diphenol oxidase (DPO)
- tyrosinase
The abundance of _________________ in plants may be the reason for naming this enzyme PPO.
Phenolics
_____________ can affect color, flavor, and nutritional value of foods, hence, can cause tremendous economic losses.
Enzymatic browning
By-product of ortho-quinone and is the brown pigmentation in enzymatic browning.
Melanins
_______________________ catalyze the oxidation of phenolic constituents to quinones, which finally polymerize to colored melanins.
Polyphenol oxidase enzymes (PPOs)
Products of enzymatic browning play key _____________ roles.
Physiological
________________ produced as a
consequence of PPO activity.
Melanins
Melanins may exhibit the following properties:
- Antibacterial
- Antifungal
- Anticancer
- Antioxidant
Affect fruits, vegetables, and seafood in either positive or negative ways.
Enzymatic browning
Positive effect of Enzymatic browning.
These reactions, for instance, may contribute to the overall acceptability of foods such as tea, coffee, cocoa, and dried fruits (raisins, prunes, dates, and figs).
The polyphenolic compound that contributes to browning and stringent and bitter flavor of tea
Tannin
Undesirable during processing of fruit slices and juices
Negative effect of Enzymatic browning
Negative effect of Enzymatic browning
Lettuce, other green leafy vegetables; potatoes and other starchy staples, such as sweet potato, breadfruit, and yam; mushrooms; apples; avocados; bananas; grapes; olives etc. Crustaceans are also extremely vulnerable to enzymatic browning
Is an oxidoreductase able to oxidize phenol
compounds, employing oxygen as a hydrogen acceptor.
PPO (EC 1.10.3.1; o-diphenol oxidoreductase)
Directly responsible for enzymatic browning reactions in damaged fruits during postharvest handling and processing.
Polyphenol oxidase (PPO)
PPO catalyzes two basic reactions:
- hydroxylation to the o-position adjacent to an existing hydroxyl group of the phenolic substrate (monophenol oxidase activity).
- oxidation of diphenol to o-benzoquinones (DPO activity).
Properties of PPO
- PPO, active between pH 5 and 7, does not have a very sharp pH optimum.
- At lower pH values of approximately 3, the enzyme is irreversibly inactivated.
- Reagents that complex or remove copper from the prosthetic group of the enzyme inactivate the enzyme.
Substrates of PPO
- Tyrosine
- Other phenolic compounds such as caffeic acid and chlorogenic acid also serve as substrate.
The major substrate for certain phenolases
Tyrosine
Common phenolic compounds present in foods:
- Quinic acid
- Dopamine
- Ferulic acid
- Catechol
- Chlorogenic acid
Three Control of Enzymatic Browning:
- Direct Inhibition
- Chemical Modification
- Non-enzymatic Reduction
Most preferred control of enzymatic browning.
Direct Inhibition
Factors why direct inhibition is preferable.
- Reducing agents
- Acidulants
- Chelating agents
- Complexing agents
- Enzyme inhibitors
- Enzyme treatments
Three direct inhibition.
- Inhibition of PPO.
- Inhibition of Substrate.
- Inhibition towards the products.
Two methods of inhibition of PPO.
- Physical methods
- Chemical methods
Physical methods of inhibition of PPO includes:
- Heating
- Cooling
- Irradiation
Inhibition of PPO: HEATING
*Blanching and high temperature- short time (HTST) pasteurization used respectively in the pre-treatment of vegetables for canning
* Denature enzyme by blanching - 100°C for 2-7minutes for complete inactivation
* Blanching could cause undesirable generation of color or flavor and softening of texture – rarely used for frozen fruits consumed without cooking.
Inhibition of PPO: COOLING
- Rate of reaction drops at low temperature
- Refrigeration temperature of less than 7°C
Inhibition of PPO: IRRADIATION
- UV-C, gamma rays, x-rays, and electron beams
Chemical methods of inhibition of PPO:
- Chelating agents
- Acidification
- Antioxidants
Inhibition of PPO: CHELATING AGENTS
- PPO has a co-factor copper
- Citric acid, sorbic acid, polyphosphates, hinokitiol, kojic acid, EDTA, porphyrins, polycarboxylic acids.
- Hinokitiol - used in coating materials for food packaging
Inhibition of PPO: ACIDIFICATION
- Optimum pH for PPO is 5-7
- Inhibited at pH below 3
- Addition of acidity regulators such as citric acid, ascorbic acid, malic acid
Inhibition of PPO: ANTIOXIDANTS
- React with oxygen and suppress the initiation of the browning process, thus, inhibit melanin formation.
- Ascorbic acid, N-acetylcysteine, L-cysteine, 4-hexylresorcinol, erythorbic acid, cysteine hydrochloride, glutathione
Methods for inhibition of substrate:
- Removal of oxygen
- Removal of phenols
Inhibition of substrate: REMOVAL OF OXYGEN
- Vacuum treatment: Withdrawing air or replacing it with other gases (e.g., N2 or CO2) during preservation, such as in vacuum-packaging or modified atmosphere packaging (MAP)
- Brine immersion – 0.1% sodium chloride significantly inhibited browning.
- Coating fruits with sugar
Inhibition of substrate: REMOVAL OF PHENOLS
- Soaking or immersion in water
- Fermentation
- Activated carbon
Methods of inhibition towards the products:
- Natural agents
- Genetic Modification
Inhibition towards the product: NATURAL AGENTS
- Different natural products and their extracts, such as onion, pineapple, lemon, and white wine, are known to inhibit or slow the browning of some products.
- Pineapple juice is high in sulphur- which prevents melanin formation; however, sulfur / sulphites are known to cause food allergies
Inhibition towards the product: GENETIC MODIFICATION
- Arctic apples have been genetically modified to silence the expression of PPO, thereby delaying the browning
effect, and improving apple eating quality. - The apple’s own genes were used to “turn off” the enzyme that makes apples brown when cut.
Effects ofE nzymatic Browning in food
DESIRABLE
* Melanin – antimicrobial effects which prevents infection
* Produces desirable color and flavor to some foods such as coffee, tea, black raisins, black figs
UNDESIRABLE
* Softens the product
* Affects sensorial properties, making the food unappealing
* Decreases shelf life
* Black spot- shrimp, lobsters, crabs
The most complex reaction in food chemistry.
Non-enzymatic browning
Reaction through different pathways with a large number of food components.
Non-enzymatic browning
Referred to as the Maillard reaction.
Non-enzymatic browning
Maillard reaction was first described in 1912 by the French physician and chemist ________________.
Louis-Camille Maillard
Referred to as the Maillard reaction when it takes place between _____________________________________________________.
free amino groups from amino acids, peptides, or proteins and the carbonyl group of a reducing sugar.
Maillard browning begins with reaction of the amino group of amino acids with a glycosidic hydroxyl group of sugars; the sequence terminates with the formation of __________________________.
brown nitrogenous polymers or melanoidins.
One of the main reactions causing deterioration of proteins during processing and storage of foods.
Maillard browning
Can promote nutritional changes such as loss of nutritional quality (attributed to the destruction of essential amino acids) or reduction of protein digestibility and amino acid availability.
Maillard browning
It also contribute to development to desirable flavor, aroma and color in baked and fried products.
Maillard browning
Three stage of Maillard browning:
- Early stage
- Intermediate stage
- Final stage
EARLY STAGE
Condensation of primary amino groups of amino acids, peptides, or proteins with the carbonyl group of reducing sugars (aldose), with loss of a molecule of water, leading, via formation of a Schiff’s base and Amadori rearrangement, to the so- called Amadori product (1-amino-1deoxi-2-ketose) - a relatively stable intermediate.
Are precursors of numerous compounds that are important in the formation of characteristic flavors, aromas, and brown polymers.
Amadori compounds
Formed before the occurrence of sensory changes; therefore, their determination provides a very sensitive indicator for early detection of quality changes caused by the Maillard reaction.
Amadori compounds
Leads to breakdown of amadori compounds (or other products related to the Schiff’s base) and the formation of degradation products, reactive intermediates (3- deoxyglucosone), and volatile compounds (formation of
flavor).
INTERMEDIATE STAGE
FINAL STAGE
Characterized by the production of nitrogen-containing brown polymers and copolymers known as _______________.
Melanoidins
Responsible for browning of non-enzymatic browning.
Melanoidins
Have been described as low-molecular weight (LMW) colored substances that
are able to cross-link proteins via ε-amino groups of lysine or arginine to produce high-molecular
weight (HMW) colored ______________.
Melanoidins
Control of maillard browning.
Removal of the substrate such as simple sugars.
The enzyme ___________________ are used to convert sugars into their aldonic acid derivative preventing the initiation of maillard reaction.
glucose oxidase and ribose oxidase
Metal ions such ________________ speeds up the maillard reactions.
copper and iron
is used to chelate these metals preventing them to speed up the maillard reactions.
EDTA (chemical reagent)
Can also be used to reduce HMF to form stable colorless compounds.
Sulfites
Non-enzymatic browning involving the degradation of sugars and generally proceeds simultaneously with the Maillard reaction
Caramelization
Contributes markedly to the production of brown pigments and may lead to an overestimation of the Maillard reaction and its associated properties in foods.
Caramelization
CARAMELIZATION
When sugars are heated above their melting points they darken to a brown coloration under alkaline or acidic conditions. If this reaction is not carefully controlled it can lead to the production of unpleasant, burned, and bitter products. Consequently, it is important to control this reaction during food processing while still retaining the pleasant qualities of caramel.
Caramelization of fructose accounted for __________ of total brown development between pH _________.
- 10–36%
- 4.0 and 7.0.
Remains one of the most widely used coloring agents in food and pharmaceuticals
Caramel
Caramelization: Formation of brown pigment
Starts with the opening of the hemiacetal ring followed by enolization, which proceeds via acid- and base-catalyzed mechanisms, leading to the formation of isomeric carbohydrates.
Dehydration of sugar.
furaldehyde or furfural
Low amounts of isomeric carbohydrates are formed; dehydration is favored, leading to furaldehyde compounds.
Acid media
Dehydration reactions are slower than in neutral or acid media.
Alkaline media
Ascorbic acid browning
Thermal decomposition of ascorbic acid under both aerobic and anaerobic conditions, by oxidative or non-oxidative mechanisms, either in the presence or absence of amino compounds.
Nonenzymatic browning is one of the main reasons for the loss of commercial value in citrus products .
Ascorbic acid browning
Ascorbic acid browning examples.
Non-citrus foods such as asparagus, broccoli, cauliflower, peas, potatoes,
spinach, apples, green beans, apricots, melons, strawberries, corn, and
dehydrated fruit are also affected with this type of browning reaction.
Pathway of ascorbic acid browning
- When oxygen is present in the system, ascorbic acid is degraded primarily to dehydroascorbic acid (DHAA). DHAA is not stable and spontaneously converts to 2,3-diketo-lgulonic acid .
- In the presence of amino acids, ascorbic acid, DHAA, and their oxidation products furfural, reductones, and 3-deoxypentosone may contribute to the browning of foods by means of a Maillard-type reaction .
- The presence of metals, especially Cu and Fe causes great losses of vitamin C. Catalyzed oxidation goes faster than the spontaneous oxidation.
Control of ascorbic acid browning
- Rapid removal of oxygen from the packages is an important 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.
- Modified-atmosphere packages
- Microwave heating
- Ultrasound-assisted thermal processing
- Pulsed electric field processing
- Carbon dioxide-assisted high-pressure processing
Arises from free radical or reactive oxygen species (ROS) generated during food processing and storage, hydroperoxides being the initial products.
Lipid browning
Long-chain fatty acids responsible for important characteristics and functions.
Lipid browning
Can act on lipids to produce polyunsaturated fatty acids that are then oxidized by either lipoxygenase or cyclooxygenase to form hydroperoxides or endoperoxides, respectively.
Lipolytic enzymes
