Fruits and vegetables Flashcards
[PART 1]
[PART 2]
Why is it important to reduce respiration rate when processing fruits and vegetables after harvest? [3]
- The longer the time of respiration, substrates such as sugar will be lost, affecting flavour (esp sweetness).
- Respiration produces heat, and if heat is not removed, may cause heat stress
- For climacteric fruits, if respiration proceeds after ripening, production of ethylene will cause them to soften/rot/reach senescence at a faster rate.
[PART 2]
At which part of fruit development will high respiration rates pose a big problem?
After ripening, where high respiration rates would increase the rate at which commodities reach senescence (death).
[PART 2]
What are the 2 broad categories of fruits and their differences? State examples for each category of fruit
1. Climacteric fruits
- fruits that show a dramatic rise in respiration rate during ripening, and peak in respiration rate coincides with sharp increase in ethylene production.
- ripen even after harvesting
- e.g. banana, avocado
2. Non-climacteric fruits
- fruits that have a relatively stable respiration rate, with a steady decline as it approaches senescence. Do not respond to ethylene treatments
- must fully ripen before being harvested
- e.g. strawberry, blueberry
[PART 2]
What are the 4 extrinsic factors affecting rate of respiration?
- Temperature
- CO2 and O2 concentrations
- Stress and injury
- Ethylene concentration
[PART 2 – respiration – temperature]
What does the Van’t Hoff rule state about the relationship between temperature and rate of respiration?
Velocity of a biological reaction increases 2 to 3 fold for every 10°C rise in temperature
[PART 2 – respiration – temperature]
What are the storage temperatures for :
(a) tropical / subtropical crops
(b) temperate crops
(c) most vegetables
(a) tropical/subtropical : 10 to 15°C (chill sensitive)
(b) temperate : -1 to 5°C
(c) most vegetables : 1 to 5°C
Temperate fruits and vege stored in chiller conditions (0-4°C)
[PART 2 – respiration – temperature]
Which kinds of fruits / vegetables are susceptible to chill injury, and which kinds are chill-insensitive?
Chill sensitive : fruits grown in tropical / sub-tropical areas (warm climate)
Chill insensitive crops : most vegetables, temperate fruits
[PART 2 – respiration – temperature]
At what temperatures does chill injury happen (for chill sensitive crops)?
above freezing temperatures (0°C) but below optimal temperature (10-15°C)
[PART 2 – respiration – temperature]
What are some symptoms of chill injury? [7]
(not as impt,, memorise at least 3/4)
1. Internal browning of flesh and seeds
2. Break down of tissue structure, mushy
- imagine cuz sensitive to cold, fruits freeze faster –> ice crystals pierce through tissue and cause disintegration + leakage of enzymes causing tissue / cell wall degradation
3. Failure to ripen properly
4. Accelerated senescence but normal appearance
- Compositional changes with altered flavour or taste
- Increased susceptibility to decay
- Surface lesions
easier to rmb first four
[PART 2 – respiration – temperature]
What happens when plants get heat injury (temp & resp rate and proteins)?
respiration increases as temperature increases.
Beyond a certain temp, protein denaturation occurs and leading to rapid decline of respiration rate, leading to thermal cell death.
[PART 2 – respiration – temperature]
What are some symptoms of heat injury? [5]
- localised bleaching
- Necrosis / sunscald (“sunburn”)
- Collapse of tissues (denaturation of protein)
- Uneven ripening
- Sensitivity to ethylene
[PART 2 – respiration – O2 and CO2]
Why is it important to not have too little [O2] and too much [CO2]?
Too little [O2] / too much [CO2] leads to anaerobic respiration (fermentation), producing metabolites such as lactic acid or ethanol, leading to off-flavours
[PART 2 – respiration – O2 and CO2]
Within a fruit, flavour throughout the fruit is the same. True or False?
False, there are different [CO2] in different parts of the fruit due to internal barriers to gas diffusion and tissue respiration.
Thus, this leads to different extent of anaerobic respiration and leading to variation of flavour within the fruit.
[PART 2 – respiration – stress, injury]
How does physical stress lead to lower post-harvest quality of fruits and vegetables?
Mechanical injury such as stacking, dropping etc
- Causes cellular enzymes to leak out and come into contact with substrates → degradation of tissue
-Stress also increases ethylene production, increasing rate of senescence
[PART 2 – respiration – stress, injury]
How does water stress (low RH) lead to lower post-harvest quality of fruits and vegetables?
Lower than optimum RH → relative humidity in air is low → Movement of water out of tissue and increased transpiration
- Loss in turgor → softening
- Wilting (dry up)
[PART 2 – respiration – stress, injury]
Other than physical stress and water stress, what are the 5 different kinds of stress/injury a commodity can face?
- Biological stress (disease, pest)
- Chemical stress (fumigants)
3. Irradiation
[Temperature]
4. Chill injury
- Heat injury
[Part 2 – respiration – ethylene concentration]
Ethylene causes senescence only in climacteric fruits. True or false?
False.
Ethylene (“death hormone”) promotes senescence in all types of produce (climacteric fruits / non-climacteric fruits / vegetables)
- Its just that ethylene does not induce ripening and increase in respiration in non-climacteric fruits.
[Part 2 – respiration – ethylene concentration]
What are some main effects of ethylene? [3]
- Promotes ripening (climacteric fruits)
- yellowing and softening in bananas - Promotes yellowing and softening
- as in causing wilting in vegetables etc
- yellowing : caused by degradation of chlorophyll - Promotes senescence
[Part 2 – respiration – ethylene concentration]
What is a competitive ethylene inhibitor commonly used?
1 -Methylcyclopropene (1-MCP)
[Part 2 – respiration – ethylene concentration]
Why is 1-MCP said to be a competitive inhibitor of ethylene?
1-MCP has similar structure to ethylene (substrate), and thus 1-MCP competes with ethylene by occupying the active site of the receptors.
[Part 2 – respiration – ethylene concentration]
What happens when climacteric tissues are exposed to ethylene during the pre-climacteric stage? (before respiratory/ethylene peak)
- Ethylene [from environment] induces climacteric rise in respiration
- Once respiratory rise starts, endogenous ethylene [produced within fruit] production rate increases (autocatalytic production of ethylene)
- fruit ripens
[Part 2 – respiration – ethylene concentration]
When exposed to ethylene in the atmosphere, non-climacteric fruits experience respiratory rise and start autocatalytic production of ethylene. True or False?
False, non-climacteric fruits have relatively stable rate of respiration and do not have auto-catalytic production of C2H4
- but ethylene in the environment increases the rate of senescence of non-climacteric fruits
[Part 2 – strategies for shelf-life extension : MAP]
What is MAP, and what are the concentrations of gases?
Modified atmosphere packaging
- O2 : <10%, but should not fall below 2%
- CO2 : increase composition but should not reach 20%
[Part 2 – strategies for shelf-life extension : MAP]
What does the idealised MAP system consist of? (including storage condition, gas composition etc) [5]
- H2O scrubber – reduce water activity and prevent mold growth
- C2H4 (ethylene) scrubber – slow down rate of senescence
- Impermeable package to reduce exposure to surrounding O2 (irl have vents to prevent condensation and mold growth, bc don’t have H2O scrubbers)
- Gas composition : 0.03% CO2 + 21% O2
- Cold storage to reduce rate of biochemical reactions
[Part 2 – strategies for shelf-life extension : 1-MCP]
How is 1-MCP commercially applied?
1 – MCP is complexed with α-cyclodextrin, a water soluble powder
When powder is mixed with water, it disperses around the produce and 1 – MCP is generated as gas
[Part 2 – strategies for shelf-life extension : 1-MCP]
Inhibitory action of 1-MCP can be temporary or persistent. Temporary / persistent inhibitory action is suitable for what kind of produce?
Temporary : climacteric fruits
- climacteric fruits need to ripen later on –> delay ripening during post harvest + transport etc, until reach supermarkets then let it continue ripening
Persistent : vegetables / non-climacteric fruits
[Part 2 – strategies for shelf-life extension : 1-MCP]
1-MCP can impair ripening. True or false?
True, since it binds to receptor and ethylene cant bind –> signal transduction cascade cannot be passed down –> ripening inhibited
[Part 2 – strategies for shelf-life extension : Pre-cooling]
What is pre cooling?
process to remove field heat of freshly harvested produced, usually within 24h or less after harvest
[Part 2 – strategies for shelf-life extension : Pre-cooling]
What is field heat?
difference in temperature of harvested produce and optimal storage temperature
[Part 2 – strategies for shelf-life extension : Pre-cooling]
What are some of the benefits of pre-cooling? [4]
- Suppress enzymatic degradation (leading to softening) and respiratory activity [aka slow down rate of biochemical reactions] [BIOCHEMICAL]
- Slow / inhibit water loss (wilting) – If temperature high, resp high → stomata open → transpiration also high → loss of water
- Slow / inhibit growth of decay-producing microbes [BIOLOGICAL]
- Reduce production of ethylene (due to reduced respiration) or minimise produce’s reaction to ethylene [CHEMICAL]
[Part 2 – strategies for shelf-life extension : Pre-cooling – room cooling]
What is the idea behind room cooling? What kinds of produce is it suitable for?
- Produce is placed in an insulated room equipped with refrigeration units to chill air
- Produce packaged in well-vented packaging and stacked in small crates
- Cool air is circulated by convection, with help of fans
Suited for types of fresh produce that are tolerant of slow heat removal (does not dehydrate easily)
[Part 2 – strategies for shelf-life extension : Pre-cooling – room cooling]
What are the 3 advantages of room cooling?
- Clean & simple
- Low installation and maintenance cost
- Provides temporary storage space after pre-cooling
[Part 2 – strategies for shelf-life extension : Pre-cooling – room cooling]
What are the 3 disadvantages of room cooling?
- Slow
- Uneven cooling at the beginning
- May not be suitable for produce that are sensitive to dehydration due to a prolonged cooling period
[Part 2 – strategies for shelf-life extension : Pre-cooling – forced-air cooling]
What is the idea behind forced-air cooling (pressure cooling)? What kinds of produce is it suitable for?
A kind of modified room cooling.
- Exposing produce to higher air pressure on one side than on the other side
- Cold fan at one side to blast cool air in (high pressure)
- Fans to pull out the warm air, creating a lower pressure on top and causing hot air to rise and get pulled out
Suitable for most leafy, fruited and root produce
[Part 2 – strategies for shelf-life extension : Pre-cooling – forced-air cooling]
What are the 4 advantages of forced-air cooling?
- Clean & simple
- Low installation and maintenance cost
- Rapid (faster than room cooling)
- High rate of heat transfer
[Part 2 – strategies for shelf-life extension : Pre-cooling – forced-air cooling]
What is the main disadvantage of forced-air cooling?
Slower than vacuum cooling
Part 2 – strategies for shelf-life extension : Pre-cooling – vacuum cooling]
What is the idea behind vacuum cooling? What kinds of produce is it suitable for?
- produce are subject to low pressure with fans sucking hot air out
- reduces pressure on the headspace of produce, causing vaporization of water at low temperatures
- water from produce evaporates, thus produce loses heat (since it has a high heat capacity and can carry a lot of heat)
Suitable for mushrooms and (some) leafy vegetables
Similar forced air cooling, just that no additional cool air
[Part 2 – strategies for shelf-life extension : Pre-cooling – vacuum cooling]
What are the 3 advantages of vacuum cooling?
- Rapid
- Uniform cooling
- High energy efficiency
[Part 2 – strategies for shelf-life extension : Pre-cooling – vacuum cooling]
What are the 3 disadvantages of vacuum cooling?
- Risk of wilting due to moisture loss
- High cost
- Requires packaging with holes for water evaporation
[Part 2 – strategies for shelf-life extension : Pre-cooling – hydrocooling]
What is the idea behind hydro cooling? What kinds of produce is it suitable for?
- usage of chilled water or cold water to cool produce
- produce can be flooded / immersed in water, or water can be sprinkled
Suitable for most fresh fruits and vegetables
Note : contact between water and product surface must be uniform
[Part 2 – strategies for shelf-life extension : Pre-cooling – hydrocooling]
What are the 3 advantages of hydrocooling?
- Rapid
- High energy efficiency
- Provides means to clean vegetables (chlorinated water) → reduces spoilage
[Part 2 – strategies for shelf-life extension : Pre-cooling – hydrocooling]
What are the 2 disadvantages of hydrocooling?
- Additional drying step required (wet produce – susceptible to microbial growth)
- Additional effort to monitor water quality and cleanliness of cooler daily.
[Part 2 – strategies for shelf-life extension : Pre-cooling – ice cooling]
What is the idea behind ice cooling? What kinds of produce is it suitable for?
- produce is cooled with crushed / fine granular ice
- the ice is either packed around produce cartons or sacks, or it is made into a slurry with water and injected into waxed cartons packed with produce. The ice then fills the voids around the produce.
- Ice removes heat rapidly, and continue to absorb heat as it melts
Suited for broccoli, carrots, Chinese cabbage, green onions, and commodities that have a high respiration rate (prone to drying)
- High respiration rate = stomata open
- If use room cooling / forced-air cooling / vacuum cooling → will drive transpiration as water in air is taken away → causing wilting
[Part 2 – strategies for shelf-life extension : Pre-cooling – ice cooling]
What are the 3 advantages of ice cooling?
- Rapid
- High heat transfer performance
- Prevents moisture loss
[Part 2 – strategies for shelf-life extension : Pre-cooling – ice cooling]
What are the 2 disadvantages of ice cooling?
- Additional weight from crushed ice and high water content from melting ice → increased risk of physical injury + rotting
- Risk of chilling injury if period of icing is prolonged.
[PART 3]
What is the definition of relative humidity and equation?
It is the amount of atmospheric moisture present relative to the amount of moisture that would be present if air was saturated
- R.H. = (actual vapour density / saturation vapour density) x 100%
[PART 3]
What is dew point?
Temperature at which air becomes saturated with water
[PART 3]
What happens if temperature falls below dew point?
When temperature decreases, water-holding capacity (WHC) of air decreases. Thus, with the same amount of water but lower capacity (air cannot hold as much water), condensation occurs
[PART 3]
What are the effects of low relative humidity in the surrounding atmosphere of produce? [3]
- Water loss leads to loss in turgor pressure and freshness, esp for leafy greens
- Wilting, shrivelling, poor appearance
- Increased respiration (moisture stress), shorter shelf life
- Cuz water travels from high humidity to low humidity, transpiration increases, stomata open, thus respiration rate also increase to produce water (C6H12O6 + 6O2 → 6CO2 + 6H2O)
[PART 3]
What is meant by water stress / moisture stress in produce?
stress that occurs when the moisture levels in plant tissues are reduced
[PART 3]
What are the effects that could arise from moisture stress due to low R.H.? [3]
- Increased ethylene production and increase in ACC + ACC oxidase activities
- 1-Aminocyclopropane-1-carboxylic acid oxidase (ACC oxidase) : enzyme that catalyzes the final step in the production of the plant hormone ethylene - Faster ripening due to increased respiration (+ increased ethylene)
- Hasten senescence : increased formation of free radicals, deterioration of cell membranes, softening, decreased insoluble pectin + increased soluble pectin
[PART 3]
What are the effects of high R.H. in the surroundings on produce? [3]
- Condensation on surface of fruits occurs → microbial growth (mould etc)
- Surplus turgor pressure may lead to cracking of fruit
- Inhibition of gas exchange (water covers the leaves / skin of fruit → blocking stomata, making it harder for gaseous exchange)
[PART 3]
What is transpiration
mass transfer process in which water vapour moves from surface of plant / organ to surroundings through openings present in tissue (stomata)
[PART 3]
What are lenticels?
small, lens-shaped openings on the surface of fruits, stems, and tubers to allow air to pass through
(water vapour can be lost through lenticels during transpiration)
[PART 3]
What are the factors affecting water loss? 3
- Pressure gradient in intercellular spaces and in ambient atmosphere
- Surface area of plant organ
- Resistance (thickness of surface layer)
Like FST2109 – pressure : driving force ; resistance : opposing force that prevents water vapour from escaping
[PART 3]
What is water vapour pressure?
Pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases at a given temperature in a closed system.
- It’s an indication of a liquid’s thermodynamic tendency to evaporate.
[PART 3]
What is water vapour pressure deficit (VPD)?
The difference in water vapour pressure (wvp) of normal air and wvp of saturated air
[PART 3]
What effect does temperature have on water vapour pressure?
- increased temperature → increased free energy of water molecules (breaking of H bonds between H2O molecules, becoming from liquid state in fruit and loss as vapour) → increased potential to transfer and evaporate away as vapour
- this increased temperature → high WVP
[PART 3]
what are the recommended RH for fruits and vegetables?
- Most fruits (tropical / subtropical / temperature / fruit-type vegetables) : RH 85-95% (~90%)
- Most vegetables: 95-100% RH, esp leafy greens
Dormant storage organs – don’t need so high RH as there is a thick layer of dry skin outside to protect from water loss
- Roots/tubers: 85-90% RH
- Bulb (garlic/onion), ginger, jicama, pumpkin, winter squashes : 65%-75% RH (~70%)
[PART 3]
What are the broad categories of factors affecting transpiration? [4]
1. Produce characteristics
- Surface area to volume ratio, properties (pore size, presence of protective wax), size, respiration rate
2. Postharvest handling
Injuries and damage to protective layers increases respiration and transpiration
3. Environmental factors
a. Air Relative Humiditty (RH) – affects VPD
**b. Temperature **– affects WHC (water holding capacity) of air, VPD, transpiration
c. Air pressure – partial vacuum reduces water molecules in air, increases VPD (lower RH) and is thus the driving force of transpiration
VPD : (amount of water in saturated air - actual amount of water in air) x 100%
If actual amount of water in air decreases, the difference increases
4. Air movement
Strong air current near produce surface (e.g. fan blowing only at a particular fruit) takes away water from air, reducing RH and increasing transpiration
Air circulation in an enclosure promotes uniform temperature distribution, reducing VPD
[PART 3]
What is tuber skinning injury?
Superficial wound caused by mechanical forces during harvest and post-harvest handling
[PART 3]
What are some signs of tuber skinning injury?
- Discolouration of wounded area
Loss of water in tuber → loss of turgor - Increased susceptibility to pressure bruising and blackspot development
- Pressure from machinery → bruise (like blueblack) → when bruise is healing it turns black
- Creation of areas open to infection
[PART 3]
[PART 3]
What is a method to prevent tuber skinning injury?
Artifically induce maturation by killing potato (tuber) plants 7-21 days prior to harvest
- when plant dies, growth stops and activity of the phellogen stops (phellogen adds skin to potato)
- thus, the skin wont continue expanding. Since the skin cells are dead, they adhere to tuber flesh in a process called skin-set, making it more resistant to damage
VS if skin cells (phellogen) still alive when harvesting, tuber will get injured more easily
[PART 3]
If the tuber has already been injured from tuber skinning injury, what process is used to treat tuber skinning injury? State the rationale behind this method in preserving quality of tubers.
Curing.
Rationale : promotes skin thickening, thus tuber less likely to:
- Loose water via transpiration
- Greater resistance to post harvest diseases (thicker skin limits microbes accessibility to moisture and substrates within tubers)
Thus, curing extends shelf life
[PART 3]
Curing can be done after very long periods after harvest (e.g. 1 day later). True or false?
False.
Curing should begin as soon after harvest as possible, preferably within 12h of harvest
[PART 3]
What are the conditions for curing? [3]
- Warm temperatures Warm temperatures (30-40℃)
- High relative humidity (>85% RH) + good ventilation to remove CO2 and replenish O2
- Tubers should not be washed prior to curing → washing introduces pressure, forming more injuries, surface for curing becomes more uneven and making it harder to heal
[PART 3]
Cold-induced sweetening is an adaptation in vegetables that grow in cold temperatures.
Explain how cold-induced sweetening allows these vegetables to survive very cold weathers such as in winter.
Starch is broken down into sugars, increasing the concentration of solutes. This lowers the freezing temperature of water in sap and thus the plant won’t freeze in the winter.
[PART 3]
Cold-induced sweetening is desirable in cold-growing vegetables but not in potato tubers. Explain why.
For cold-growing vegetables, cold-induced sweetening is desirable for their sweet flavour
For potato tubers, accumulation of reducing sugars lowers its processing quality
- if tubers are fried (e.g. potato chips), more reducing sugars result in darker fry colour due to Maillard reaction → if very dark brown, not nice
- Risk of acrylamide production (which may cause cancer in humans, not sufficient human studies to prove this yet)
[PART 3]
What is controlled atmosphere storage?
Modification of the gas composition within the storage room where produce is stored
- Continuous monitoring and precise adjustment of gases within storage space to a predetermined level are carried out
[PART 3]
Explain in rationale behind controlled atmosphere storage (CAS) in preserving the post-harvest quality of fresh produce.
Involves reduction of O2 and increasing CO2 as compared to ambient temperature (may involve removal of ethylene)
- to reduce respiration rate so less ethylene produced (for climacteric fruits) + slow rate of senescence
[PART 3]
What are 2 challenges faced in trying to do controlled atmosphere storage?
- Changes in gas composition in the room is dynamic and always changing due to commodities respiring and producing ethylene, but controlled atmosphere storage conditions are predetermined and fixed → thus requires very intensive monitoring and controlling of parameters
- A few over-ripe/diseased/damaged fruits can impair effectiveness of the controlled atmospheric environment → a lot of ethylene produced → leads to accelerated ripening of other fruits and CA storage conditions becomes ineffective.
[PART 3]
Describe the idea behind modified atmosphere storage (MAS)
Controlling the gas composition within the packaging of the produce itself.
- The gas composition is not actively controlled and the atmosphere within package is created by the respiratory activity of the fresh produce.
[PART 3]
A type of modified atmosphere packaging (MAP) is passive atmosphere modification (PAM). Describe the rationale on how it works.
- Involves using of a film. Its permeability to gases and water vapour should match respiration rate of produce → atmosphere reaches an equilibrium over a few days
- The desired atmosphere develops naturally due to the products’ respiration and the diffusion of gasses through the film.
[PART 3]
In passive atmosphere modfication, what is the permeability of the ideal film to CO2/O2?
CO2 permeability 3-5 times more than O2 permeability to avoid excessive build up of CO2 (which may lead to anaerobic resp and off-flavour)
[PART 3]
What is active modified atmosphere packaging (MAP)?
MAP is a method that establishes a modified atmosphere in a package by creating a light vacuum and replacing the atmosphere with a desired gas mixture.
[PART 3]
What are 2 features in active MAP?
- Film that has desired permeability for gases and water vapour
- Scavengers / scrubbers packed in sachets to actively adjust atmosphere within packaging.
[PART 3]
What is smart packaging?
Packaging system that can carry out intelligent functions like detecting, containment, convenience, tracing, communicating and applying scientific logic
[PART 3]
What is the rationale behind smart packaging?
- putting a small sensor to the packaging of produce → ability to track and trace the exact location of a product in the supply chain using unique barcodes, QR codes, or RFID technology.
Also involves usage of time-temperature indicators (TTIs), ripeness indicators, chemical sensors etc
[PART 3]
What is the main purpose of using coatings in produce?
What kinds of produce are coatings applied on?
Purpose : to decrease movement of water vapour out of cells due to the wax serving as a physical barrier, reducing transpiration
- Applied mostly on whole and cut fruits
[PART 3]
What are edible coatings made of?
Hydrocolloids (water-soluble polymers)
- protein / polysaccharide films
[PART 3]
The hydrophilic network of edible coatings formed by hydrocolloids is useful for?
Acting as gas barriers for O2 and CO2
- more hydrophilic =also permeable to water vapour and less permeable to gas
[PART 3]
Water weakens the hydrocolloid edible coating, weaking its barrier ability to gas. True or false?
True.
- Water leads to swelling of coating, creating gaps and making easier for gases to pass through
- water disrupts the structure of hydrophilic network as hydrocolloids start to react with water
- Water may weaken mechanical properties of coating, leading to cracks or tears
[PART 3]
What is usually used to make non-edible coatings?
Petroleum based waxes and oils
- E.g. paraffin wax, polyethylene wax, mineral oils
[PART 3]
What is the rationale behind calcium treatment?
Calcium treatments provides stability and rigidity to cell wall as they cause cross-linking of pectin
[PART 3]
Which calcium compound when used as calcium treatment, leads to bitter off flavours?
CaCl2
[PART 3]
Which organic calcium salts are commonly used for calcium treatments?
calcium lactate, calcium propionate,calcium gluconate
[PART 3]
Overtime, what are 2 enzymes that will cause depolymerisation of pectin? What do these enzymes break pectin into?
- Pectin methylesterase : breaks down the ester bond in the –COOCH3 bonds to give carboxy groups (COO-)
- Polygalacturonase (PG) : breaks down pectin into its monomer, galacturonic acid
[PART 3]
What is the action of Pectin methylesterade (PME) and polygalacturonase (PG) leading to the softening of pectin?
- Pectin methylesterase (PME) break up methoxygroups (COOCH3) in the pectin molecules which creates free carboxyl groups
- Polygalacturonase then can access the glycosidic bonds which breaks up the pectin chain, leading to a less rigid cell wall
[PART 3]
What are the 2 types of calcium treatment?
- Calcium impregnation
- Calcium dipping
[PART 3]
What is the rationale behind calcium impregnation and what kind of fruits/vegetables is it used on?
Calcium impregnantion involves placing the produce in a vacuum to draw out air. This reduces the pressure, thus causing the calcium treatment solution to flow into the lenticels and penetrate deep into the fruit (effective penetration of calcium treatment rather than just applying on surface)
- Used on whole and cut fruits and vegetables which are not so soft (e.g. apple/tomato/brocolli)
[PART 3]
What is the concentration of calcium used in calcium impregnation?
1.5-7.5%
[PART 3]
How is calcium dipping carried out and what kinds of produce is it used on?
Calcium dipping involves soaking of product, applying mechanical agitation (or may not) and removal of excess solution
- it is a more gentle treatment used on more perishable fresh products like leafy vegetables
[PART 3]
Calcium treatment can be complemented with heat treatment, where produce may be heated before calcium treatment or heating + calcium treatment happen simultaneously.
What is the rationale of using heating treatment with calcium treatment? [3]
- Reduces microbial load
- Enhances PME activity:
- This provides more carboxylate groups for interaction with calcium ions. - Reduces PG activity:
Reduces pectic acid cleavage
[PART 3]
One rationale behind using polyamines is to reduce the production of ethylene. Explain how.
PAs reduce production of ethylene by restricting transcriptional synthesis of aminocyclopropane-1-carboxylic acid synthase (ACS).
- ACS : enzyme that converts methionine to 1-aminocyclopropane-1-carboxylic acid (ACC), which is the direct precursor of ethylene.
[PART 3]
What are the other 2 benefits of using polyamine treatments?
- Delays fruit ripening and softening
- Polyamines are positively charged at physiological pH (NH3+), causing cross-linking of COO- in pectin chains - Polyamine treatments can reduce chill injury as positively charged amino grooups associate with negatively charged phosphate groups in lipid bilayer.
- polyamine embed in lipid bilayer, increasing membrane fluidity so that membranes will not be so rigid and prone to damage (chill injury)
