Fruits Flashcards
What is the structure of the cytoplasm?
- Jelly-like substance
- Enclosed by cell membrane
- Contains;
o Plastids which contain fat soluble pigments (chlorophylls and carotenoids) and starch
o Nucleus (genetic material)
o Mitochondria (contain enzymes)
What is the structure of the cell membrane?
- Composed of lipoproteins (protein-fat complex)
- In living cells (raw food) cell membrane is semi-permeable, allowing osmosis to occur
- Cooking denatures cell membrane making it totally permeable, allowing diffusion to occur
What is the structure of the vacuole?
- Sac-like space containing cell sap (mainly water)
- When vacuole is full of cell sap, vegetable is said to be crisp (turgor pressure)
- When osmosis occurs out of vegetable, you get wilted product
What is the structure of the cell wall?
- Cellulose
o Main constituent
o Deposited in fibres
o Complex carbohydrate which is a polymer of glucose
o Source of fibre in diet (beta linkage cannot be broken by humans) - Hemicellulose
o Cellulose is imbedded in this
o Complex carbohydrate which is a polymer of xylose molecule
o Indigestible usually but breaks down in alkaline conditions (important factor for vegetables) - Pectic substances
o Complex carbohydrates which are polymers of galacturonic acid
o Also found in the spaces between cells (intercellular spaces)
o 3 groups include protopectin, pectin, and pectic acid
♣ Protopectin is cement (glue) between cells, allowing a fruit to maintain its shape
♣ Solubilization of protopectin is the breakdown during ripening or cooking of fruit (heat/acid): protopectin > pectin > pectic acid
What is the effect of cooking the cell wall?
- Heat and water together causing softening not degradation of cellulose/hemicellulose (hard/firm/crisp to not)
- Acid slows down softening process of cellulose/hemicellulose, allowing fruits/vegetables to retain firmness longer
- Alkaline conditions cause hemicellulose only to break down
o Results in mushy/slimy veggies - Acid speeds up rate of protopectin solubilization
o Fruit loses shape faster - Sugar slows down protopectin solubilization by forming hydrogen bonds with protopectin
o Fruit retains shape longer
What is the fruit structure?
- Low in fat usually, protein
- High in water content (soluble materials)
- Contains 10-15% carbs
- Have higher proportion of pectin substances
- Low in cellulose and hemicellulose (fibre) so ignore these when talking about cooking fruits
What happens when you cook fruit in a hypotonic and hypertonic solution?
Hypotonic medium – water (eg. applesauce)
- Softening: cellulose and hemicellulose absorb water
- Loss of shape (protopectin solubilizes, organic acids in fruit speed this process)
Hypertonic solution - water AND sugar (eg. apple compote)
- Softening: cellulose and hemicellulose absorb water
- Retention of shape: sugar hydrogen bonds to protopectin
to slow down solubilization
Explain the increase in translucency when you cook fruit.
- Intercellular space contains pectic substances and oxygen
- Oxygen gives raw fruits and vegetables opaque appearance
- Cooking in medium containing water means some oxygen is replaced by water (diffuses out)
- Allows light to penetrate through with less oxygen
- Refractive index is closer to value of 1 (pure water)
What is osmosis?
- Occurs in living cells (raw, uncooked, not dried foods)
- Movement of water only from a higher to lower concentration
- Across a semi-permeable cell membrane
- Results in increase or decrease in turgor pressure
- Place vegetables in cold water to firm up again
- Salts and acids increase the rate of osmosis
What is diffusion?
- Occurs in foods exposed to heat (cooked or dried foods)
- Movement of water and solutes (sugars, salts, acids, minerals, water soluble vitamins and pigments)
- Each substance moves from higher to lower concentration
- Across a totally permeable cell membrane
- Eventually reach an equilibrium state
Explain applesauce, apple compote, and dried apples using transportation of molecules.
- Applesauce
- Hypotonic solution means water moves into apples, sugar out - Apple compote
- Hypertonic solution means vice versa - Dried apples
- Dehydrated by heat, sweeter due to greater concentration
- Cooked in hypotonic solution so water moves into fruit by diffusion
- Sugar moves out
- Some protopectin solubilization (more tender), however limited because of concentration of natural sugars in dried fruit
What is oxidative enzymatic browning?
- Mitochondria contains the enzyme polyphenoloxidase
- Vacuole’s cell sap contains the substrate called polyphenolic compounds
- Oxygen comes from atmosphere, intercellular spaces, and water
- Cutting, peeling, bruising damages cell membrane and allows compounds to be released
- The substrates react with oxygen to create intermediate products which further react to product melanin
- The reaction usually takes 24 hours, however due to enzymes the reaction is catalyzed to take around 30 minutes
What are enzymes? Explain the conditions of polyphenoloxidase in particular.
- Organic catalyst which speeds up rate of chemical reaction
- Composed of protein
- Denatured by heat and acid
- Functions within narrow pH and temperature range
- The enzyme, polyphenoloxidase, is not necessary for reaction
- pH = 7, decreased activity at pH < 4, denatured at pH < 2.5 (bad flavour though)
- Optimum temperature = 15-20 C, decreased activity at < 4 C, cooking denatures this enzyme
List all the ways to prevent oxidative enzymatic browning.
- Heat (cooking or blanching): denatures polyphenoloxidase, change texture and flavour
- Acid (eg. citric acid in lemon/orange juice, wine): reduces pH below optimal level (70) for polyphenoloxidase to function (decreased activity at pH < 4.0)
- Cold – fridge (4 C) or freezer (-18 C) temperature: below optimum (15-20 C) for polyphenoloxidase to function effectively, slower rate (use additional method with this)
- Sugar syrup: coats fruit creating physical barrier excluding atmospheric oxygen, cannot exclude oxygen in intercellular spaces (therefore minimal browning occurs but not noticeably so)
- Salt (NaCl) solution: coats vegetable creating physical barrier excluding atmospheric oxygen, Chloride ion inhibits enzyme polyphenoloxidase
- Antioxidants: prevent intermediate products (colourless oxidized polyphenols) from being converted into melanins (brown pigments) as antioxidants are capable of donating electrons to intermediate products
i. Sulfhydryl compounds in pineapple
ii. Sodium metabisulfite solution
iii. Sulfur dioxide gas
iii. Ascorbic acid
What characteristics of fruit make it susceptible to oxidative enzymatic browning? What are examples of these fruits?
- Characteristics of fruit include light in colour, low in acid, raw
- Apples, bananas, peers, peaches, green grapes, eggplant, potatoes, mushrooms, celeriac
What is a fruit?
- Ripened seed of plant and surrounding tissues
- Legumes, nuts, grains are all fruits
- However, in cooking we use the common usage of the term fruit
Which fruits are good sources of which vitamins?
Yellow fruits are good sources of vitamin A
Citrus fruits are good sources of vitamin C
Most fruits are low in B vitamins and minerals/iron
What is the pH and flavour of fruits? Are there organic acids in fruits?
- Contains organic acids (citric, ascorbic, malic) which are dissolved in cell sap
- Most fruits have pH of less than 4.5
- Pleasant flavour of ripened fruit is result of ratio achieved between sugar and acid content
- Polyphenolic compounds: astringent quality (mouth puckering)
What changes occur during the ripening period?
Colour: decrease in green colour as chlorophyll pigment breaks down and development of yellow/orange and red/blue colours due to synthesis of carotenoid and anthocyanin pigments
Texture: softening of flesh due to degradation of protopectin into pectin and then pectic acid
Flavour: development of characteristic flavours involve decrease in acidity and sometimes starch and increase in sugar, volatile substances, essential oils
How do you store fruits?
- Most fruits are perishable and need refrigeration
- Even after harvesting, fruits continue to take in O2 and give off CO2
- Colder temperatures slow down this rate of metabolism and riping = lengthen storage
- Temperature, humidity, and percentage of CO2 in atmosphere is carefully monitored and controlled
o Covering fruits prevent drying out
o Soft fruits like berries keep better if spread out on flat surface
o Bananas and avocados are injured by chilling (won’t ripen properly) and should be kept in warmer temperatures until ripe (avocadoes can then go in fridge at 4 C)
How do you grade fresh fruits?
- Fresh apples and pears: Canada Extra Fancy, Canada Fancy, Canada Commercial
- Other fresh fruits: Canada No. 1, Canada Domestic
- Fresh vegetables: Canada No. 1, Canada No. 2
How do you grade processed fruits?
- Canada Fancy: clean, at proper maturity, nearly perfect, suitable when appearance, uniformity of size, and colour are important
- Canada Choice: slight variation in size and colour, suitable for general use when perfect uniformity in size and colour aren’t important
- Canada Standard: good flavour, appearance is not important, more or less ripe, less tender
Where do most calories of fruit come from?
Carbs
Why do fruit sweeten as they ripen?
Starch is converted to sugar
What contributes dietary fibre in fruit?
Complex carbs (cellulose, hemicellulose, lignon, pectin)
