3: Proximate analysis i (moisture, ash, fat)

methods for quantifying moisture contents of foods, determination of ash contents in foods, lipid extraction & quantitation

why is measuring moisture impt (3)

1. align how much water must / need to be present inside the food

2. to ensure food safety bc water activity can affect mb growth, chem & enzymatic reactions, and physical properties

3. to maintain food quality bc diff in water activity can influences moisture migration

   • can be btwn food components or btwn food & env

moisture content consists of __ & __

water activity + water content

overview of methods for moisture

DIRECT & INDIRECT methods

• direct → drying (oven, microwave, infrared), distillation, karl-fisher titration

• indirect → refractive index, density

general pros and cons about direct n indirect methods

direct → high accuracy, reproducible ,, time-consuming

indirect → lower accuracy ,, fast

(how accurate + speed)

direct is usually the __ method while indirect is __ method

direct → official

indirect → secondary

steps in drying methods for liquid and solid foods

liquid

• evaporate

• drying

solid

• air dry

• ground

• drying

prob of drying methods

decomposition of other food components

• includes carbs, sucrose, volatile constituents

how are these food components lost during drying

• carbs → dehydration ,, where carbs decompose & release water

• sucrose → hydrolysis ,, bc it’s a chem R. that uses moisture

• volatile constituents → vapourise

what r the types of oven used

convection, forced draft, vacuum

usage of aluminum pan for sample drying (how it works + functions)

sand disperses sample into smaller aprticles → increase SA

• moisture can escape more easily + prevent crusting at the surface

this is a sand pan technique that controls surface crush formation

what is the sand n sample ratio

10:1 (sand > sample)

pressure & temp requirements for vacuum oven

pressure: 25-100 mmHg

temp: 95-102 degrees (non heat sensitive) ,, 60-70 degrees(heat sensitive)

what is ASH in food

total amt of minerals + inorganic residue remaining aft combustion / complete acid-facilitated [O] of organic matter in food

WHY measure ash in food

1. nutritional evaluation

   • nutri values of minerals

2. quantification of toxic metals

   • [arsenic contents in rice]

general methods of ashing

dry, wet, microwave, others

describe dry ashing conditions (temp n heating)

temp: v high ,, 500-600 degrees

conventional / microwave heating

DESCRIBE the general procedure of dry ashing

1. weigh 5-10g of sample into a tared crucible

   • pre-dry sample if v moist

2. place crucible in a cool muffle furnace

   • use tongs, gloves, protective eyewear if furnace is warm

3. heat samples for 12-18h / overnight at ~ 550 degrees

4. turn off furnace, wait until temp drop to 200 degrees then open

   (open door slowly → X lose powdery ash that may be disturbed by air movement)

5. using safety tongs, quickly transfer crucibles to a desiccator w a porcelain plate & desiccant

6. cover crucibles, close desiccator, allow crucibles to cool prior to weighing

7. calculate % ash (dry basis)

   • (WAA - TWOC) / (OSW x DMC) x 100

   • top gives u weight of ash remaining ,, bottom gives u dry weight of original sample

     • therefore gives u % of ash in dry basis ..?

what to do if incomplete ashing

either: add some h2o / nitirc acid → re-ash

or: suspend ash in h2o → filter (ashless filter paper) → dry the filtrate → place paper + dried filtrate in furnace → re-ash

what would require special considerations when ashing

high fat samples

glycerin, alcohol. h2o2

jellies

salt rich foods

cereals

ashing of high fat samples

use either

1. crude fat determination procedure

2. burning off fats prior to closing the furnace

what will accelerate ashing

glycerin, alcohol. h2o2

jellies

will spatter

• mix w cotton wool to avoid this

salt-rich food

ashing of water-insoluble components & salt-rich water extract done separately

to prevent spattering, use __

crucible cover

to __ the ashing of cereals, __ can be added + what needs to be done

to accelerate the ashing of cereals, an alcoholic solution of magnesium acetate can be added

• an appropriate blank determination will be necessary

wet ashing is a __

acid-facilitated oxidation

wet ashing is used when __

minerals may volatilise & be lost during dry ashing

wet ashing is done by __

using acids

• hcl , h2so4 , hno3

• hyperchloric acid hclo4

• mixture of diff acids ; often used → complete & rapid [O] of organic material

procedure of wet ashing

1. accurately WEIGH a dried, ground 1g sample in a 125 mL Erlenmeyer flask

   - before: acid wash & dry flask

2. prepare BLANK

   • 3 mL h2so4 + 5 mL hno3

   • treated like samples

   • perform in FUME HOOD (cus strong acid)

3. ADD 3 mL h2so4 , then 5 mL hno3 to sample in the flask

4. HEAT sample on a hot plate at ~200 degrees , it shld be boiling

   • brown-yellow fumes will be observed

5. brown-yellow fumes cease + white fumes from h2so4 decomposing → sample is darker

6. REMOVE flask from hot plate (X cool to room temp) & SLOWLY add 3-5 mL of hno3

7. put flask back on hot plate → hno3 boil off

   • when all hno3 boil off → colour; clear to straw yellow

   • if not all boiled off (still dark in colour) → add another 3-5 mL hno3 & boil,, repeat until clear to straw yellow

8. while on the hot plate, REDUCE VOLUME appropriately → ease of final transfer

   sample COOL to room temp, then quantitatively transfer sample to an appropriately sized volumetric flask

microwave ashing

FAST (minutes rather than hrs)

for dry n wet ashing

other ash measurements

soluble n insoluble ash

ash insoluble in acid

alkalinity of ash