Storage of fruit is a way of maintaining its quality after harvest for a specific duration before sale or consumption. Systemic storage is a part of a post harvest handling chain and is an important tool for successful marketing of fruits either for local or export market. Fruits for export have to remain green or their normal conditions until they reach their destination. This period can vary from a few days to a few weeks depending on the distance and mode of transport. During this period, fruits should remain normal conditions and firm in order to meet the demands of consumers.
Control atmospheric storage and modified atmospheric storage are fast becoming popular techniques for extending the storage life of fresh fruits and vegetables. This is particularly true for tropical and subtropical fruits where the benefit of refrigeration is limited because of high cost. Chilling injury is another constraint. Under controlled atmospheric (CA) conditions the level of oxygen and carbon dioxide are maintained at predetermined levels. Modified atmosphere (MA) is basically similar to CA storage except that atmospheric composition is not precisely controlled. The desired atmosphere is obtained through the combined effects of natural respiration of fruits and the sealed semi-permeable enclosure especially low-density (LDPE) polyethylene bags. Respiration is determined by the type of product, the cultivar, its maturity and the temperature. The permeability of the packaging material is dependent on the thickness and surface area of the film used. Good temperature management is therefore critical with MA packaging to ensure that respiration remains within predetermined levels.
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Low density polyethylene (150, 300 gauge)
Weigh the samples (banana/apple), wash them with water and keep for 10 minutes on a laboratory bench to drip-dry.
Enclose the samples in LDPE bags, close up the bags & place the samples at different temperatures as shown in the table below.
Keep the samples as long as you can and terminate the storage period by the first appearance of an abnormal condition of a fruits.
Carefully examine & record the surface appearance of fruits and compare with the initial observations
Determine the weight loss of samples
Determine the suitable thickness, surface area and temperature for storage of Lettuce and Banana.
Discuss the terms "green ripening" & "chilling injuries"
EXPERIMENT 2: FREEZE DRYING
A processing method that uses a combination of freezing and dehydration is called freeze drying. Foods that already have been frozen are placed in vacuum-tight enclosure and dehydrated under vacuum conditions with careful application with heat. The water molecules in frozen food are removed by direct sublimation of ice to water vapor, resulting in very little damage to the food. The rate of freeze drying is primarily depend on
The rate of water vapor transfer from the embedded ice crystals through the porous layer of dry material to the food surface
The rate of heat transfer from the food surface to the embedded ice crystals.
Other factors include,
Total surface area
Pressure within the system
Condenser temperature and capacity
Products that are very heat sensitive are ideal candidates for freeze drying.
Sublimation occurs when a molecule gains enough energy to break free from the molecules around it. Water will sublime from a solid (ice) to a gas (vapor) when the molecule has enough energy to break free but the conditions are not right for a liquid to form. There are two major factors that determine what phase (solid, liquid or gas) a substance will take: temperature and atmospheric pressure. For a substance to take any particular phase, the temperature and pressure must be within a certain range. Without these conditions, that phase of the substance can't exist. The following graph shows the necessary pressure and temperature values for different phases of water.
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Food Sample: Carrot (sliced to 1~2 mm thickness)
Weigh the samples and test for initial moisture content using the Moisture Balance. Carefully examine and record the initial organoleptic properties of the samples.
Freeze the sample at -20 °C for a week.
After one week, place the frozen sample on the rack and lower it into the specimen chamber.
Seal the lid of both chambers using only finger tightness, taking care not to tighten the screws too much. The drain valve on the side of the apparatus should also be closed. Make sure the switch on vacuum pump motor is in the ON position.
Set the temperature of the specimen chamber at -30 °C and allow the chamber to reach this temperature. This will take 30 ~ 40 minutes. Thereafter the specimen chamber refrigeration can be turned off.
At the same time the condenser chamber refrigeration should be turned on and the temperature of this chamber should go down to -50 °C. This will take about 20 minutes.
Once the condenser chamber gas reached -50 °C and the vacuum is below 0.2 millibar, the sample probes should read temperatures below -30 °C. Sublimation of moisture (in the form of ice) from the samples will begin at this point.
Specimen chamber heater should then be turned on and its temperature gradually rissen to about 50 °C. The condenser chamber refrigeration should be kept on continuously and its temperature maintained at -50 °C.
Record the following values from the start up to this point in 5 minutes intervals.
specimen chamber temperature
Condenser chamber temperature
Beyond this point, the above readings should be taken at 30 minutes intervals.
While sublimation of ice is occurring the probe temperatures will remain relatively steady (at a temperature below -30 °C).
When sublimation ice front reaches the probe, the probe temperature will start to rise. At this point the freeze-drying will be complete and the Vacuum Pump can be switched off.
Slowly release the vacuum in the system by gradually opening the drain valve. Thereafter open the lid of the specimen chamber and remove the samples. Weigh and record the final weights of each sample. Measure the final moisture content of the samples using the moisture balance.
Turn the refrigeration of the condenser chamber off and switch on the defrost to melt the ice formed during freeze-drying. Keep the drain valve open and collect the water.
Calculate the percentage moisture lost from the samples
Observe the organoleptic properties.
3. Observe whether there is any difference in microstructure of original samples with compared to the freeze dried sample.
EXPERIMENT 03: OHMIC HEATING
Ohmic heating is also termed "resistent heating or electro heating". this metod of processing is a more recent development in which an alternating electric current is passed through a food, and the electrical resistance of the food causes the power to be translated into heat.
The process can be used for sterilization of food, and especially those that contain large particles (up to 2.5 cm) that are difficult to sterilize by other means.
Electrical circuit reprecentation of Ohmic heating
In Ohmic heating heat generation depends on
Electrical resistent of product.
Geometry of the heater.
R = Total resistance (Ohms)
r = Resisitivity (Ohms m)
A = Area of electrodes (m2)
L = Distance between electrodes (m)
The electrical resistance of a food is the most important factor in determining how quickly it will heat. The resistance is converted to conductivity, using the following equation.
where s = Conductivity (Sm-1)
The resistance detrminines the current that is generated in the product.
where v = Voltage applied (Volt)
I = Current (Amps)
The rate of heating Q is given by,
where m = mass (kg)
cp = specific heat capacity (J/Kg. C0)
Dq = Temperature difference (C0)
The power by
The temperature rise in heter is ,
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where = Temperature rise
= Average product conductivity through out the temperature rise
A = Area of electrodes. (m2)
X = Distance between electrodes (m)
m = mass (kg)
=Specific heat capacity of the product. (J kg-1C0 -1)
t = Time (S)
Voltage Regulator (Rheostat)
Blended fruits (eg. Pine apple)
Connect the ammeter, voltmeter, rheostat and electrodes as shown above.
Fill the food item in between the two electrodes.
Regulate the voltage to 50 V and the current approximately above 4.5 A
Measure the temperature increase of the food item in 1 minute intervals in Ohmic heating.
Measure the temperature rise in the food by conventional heating.
Draw the thermo gram (Temp vs Time) for both ohmic heating and the conventional heating
Observe and record whether therer is any difference in the microstructure, texture and the appearance of the ohmic heated sample and conventionally heated sample with compaired to original sample.
Derive the equation and discuss the factors which determine the efficiency of the ohmic heating method.
Discuss the advantages and disadvantages of the Ohmic heating method.
Calculate the energy consumption by the food in Ohmic heating and conventional heating.
EXPERIMENT 4: BREAD MAKING
Determination of the gluten development in Dough Mixing in Bread Baking
Bread is a fermented product, its leavening action arising from the fermentation of sugar to carbon dioxide and other components affect flavor and aroma characteristics. The ingredients and formulations used in the preparation of bread are extremely important with respect to the final product.
Different flour contain different amount of protein. Wheat flours contain about 14% protein. Gluten is a kind of protein in flour. When you knead dough you help two proteins in wheat flour named gliadin and glutenin to form gluten. Gluten absorbs water and forms an extensible and elastic membrane, which enclose the gas bubbles that make bread rise. In baking, gluten is mainly used for the light texture and distinct taste gives for the bread. The amount of gluten, its water absorption capacity as well as its elasticity and extensibility define the processing properties of the dough. The quality of the gluten dictates how much gas is retained in the dough.
In bread baking many improvers are added to improve the baking properties. Sugar is added to enhance the gas fermentation. Yeast can easily ferment sugar into alcohol and carbon dioxide. It causes the rapid leavening of the dough. Also sugars give rise to the formation of a brown colour during baking. Ascorbic acid acts as an oxidizing agent and improves gluten quality and dough stability and reduces the pH of the medium so that it is favorable for the yeast activity.
1 kg wheat flour
750 g wheat flour
750 g wheat flour
250 g rice flour
250 g atta flour
17 g salt
25 g sugar
17 g salt
17 g salt
20 g dough fat
25 g sugar
25 g sugar
One ascorbic acid tablet
20 g dough fat
20 g dough fat
450 ml water
One ascorbic acid tablet
One ascorbic acid tablet
450 ml water
450 ml water
Mix wheat flour, ascorbic acid, and yeast at low speed for 3 minutes as given in recipe 1.
Dissolve salt in water.
Add water, salt dough fat and mix for about 10 minutes at medium speed.
Estimate gluten development during mixing in one minute intervals.(method is given below)
Keep the dough for 45 minutes to proof.
After completing the proving, remove the air well from the dough by kneading the dough well.
Weigh 200g of dough and place it in oiled baking tray.
Allow the dough to prove at 310C for 1.5 hours.
Record the proving time of the dough.
Prepare dough by mixing 750g wheat flour with 250g rice flour and 750g wheat flour with 250g atta flour as given in recipe 2 & 3 by repeating the above method.
After completing the dough proving, bake in an oven at 180 0C for 15 minutes.
Separation of gluten
Weigh the cheese cloths.
Wrap the sample (15g) in a small piece of cheese cloth.
Immerse the samples in water for 20 minutes, and then wash them gently by a stream of running water.
Continue working the dough further starch separation.
Dry the samples of gluten and weigh the produced gluten.
Calculation and Discussion
Calculate the gluten percentage of each of the dough samples.
Record the texture and the volume of the baked bread.
EXPERIMENT 5: FOOD HYDROCOLLOIDS
Hydrocolloids are polymers that can be dissolved or dispersed in water and that produce thickening and gelling. Most food hydrocolloids are polysaccharides, although some proteins (e.g., gelatine) also fit the definition. Hydrocolloid is the scientifically preferred term for these materials, but gum is a common synonym and mucilage is also used. Hydrocolloids are used extensively as food additives to perform a variety of functions (Table 1).
Table1. Some functional properties of Food Hydrocolloids
Function Examples of Food Application
Thickening Jams, sauces, pie fillings
Particle suspension Chocolate milk
Stabilization Salad dressings, ice cream
Emulsification Salad dressings
Clouding Fruit drinks
The basis for many of the functional properties of hydrocolloids is their remarkable capacity to increase viscosity and form gels in aqueous systems at low concentrations. Polysaccharide hydrocolloids differ in molecular weight, chain branching, charge and hydrogen bond forming groups. The effectiveness of hydrocolloids in providing functionality to foods varies with the hydrocolloid and the food. Thus food technologist must be able to select the right hydrocolloid for a specific application.
Most hydrocolloids tend to form clumps when the powdered hydrocolloid is mixed with water. Since hydrocolloids must be in solution to provide desired functions, it is imperative that mixing with water is done properly. Gradual addition of powders to water with high-shear mixing is one way to avoid clumping. Another is to mix the dry hydrocolloid with a liquid nonsolvent, such as vegetable oil, alcohol, or corn syrup before mixing with water.
Alginate, carrageenan and xanthan gum are hydrocolloids that are widely used in foods.
Magnetic stirrer and stir bars
Beakers, 250, 400, 600 and 1000ml
Graduated cylinders, 100 and 250 ml
Screw-top test tubes and caps
Hydrocolloids: Sodium - calcium alginate, sodium alginate and xanthan gum
Sodium hexametaphosphate (powder)
Fine granulated sugar (sucrose)
Adipic acid (powder)
Sodium citrate (powder)
Dicalcium phosphate, anhydrous (CaHPO4)
Effects of Concentration on Viscosity
Weigh out 12.0 g sodium-calcium alginate
Pour 800 ml distilled water into the mixing bowl for the mixer
Gently agitate the water by operating the mixer at a low speed
Add the gum gradually to the center of the bowl and mix until all of the gum is hydrated. Pour the gum solution in to a 1000 ml beaker.
Transfer 400g, 266g, and 133g to separate 600 ml beaker. Bring the total volume in each beaker to 400 ml with distilled water. What are the resulting gum concentrations?
Measure the viscosity of each solution.
Add 4g Sodium hexametaphosphate to each of the sodium-calcium alginate solutions, mix, and repeat the viscosity measurement.
Repeat steps 1 through 6 using xanthan gum
Draw plots of the viscosity (cps) versus concentration (% gum, w/v)
Food Application: Emulsion Stability
Mark three test tubes at the 5 ml level
Pour 5 ml of each of the following into separate test tubes: water, 0.5% sodium-calcium alginate, and 0.5% xanthan.
Add 5 ml vegetable oil to each test tube
Shake each test tube vigorously for 30s
Record the time required for the water and oil phases to separate.
Diffusion Setting and Internal Setting Alginate Gels
Diffusion Setting Gel
Transfer 236 ml cold water to a 400 ml beaker. Thoroughly mix 45 g fine granulated sugar and 1.7g low residual calcium sodium alginate. Gradually add the dry ingredients, stirring constantly, to the water.
When the dry ingredients are completely dissolved, add 5 drop of food coloring and 1.0 ml 2.6 M CaCl2 (0.1g Ca) to the solution. Stir for 1 min. Cover the beaker and leave it at room temperature overnight.
Internal Setting Gel
Transfer 236 ml cold water to a 400 ml beaker and add 5 drops of food coloring. Combing 45g sugar, 1.7g low residual calcium sodium alginate, 1.6g food grade adipic acid, 1.9g sodium citrate and 0.18g anhydrous CaHPO4 (0.18g CaHPO4 contains 0.1 g calcium). Mix thoroughly.
Add the dry ingredients to the water and stir briskly for 1 min. Cover the beaker and leave at room temperature overnight.
Compare the appearance, strength, and texture of the two gels.
Define the term viscosity and explain why it is an important property of foods.
Distinguish between a diffusion setting gel and an internal setting gel.