Sample Preparation for Training

For accurate judging of the sensory quality of dairy products, it is necessary that the evaluator has the awareness of desirable attributes and likely defects in them.The sensory panelists are provided a control sample along with other samples having all possible defects. Under real situation it may not be possible to procure all such defective samples from either a processing plant or from the market.Various defects, particularly the flavour defects in dairy products have, therefore,to be simulated in the laboratory to impart meaningful training to the panel members. The most common flavour defects of dairy products and causes for the same are given below:

i) Simulation of Different Flavour Defects in Milk and Milk Products:

The flavour defects and their preparation, which are of general nature and found in most of dairy products, are discussed here.

ii) Cooked Flavour: When milk and milk products are heated beyond minimum conditions of heating (>76.7oC) the flavour properties are generally altered.This flavour alteration is called cooked. In fact cooked is a generic name that is applied to heat induced flavour defect described as heated, nutty, burnt, scorched and caramel. The type of these developed flavours depends on the intensity of heat treatment, the time of exposure, composition of dairy product and extent of burn-on the heating surfaces. The cooked flavour is desirable to some extent in milk and many dairy products, but its higher intensity is undesirable.Cooked and related off flavours in milk and milk products can be simulated simply by heating them at a temperature higher than that recommended for their processing.

iii) Light Induced and Sunlight Off-Flavour: This defect is more common in milk and develops due to exposure of milk to sunlight or extended exposure to fluorescent light. The related terms of light activated off flavour are cabbagelike, mushroom-like, burnt protein, chemical like or burnt feather. Methionine amino acid is degraded by the light in the presence of riboflavin, into methional, which imparts the light induced off-flavour.For simulation of sun light off-flavour, expose capped milk sample bottles to direct sunlight ranging from 30 min to 3 hours, depending on the intensity of off-flavour needed.

iv) Oxidized Off Flavour: An oxidized off flavour results from the action of oxygen on certain compounds of milk fat (particularly unsaturated fatty acids) and reaction is catalyzed by divalent cations (copper and iron). The end products are short chain volatile aldehyde/ ketones. The flavour sensation produced are: cardboardy, cappy, tallowy, fishy and oily. Samples with this defect can be prepared by bubbling oxygen gas from a cylinder for 10 to 20 minutes or by excessive agitation and storage at 37oC.

v) Metallic: This off flavour is due to prolonged exposure of milk and dairy products to metal surfaces, like iron, copper, etc. Initially the product gives perception of a rusted iron nail, but on longer storage this results into oxidized off-flavour.Prepare a 0.3% solution of ferrous sulphate in distilled water. Add 0.5 to 2 ml of this solution to 500 ml milk or equivalent amount of product to simulate varying degree of metallic flavour.

vi) Rancid Off-Flavour: Also called as hydrolytic rancidity, this off flavour results due to hydrolysis of triglycerides and diglycerides of milk fact by action of lipase enzyme. The end products of the reaction are short chain fatty acids (butyric, caproic and caprylic) and their salts. The sensory perceptions of the flavor defects are: soapy, bitter, goaty, butyric.For simulation of rancid off-flavour add lipase solution (50 mg dissolved in 10 ml distilled water) to milk or other products @ 0.5 to 2 ml/100 gm and incubate for 1 to 2 hr. In case of milk incubate for about 24hours at 10oC. Alternatively homogenize raw milk or cream and leave for some time to develop rancid defect.

vii) Acidic/Sour: High acid is probably the most commonly occurring flavour defect in milk and dairy products under Indian conditions. This is usually due to uncontrolled growth of lactic acid fermenting bacteria at room temperature.Undesirable acidic/sour defect can be developed by storing the samples at room temperature (preferably 37oC) for different periods.

viii) Flat or Lack of Flavour: Flat suggests different meaning to different products. For milk, flat denotes lack of richness or mouth feel and simply prepared  by adding water @ 20-25% to milk or removing cream from milk. In case of other products flatness means lacking in typical pleasant flavour. Accordingly flat samples may be prepared by reducing the fat content (khoa and paneer),insufficient heating of raw material (khoa and ghee), excessive washing of butter grains (table butter) and improper ripening of curd (cheddar cheese).

ix) Bitter Off Flavour: Bitterness in dairy products is produced due to excessive rancidity or by certain weeds or by some psychrotrophic bacteria. Proteolysis results into low molecular weight protein fragments (peptides and some amino acids), which are responsible for bitter flavour in cheeses.Bitter samples of milk and dairy products may be prepared by adding 1-2 ml of 1% quinine sulphate solution to about 1 kg of milk/product. The quantity can be changed depending on the intensity of bitterness required.

x) Astringency Defect: This is a textual sensation (perceived by the sense of touch) but is detected only when the sample is put into mouth. Other descriptions of astringency defect include: mouth coating, dry, puckery, chalky.Alum exhibits extreme astringency defect. So for preparing samples of astringency defect, aqueous solution of alum (aluminum sulfate 0.5% or more may be added depending on the intensity required) should be added.

xi) Other Off-Flavour: Some other flavours defects typical to a dairy products will be discussed separately in Unit 3.