• Good quality milk is pleasantly sweet and has clean flavor with no distinct aftertaste

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• But, due to the perishability of milk and nature of production and handling procedures, development of off-flavors is not uncommon

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• To prevent these defects in milk, proper milk handling procedures from the farm to the consumers’ glass are essential

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• These defects may be classified according to the ABC's of off-flavors:

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• Absorbed/Transmitted
• Bacterial/Microbial
• Chemical/Enzymatic/Processing

Flavor defects in milk and its prevention

Absorbed off- flavor

(feedy, barny, cowy, weedy, unclean, lacks freshness, stale, refrigerator)

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• Milk absorbs off- flavors during production, storage and distribution

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• On the farm, off-flavors can be developed through the body system of the cow (from the rumen) into the milk (e.g., onion/garlic, feedy, barny, cowy)

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• Similar off-flavors may be absorbed into the milk during farm storage if ventilation is poor and the milk is not protected

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• Pasteurized milk too can absorb flavors during refrigeration storage; volatile compounds of fruits or vegetables or unclean odors associated with poorly cleaned milk coolers

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• Absorption of flavors by packaged milk can occur at the plant, in the supermarket or in the consumers’ home refrigerators

Bacterial off- flavor

(acid, bitter, malty, lacks freshness, unclean, fruity/fermented, putrid and rancid)

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• Bacterial and other microbial (yeast or molds) off-flavors result from the growth of microorganisms that are present in milk due to poor sanitation and milk handling practices

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• Bacteria that are able to grow at refrigeration temperatures (≤45°F/7.2°C) or psychrotrophs, are most often responsible for spoiling refrigerated milks

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• The spoilage (fruity, rancid, acid) depends on the type(s) of bacteria present and generally evident when bacterial numbers (Standard Plate Count) exceed 1 to 10 million/ml

The time it takes for bacterial counts to reach spoilage levels depends on:

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• Initial numbers of bacteria and
• Temperature of storage

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Warmer the storage temp., the quicker bacteria grow and produce off-flavors and the shorter the shelf-life is (of milk)

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If the raw milk quality is good and post-pasteurization contamination is prevented during processing, no. of microorganisms do not reach spoilage levels before 14-21 days when milk is at proper refrigeration

Table. Legal microbial standard of processed and packaged milk of Nepal

Chemical off- flavor

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(cowy/ketosis, salty, rancid, bitter, oxidized, sunlight, foreign, astringent, medicinal, flat, cooked)

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• Chemical, enzymatic and alien agent defects can occur in both raw and pasteurized milk

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• The cows may be suffering from ketosis or mastitis, which can affect milk flavor

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• Abusive handling of raw milk may result in a rancid flavor from the action of the naturally occurring lipase, which breaks down milk fat to free fatty acids (butyric acid is perceived as “bitter and rancid”)

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• Oxidized flavors can be induced by heavy metals, particularly copper, or by exposure to sunlight and fluorescent lights

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• Chemical or foreign off-flavors can also occur due to contamination with cleaning chemicals, sanitizers, medicines, or other substances during production or processing

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• Processing parameters, if not managed properly, can result in off-flavors including cooked (from high heat) or flat (from added water)

Normal Milk No criticism. Very little odor, pleasantly sweet and clean with no aftertaste

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Acid Acidic taste sensation. Sour, tart, may cause tingling sensation on tongue. “Cultured milk” or “sour” odor may be present

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Cause - Growth of lactic acid producing organisms such as Lactococcus lactis, due to poor refrigeration, especially when temperatures exceed 70°F (21°C). “Malty” milks may be acidic too

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Astringent Peculiar mouth-feel (e.g., cranberry juice)

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Cause - Associated with denatured proteins due to high heat treatments or with staleness (e.g., milk powder). May be more pronounced in skim milks and in Ultra High Temperature (UHT) or Ultra-Pasteurized (UP) products

CHARACTERISTICS OF MILK OFF-FLAVORS

Barny Unpleasant odor and taste of a poorly maintained barn or unpleasant feed. May be perceived as “unclean.” “Cowy” or “cow's-breath” may present a similar defect but generally with an unpleasant medicinal or chemical (i.e., acetone) aftertaste

Cause - absorbed, transmitted odor due to cow inhaling barn odors associated with poor ventilation and unclean barn conditions. Similar defect may be due to ketosis in cows, but with more of a medicinal or chemical aftertaste

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Bitter Basic taste sensation. Delayed taste perception and tends to persist

Cause - enzymatic breakdown (microbial or milk enzymes) of milk proteins to short bitter peptides. Certain weeds ingested by cows may also cause bitterness although this is rare

Cooked Slight sulfurous to caramelized or cabbage-like. Flavor usually becomes less intense over time

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Cause - Higher pasteurization temperatures and/or longer holding times. Intensity depends on the severity of heat treatment. Cooked flavors tend to be more pronounced in batch-pasteurized than HTST milk; most pronounced in Ultra High Temperature (UHT) or Ultra Pasteurized (UP) products

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Cowy Unpleasant odor and flavor; acetone or cow's-breath; unpleasant medicinal or chemical aftertaste

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Cause – metabolic disorder in cows such as ketosis. Rare in commingled bulk supplies. Similar defect may be transmitted/absorbed odors of poor barn conditions (i.e., barny)

Feed Odor and flavor is characteristic of associated feed; silage, hay, grass, etc. Can be slightly sweet, generally not unpleasant, although could be unclean when strong or feed quality is poor. Most feed flavors clear up readily after milk is discharged from mouth. Cows impart an odor and taste within 30 minutes of eating silage. It is strongest after about 1 hour

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Cause – Consumption of particular feed (grass or corn silage, legume hay, and brewer’s grains) or inhale feedy odors prior to milking; transmitted to the milk. Feeding should be done after milking when practical, barns should be well ventilated

Flat No odor. Lacks mouth-feel and sweetness of fresh milk. Watery characteristic

Cause - adulteration with water or low milk solids content. Older milk may be “flat”

Foreign May have odor that is not commonly associated with milk, often chemical in nature. Depends on causative agent; sanitizers, detergents, exhaust fumes, medications, etc. Chloro-phenol compounds may give medicinal or bandage-like odour

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Cause - Contamination of milk with foreign substance. May be direct contamination of milk (e.g., udder ointment/sanitizers, phenols/chlorine); may be transmitted through the cow or absorbed during raw storage or through retail packages in plant, store or home refrigerators

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Fruity/Fermented Odor and flavor is usually pronounced, similar to pineapple, apple, strawberry, etc.

Cause - growth of psychrotrophic spoilage bacteria, especially psychrotrophic Pseudomonas species or some of the spore-forming organisms (e.g., Bacillus, paenibacillus)

Garlic/Onion Characteristic pungent odor and flavor; highly objectionable

Cause – Ingestion of wild onion or garlic weed; may also be absorbed through packaging during refrigeration storage with onion or garlic containing foods

Lacks- Freshness

Lacks fine, pleasing flavor. Mild off-flavor that lacks specific characteristic to make identification easy. May be “stale” or less sweet (e.g., “flat”). Generally not intense enough to fail product

Cause - Usually due to age, staleness, residual milk enzymes or initial stage of microbial spoilage (e.g., psychrotrophic bacterial off-flavors such as unclean, bitter and rancid)

Malty Malt-like aroma or taste. Microbial spoilage, milk is often acidic

Cause - Growth of Lactococcus lactis var. maltigenes (or possibly other organisms) due to poor refrigeration

Oxidized/ Light-Induced

Odor and taste of burnt-protein, burnt-feathers. May progress to metallic or lipid oxidized type flavor due to fat oxidation

Cause - exposure of milk to sunlight or fluorescent lights resulting in protein degradation and lipid oxidation. Milk in unprotected or transparent milk bottles is more susceptible although this defect may occur in paper packaging if the light is intense and exposure time is sufficient

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Metallic- Oxidized

Wet cardboard, oily, tallowy, chalky, or fishy flavor. Have a greasy mouth-feel. Sensation comes quickly

Cause - Fat oxidation catalyzed by Cu or other metals contacting milk. May be associated with raw milk of cows fed high fat feeds (e.g., soybeans) and lack of antioxidants (e.g., vitamin E)

Carton/Paperboard Wet paper flavor

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Cause – associated with paper-board packaging with heat used to seal HDPE polymer coating

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Rancid Pungent odor when extreme. Taste soapy, unclean, bitter. Pronounced lingering aftertaste

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Cause - free fatty acids (e.g., butyric acid) released from milk fat by natural or microbial enzymes (lipase). In raw milk, it’s associated with excessive agitation or abuse. Pasteurization destroys natural enzyme (lipase), but spoilage microorganism may have similar enzymes that cause rancidity

Salty Basic taste sensation. No odor. Generally easily detected. Clean mouth-feel

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Cause - associated with late lactation or mastitic cows

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Unclean Unpleasant odor and taste. Suggestive of mustiness, putrid or other “unclean” flavors

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Cause - Growth of spoilage microorganisms in milk or on excessively dirty equipment. Can occur due to milk absorbing odors from dirty environment

Prevention of flavor problems in milk:

• Keep floors, walls and ceilings of milking and housing areas reasonably clean
• Provide adequate ventilation to eliminate stale odors
• Clip hair from the udder, teats and flanks of milking cows
• Provide adequately bedded stalls so that cows may lie down
• Wash udder and teats with a sanitizer solution and dry before attaching milking units
• Clean and sanitize all milk handling equipment between each uses
• Withhold milk from ketotic and mastitic cows
• Chill (<4°C) the milk as early as possible, preferably within half an hour after milking