Fruit, Vegetable, Dairy and Animal Product Processing

Citrus bitterness (limonin)

Limonin is a limonoid, a bitter triterpenoid derivative, and it is the main bitter compound of citrus fruit, especially grapefruit. Its precursor, limonoate A-ring lactone, is present in the intact fruit and is essentially non-bitter, so the whole fruit does not taste bitter. Once the juice is extracted, the acidic conditions of the juice promote ring closure of the lactone, which converts it to limonin. This gives grapefruit juice a delayed bitterness that becomes stronger during storage.

Do not confuse limonin with other bitter principles that appear in food and beverage questions. Quinine is a cinchona alkaloid found in tonic water, theobromine is a methylxanthine alkaloid of cocoa, and iso-alpha acids such as isohumulone come from hop resins in beer. None of these is the source of citrus juice bitterness.

• Limonoate A-ring lactone — the non-bitter precursor present in intact citrus. Example: A freshly squeezed juice may taste mild, but after acidic storage the same juice can taste bitter as limonin forms.

Milk homogenization

Homogenization is a mechanical process that reduces the size of milk fat globules so that the cream does not separate and rise to the surface. Milk is forced through a narrow homogenizing valve under high pressure, and the fat globules are disrupted mainly by intense turbulent flow and by cavitation. Cavitation is the formation and collapse of vapour bubbles caused by a rapid pressure drop across the valve. Shear and impact forces also contribute to the breakup.

Homogenization is a physical operation, not a heat treatment. Pasteurization is a separate thermal process that inactivates microorganisms, and it does not break fat globules, even though the two steps are often carried out in the same dairy line. Pressurization is also not the disruptive mechanism.

• Cavitation — vapour bubble formation and implosion caused by a sudden drop in pressure, which helps to break fat globules into smaller droplets. Example: A homogenized milk sample shows little cream separation after standing, because its fat globules are much smaller than in raw milk.

Spray drying of dairy powders

Spray drying converts a liquid feed, such as whole milk, skim milk or whey, into a powder. The liquid is atomized into a hot air chamber, where the droplets dry within seconds, and the dried particles are separated in a cyclone. The drying rate of each droplet depends on the rate of heat transfer to it and on the rate at which its water evaporates from the surface. Three factors control this rate. The first is the temperature of the incoming air, because a larger temperature difference increases the driving force for heat transfer. The second is the diameter of the droplet, because smaller droplets have more surface area per unit mass and dry faster. The third is the heat transfer coefficient of the hot air, which determines how quickly heat reaches the droplet surface.

The cyclone separator, by contrast, collects the dried powder after drying is complete, so its shape affects recovery of the powder and not the drying rate itself. Keep the drying-rate factors separate from the collection equipment when answering multiple-selection questions.

• Specific surface area — surface area per unit mass, which rises as droplet diameter falls. Example: Halving the droplet diameter doubles the surface area available per unit mass of feed, so the same droplet dries noticeably faster.

Peeling methods for vegetables

Industrial peeling removes the outer skin of vegetables with methods chosen according to the skin structure of each crop. Knife peeling uses mechanical blades to cut the skin away and is applied to cucumber in the matching exercise. Abrasion peeling passes the tubers over rough rollers that rub off the skin, and this is the method used for potato. Flame peeling exposes the surface to a brief flame that chars the skin, which is then removed by washing, and it is applied to brinjal in this matching scheme. Flash peeling uses a short pulse of steam under pressure, followed by rapid release, to loosen the skin, and it is applied to tomato.

When matching peeling methods to vegetables, remember that each method is defined by its mechanism (blade, abrasive surface, flame, or steam) and not by the vegetable alone. Abrasion is not a suitable choice for brinjal, and steam flash is not the method for brinjal.