Discuss the following in about 150 words each: 5.(d) Principles and methods of low temperature preservation of meat.

Principles of Low Temperature Preservation

The fundamental principle behind low-temperature preservation of meat lies in retarding the activities that lead to spoilage. These activities primarily include:

• Inhibition of Microbial Growth: Lowering temperatures significantly slows down the metabolic rates of spoilage-causing bacteria, yeasts, and molds. At freezing temperatures, microbial growth is effectively halted as water becomes unavailable for their biological functions.
• Reduction of Enzymatic Activity: Endogenous enzymes within the meat and microbial enzymes can cause autolysis and degradation of proteins, fats, and carbohydrates, leading to off-flavors, odors, and textural changes. Low temperatures reduce the kinetics of these enzymatic reactions.
• Slowing Chemical Reactions: Chemical processes like lipid oxidation, which causes rancidity, are also decelerated at lower temperatures, preserving the sensory attributes of the meat.

Methods of Low Temperature Preservation

The two primary methods are chilling (refrigeration) and freezing, each serving different preservation durations and purposes.

1. Chilling (Refrigeration)

Chilling involves lowering the temperature of meat to just above its freezing point, typically between 0°C and 4°C. This method is suitable for short-term preservation.

• Principle: It slows down microbial growth and enzymatic activity, thereby extending the shelf life for a few days to a couple of weeks.
• Techniques:
  • Mechanical Chilling: Involves storing meat in chill rooms where cold air is circulated to rapidly bring down the temperature. Air temperatures are typically around 0°C to 3°C, with high relative humidity (88-92%) to prevent dehydration.
  • Icing: Placing packed meat in direct contact with ice and salts for cooling.
• Impact on Quality: Chilling helps maintain the fresh appearance, color, and texture of meat. However, it does not completely stop spoilage, and psychrotrophic microorganisms (cold-tolerant bacteria) can still grow, albeit slowly.

2. Freezing

Freezing involves reducing the meat's temperature below its freezing point, usually -18°C or lower, converting water into ice. This method provides long-term preservation.

• Principle: Freezing effectively halts microbial multiplication and significantly slows down enzymatic and chemical reactions by immobilizing water as ice, making it unavailable for biochemical processes.
• Techniques:
  • Slow Freezing: Involves gradual temperature reduction over an extended period. This can lead to the formation of large ice crystals that damage cellular structures, resulting in drip loss upon thawing and poorer texture.
  • Quick Freezing (Flash Freezing): Rapidly lowering meat temperature to -30°C to -40°C. This promotes the formation of smaller, uniformly distributed ice crystals, causing less cellular damage and better preservation of texture, flavor, and overall quality. Techniques include:
    • Air-Blast Freezers: Utilize high-velocity cold air (-30°C to -45°C) sweeping over the meat.
    • Contact Plate Freezers: Meat is frozen by direct contact with cold metallic surfaces.
    • Cryogenic Freezing: Involves spraying liquid nitrogen, carbon dioxide, or freon directly onto the meat for ultra-rapid freezing, achieving very small ice crystals.
• Storage Conditions: Frozen meat requires consistent temperatures of -18°C or lower and proper packaging (e.g., vacuum packaging, freezer wrap) to prevent freezer burn and oxidation.

Comparison of Chilling and Freezing

Feature	Chilling	Freezing
Temperature Range	0°C to 4°C	-18°C or lower
Preservation Duration	Short-term (days to weeks)	Long-term (months to years)
Microbial Activity	Slowed but not stopped; psychrotrophs can grow	Effectively halted
Ice Crystal Formation	None	Present (size depends on freezing rate)
Impact on Texture	Minimal if managed well	Can be affected (especially by slow freezing)
Energy Consumption	Lower	Higher