Describe the process of manufacture of 'mother starter' and 'bulk starter' cultures. What are the defects encountered in 'starter' and what measures are to be taken to avoid these defects.

Process of Manufacture of Mother Starter and Bulk Starter Cultures

The propagation of starter cultures in dairy involves a tiered system to produce sufficient quantities of active and pure microorganisms for industrial application. This typically starts from a highly concentrated master culture, progressing to a mother starter, and finally to a bulk starter.

Mother Starter Production

The mother starter is the first stage of culture propagation from the original master culture (often freeze-dried or frozen commercial cultures). It is prepared in small, controlled volumes and serves as the inoculum for the bulk starter. The goal is to revive and multiply the culture while maintaining its purity and activity.

• Preparation of Medium: A small volume of high-quality milk, typically skim milk, is used as the growth medium. It is crucial for this milk to be free from antibiotics and other inhibitory substances.
• Sterilization: The milk is subjected to heat treatment, usually autoclaving at 121°C for 15 minutes or boiling for 30 minutes, to eliminate all competing microorganisms and ensure a sterile environment. This prevents contamination from spoilage bacteria or bacteriophages.
• Cooling: The sterilized milk is then cooled to the optimal incubation temperature specific to the starter culture. For mesophilic cultures, this is typically around 20-30°C (e.g., 25°C), while for thermophilic cultures (like those used in yogurt), it is higher, typically 40-45°C (e.g., 41°C).
• Inoculation: A small, precise amount of the master culture (e.g., one packet of freeze-dried culture or 1-2% of an existing active mother culture) is aseptically introduced into the cooled, sterile milk.
• Incubation: The inoculated milk is incubated at the optimal temperature for a specific duration (typically 12-16 hours) until it thickens and reaches the desired acidity (pH). The culture multiplies exponentially during this period.
• Cooling and Storage: Once the desired acidity and consistency are achieved, the mother culture is rapidly cooled (e.g., to 4°C) to halt further acid production and metabolic activity. It is then stored under refrigeration and used within a short period (e.g., one week) to maintain its viability and activity.

Bulk Starter Production

The bulk starter is the final, large-volume culture prepared from the mother starter, intended for direct inoculation into the main dairy product batch (e.g., cheese vat or yogurt fermenter). This is a critical step, as production failures at this stage can lead to significant financial losses due to large quantities of spoiled milk.

• Preparation of Medium: Large volumes of good quality milk (skim or whole milk, often fortified with skim milk powder for increased solids) are used. Sometimes, more complex, nutritionally rich media are employed to achieve higher bacterial cell numbers and acid-producing activity.
• Heat Treatment: The milk is pasteurized, often using vat-pasteurization (e.g., 85-91°C for 30-45 minutes) or UHT-pasteurization (e.g., 99-113°C for 3-6 seconds), to destroy undesirable microorganisms and reduce bacteriophage load.
• Cooling: The heat-treated milk is cooled to the appropriate inoculation temperature for the specific starter culture. This step requires precise temperature control.
• Inoculation: The cooled milk is inoculated with a calculated amount of the active mother starter (typically 1-5% of the total volume). Aseptic techniques are paramount to prevent contamination.
• Incubation: The inoculated milk is incubated under controlled temperature and pH conditions. Modern bulk starter tanks are equipped with precise temperature controls and pH monitoring systems, sometimes with automated stirring. The incubation time varies but can range from 0.25 to 8 hours, often with pH control to maintain optimal growth conditions (e.g., pH 5.2-6.4).
• Cooling and Storage: Once the desired activity and acidity are reached, the bulk starter is rapidly cooled (e.g., to 4°C) and stored in sterile containers until used. Immediate use is preferred to ensure maximum activity, though it can be stored for a short period under refrigeration.

Defects Encountered in Starter Cultures

Defects in starter cultures can lead to poor fermentation, off-flavours, and spoilage of the final dairy product. These can be broadly categorized into several types:

• Insufficient Acid Production (Slow or Non-Acid Curd): This is one of the most common and critical defects.
  • Causes:
    • Loss of Viability/Vitality: Genetic instability of the starter strains (e.g., loss of plasmid carrying lactose fermenting genes), frequent sub-culturing without proper rejuvenation, or poor handling/storage conditions.
    • Bacteriophage Attack: Viruses specific to lactic acid bacteria can infect and lyse starter cells, leading to complete fermentation failure. This is a major cause of sudden starter slowness.
    • Inhibitors in Milk: Presence of antibiotic residues from animal treatment, disinfectant residues from cleaning agents, natural inhibitory substances like lactenins, lactoferrin, or lysozymes in milk.
    • Abnormal Milk Quality: Milk from mastitic cows, colostrum, late-lactation milk, or milk from silage-fed animals can contain inhibitory substances or have altered composition.
    • Unsuitable Media/Conditions: Use of inappropriate milk medium, wrong incubation temperature, or inadequate heat treatment of the medium.
    • Contamination: Growth of undesirable microorganisms that compete with or inhibit the starter culture.
• Insufficient Flavour Production:
  • Causes: Improper acid production, milk with low citrate content (which is a precursor for aroma compounds like diacetyl), or overgrowth of certain bacteria that reduce diacetyl.
• Flavour Defects:
  • Sharp Acid Taste: Over-ripening due to high inoculum percentage, elevated incubation temperature, or prolonged incubation time.
  • Malty Flavour: Caused by specific strains like Lactococcus lactis ssp. lactis var. maltigenes.
  • Metallic Flavour: Over-ripening, use of poorly maintained equipment with metallic contamination, or overgrowth of Leuconostoc species.
  • Flat Flavour: Insufficient development of aroma compounds, often due to low citrate in milk or under-development of aroma-producing cultures.
  • Bitter Flavour: Caused by proteolytic activity of certain starter strains or contaminating microorganisms.
• Body and Texture Defects:
  • Hard and Lumpy Curd: Often due to over-ripening.
  • Wheying Off (Syneresis): Over-ripening, low total solids in milk, or incorrect heat treatment of milk.
  • Ropiness: Contamination by undesirable bacteria like Alcaligenes viscolactis or some ropy strains of Leuconostocs or Lactococcus lactis ssp. lactis var. hollandicus.
  • Gassiness: Production of CO2 by coliforms, yeasts, or certain strains of Leuconostocs. This can cause floating curd in cottage cheese or eye formation/bulging in other cheese types.

Measures to Avoid Defects in Starter Cultures

Preventing defects requires a holistic approach covering hygiene, raw material quality, process control, and continuous monitoring.

• Strict Hygiene and Sanitation:
  • Equipment Cleaning: Thorough cleaning and disinfection of all equipment, vessels, and pipelines with appropriate sanitizers (e.g., peracetic acid, alkaline detergents) to eliminate microbial contaminants and bacteriophages.
  • Aseptic Techniques: Employing aseptic methods during inoculation and transfer of cultures to prevent contamination from the environment or personnel.
  • Air Quality Control: Maintaining positive air pressure in culture preparation areas to prevent airborne infection by yeasts, molds, and bacteriophages.
• Raw Material Quality Control:
  • Milk Quality: Use of high-quality milk free from antibiotics, disinfectants, and natural inhibitory substances. Regular testing of incoming milk for inhibitory compounds.
  • Proper Heat Treatment: Adequate heat treatment of milk medium for both mother and bulk starters to destroy spoilage organisms and reduce phage populations.
  • Optimal Medium: Using suitable media, potentially fortified, to support robust growth and activity of the starter cultures.
• Process Control and Monitoring:
  • Temperature and pH Control: Precise control of incubation temperature and pH throughout the propagation process to ensure optimal growth and metabolic activity of desired strains. Modern systems often include automated pH control to prevent over-acidification.
  • Inoculum Level: Using the correct inoculum percentage to avoid over or under-ripening.
  • Incubation Time: Adhering to optimal incubation times to achieve desired acidity and activity without causing defects.
  • Rapid Cooling: Prompt cooling of finished mother and bulk cultures to inhibit further acid production and preserve viability.
• Starter Culture Management:
  • Culture Rotation: Implementing a rotation program for different starter strains, especially in cheese production, to prevent bacteriophage build-up and attack. This involves regularly alternating between 3-5 different mixes of strains.
  • Phage-Insensitive Media: Using media that reduce the availability of calcium ions, which are essential for bacteriophage replication.
  • Use of Mixed Strains: Employing mixed or multiple-strain starter cultures can increase robustness against specific phage attacks and provide a broader range of metabolic activities for flavour development.
  • Genetic Stability: Regularly checking the genetic stability of starter strains and replacing them if vitality is spontaneously lost.
  • Proper Storage: Storing master cultures (e.g., freeze-dried or frozen) and intermediate cultures (mother and bulk starters) under appropriate conditions (e.g., low temperature, aseptic packaging) to maintain viability and prevent degradation.
• Quality Assurance and Testing:
  • Microbiological Analysis: Regular testing of mother and bulk starters for bacterial counts, purity (absence of contaminants), and activity (acidification rate).
  • Activity Testing: Performing small-scale fermentation trials to ensure the culture performs as expected in terms of flavour, aroma, and texture development.
  • Regular Replenishment: Replacing mother cultures regularly (e.g., weekly) from fresh master cultures to maintain strength and purity.