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Y2S1_FST2110 - Microbiology and fermentation > Lect 3 : Food spoilage Pt 1 (ways of preserving/processing food) > Flashcards

Lect 3 : Food spoilage Pt 1 (ways of preserving/processing food) Flashcards

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1
Q

How is food quality and food safety linked in the preservation of foods?

A

food quality can affect food safety
- Food quality, such as fresh meats, or lower extent of processing etc can cause higher rates of growth of pathogens, which in turn affects food safety

In ensuring food safety by processing / preserving foods, food quality may increase/ decrease (e.g. sensory properties such as colour, texture, taste, aroma)

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2
Q

Acids are usually added to foods to reduce pH.

What pH are
1. non-spore forming bacteria (gram +ve/-ve)
2. Spore-forming bacteria
3. Yeasts and molds

inhibited at?

A

NON SPORE FORMING BACTERIA
- gram -ve bacteria : inhibited at pH < 5.3
- Gram +ve (more resistant) : inhibited at pH < 4.0 (LAB : 3.5)

SPORE FORMING BACTERIA
- most are sensitive and spores cant germinate at low pH

YEASTS AND MOLDS
- Acid tolerant, grow at pH < 4.0 and are resistant to addition of acids

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3
Q

Acids are added into foods either as acidulants or preservatives, with or without other chemical preservatives.

  1. What is the difference between food acidulants and preservatives?
  2. Which 3 acids are commonly added as both acidulant and preservatives?
A
  1. Food acidulants are added at relatively high concenrations, > 0.5% (food meant to be acidic) while preservatives are added in lower concentrations, around ±0.1%
  2. [CAL] Citric acid, Acetic acid, Lactic acid
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4
Q

What are the 4 factors affecting the inhibitory effect of acids which are added as presrvatives / acidulants to food? Explain.

A

1. Type of acid and conc
- Higher conc = larger inhibitory effect
- inorganic (strong) acid = lower inhibitory effect compared to organic acid (fully dissociates and thus cannot enter bacterial cell to inhibit it)

2. pH of food
- when pH of food > pKa of acid, inhibitory effect decreases ; when pH of food < pKa of acid, inhibbitory effect increases. (see equation)

3. Distribution coefficient
- Distribution coefficient = Sₗᵢₚᵢ𝒹 / S𝓌ₐₜₑᵣ
- lower D.C. = inhibitory effect increase (see notes)

4. Water activity
- foods with higher water activity = increased effect of preservatives
- bc for foods with lower water activity, the amount of free water is alr v low, won’t see too much of an effect when adding preservatives (microbes may have certain adaptations to stabilise themselves in low Aw conditions, may be more resistant to preservatives)

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5
Q

What is the non direct way of altering pH of foods to inhibit microbial growth?

A

Fermentation with lactic acid bacteria
- since most microbes are inhibited at lower pH

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6
Q

What is the oldest method of reducing water activity in food?

A

Drying, specifically solar drying

Other kinds of drying
- Tunnel and belt drying
- Fluidised bed drying
- Pan drying
- Foam mat drying
- Drum/roller drying
- Spray drying
- Freeze drying

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7
Q

What is the other way of reducing water activity in food? (apart from drying)

A

Addition of solutes, such as sugar or salt (brining / salting)

  • brining : adding high conc salt solution, so its like adding wet solution into food
  • salting : just adding salt to surface of food (dry)
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8
Q

What are the 2 ways in which the atmosphere can be controlled to inhibit microbial growth?

A

1. Controlled-atmosphere storage (external envt where food is stored)
- Widely used for fruits and vegetables
- Surrounding envt has 2-5% oxygen and 8-10% CO2, to slow respiration

  1. Modified-Atmosphere Packaging (MAP)
    - Optimal blend of pure oxygen, caarbon dioxide and nitrogen within a high barrier/ permeable package
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9
Q

in modified-atmosphere packagin (MAP), why is the level of oxygen not reduced to 0?

A

Anaerobic bacteria can grow without O2, and (fish) products pose a risk for Clostridium Botulinium growth in MAP product

  • note : though anaerobic bacteria grow slower at lower temp, it CAN STILL GROW so MAP for chilled products should also include a little bit of oxygen to inhibit them
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10
Q

How does chilling cause inhibition of MOs?

A

Rapid cooling injures cell membrane of MOs –> causes selective permeability of membranes to decrease, thus rate of uptake of nutrients and release of metaboites decrease.

  • Gram negative bacteria more sensitive
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11
Q

Is chilling food products useless, considering that psychrophiles and psychotrophs are less sensitive to cold shock and can still grow & cause spoilage?

A

No, chilling may still be useful

  • Lower temperatures still slows the growth of these microbes –> meaning that there will be longer generation time (time taken for bacteria population to double), leading to a longer shelf life
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12
Q

The Enterobacteriaceae and coliforms are able to grow at low temperatures, true or false?

A

True

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13
Q

Escherichia coli have minimum growth temperatures of __ to __℃
Klebsiella, Enterobacter and Hafnia (coliforms) can still grow at __℃

A

8,10
0

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14
Q

Most pathogens are ____

A. Psychrophiles
B. Mesophiles
C. Thermophiles

A

B. mesophiles

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15
Q

One of the most sensitive parts of the cold chain is _____ _____ (2 words)

A

domestic storage

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16
Q

Why is domestic storage in particular an area of concern in the cold chain?

A

It is proven that temperature abuse occurs frequently in domestic refrigerators
- mean temperatures of refrigerators can be in the range of 8-10℃

Howver, it is OPTIMAL TEMP for growth of psychrophiles !! Refrigerator temp must be kept <= 4℃ !!!

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17
Q

Freezing involves decreasing the temperature to below the _____ _____ of the food product

A

freezing point

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18
Q

What are the 3 lethal (death-causing) effects of freezing on bacteria?

A

Freezing causes bacteria to die by :

1.** Free water in microbial cell is converted into ice crystals**, and ice crystals puncture / pierce the cytoplasm

  1. Loss of gases in cytoplasm of microbes (cytoplasmic gases such as O2 and CO2) –> unable to respire and survive + pH altered + concentration of cellular electrolytes increasing to toxic levels

3.** cold shock** causing injury of cell membrane and altering selective permeability (ability of the cell membrane to control the flow of substances in and out of the cell –> affect rate of uptake of nutrients and metaboic activity

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19
Q

If the effects of freezing food is sub lethal on bacteria cells, can the bacteria be viable ( ability of a cell to grow and reproduce itself under a set of defined environmental conditions)?

A

Yes, bacteria can be viable BUT ONLY UNDER CERTAIN CONDITIONS
- if effect is sub lethal, it means that bacterial cells were not killed
- instead they are latently present (dormant, inactive) while being injured, and can be viable in ideal circumstances AFTER resuscitation

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20
Q

What are the 5 factors affecting antimicrobial effect of freezing?

A
  1. Type of MO and physiological situation (sensitive vs resistance to cold temp)
  2. Type of food
    - Sugar, salt, proteins, fats, colloids
    - higher Aw and low pH stimulate death (higheer Aw –> more water –> more ice crystals formed topuncture bact)
  3. Rate and temperature of freezing
    - rapid freezing gives better results
    - because faster rate of freezing means that there is LESS time given to bacteria to ADAPT to the cold environment
  4. Effect of thawing
    - thawing should happen slowly
  5. Effect of alternate thawing and freezing (freeze-thaw cycle, e.g. u freeze –> thaw –> freeze –> thaw = 2 freeze-thaw cycles)
    - death increases, antimicrobial effect increases (but note : unfavourable for quality of product)
21
Q

Give 3 reasons why force thawing, such as defrosting a piece of frozen meat in hot water / room temp is harmful

A
  1. It causes spoilage and possible growth of pathogenic bacteria, which may also produce toxins, reducing food safety (because defrosting usually takes v long, and you should not be defrosting food for >4h at danger zone of 4-60℃ !!!)
  2. Protective layers of food may be damaged, making food more susceptible to microbial attack (e.g. cell membranes of food may be damaged, releasing nutrients which bacteria love, thus increasing susceptibility og microbial attack0
  3. force thawing of PACKAGED FOOD causes condensation on surface of food product –> water avail for microbes <33 –> accelerated microbial growth
22
Q

Which 4 components of microbial cells are inactivated, reuslting in lethal damage / non-lethal injury when heating food at high temperatures?

A
  1. DNA
  2. RNA and ribosomes
  3. Cytoplamic membranes
  4. Specific enzymes
  • these components are key for reproduction, energy production, synthesis of proteins and other essential compounds
23
Q

Rate of destruction of microbes at high temperatures is _____

A

exponential (logarithmic)

24
Q

In the bacterial survivor curve (no of living bacteria vs time), what does the D-value refer to?

A

It refers to the decimal reduction time, the time taken in minutes to destroy 90% of the cells

25
Q

What are the 5 factors affecting thermal resistance of microbes?

A
  1. Type of MO
  • spores more resistant than vegtative bacteria, yeast and mold
  • spores of thermophilic bact > mesophilic > psychrophilc
  • Particular structure and composition of spores also provide additional resistance
  1. Number of cells
    - more cells, more resistant –> need longer time of heating to kill
  2. Stage of growth
    - Most heat resistance in early lag phase, late exponential phas and early stationary phase
  3. Growth temperature
    - The lower the temperature required for growth, the less the heat resistance
  4. Composition of food products (intrinsic factors)
  • low Aw : increased heat resistance –> Since less water, more heat is directed to bacteria. This means bacteria is more likely to develop traits to withstand heat in low Aw foods / lower Aw, higher conc of solutes to offer protection (see next point below)
  • Nutrients : Fat, proteins, colloidal strucutres offer protection and increase resistance
  • Low pH decreases heat resistance
  • Micorbial growth INHIBITORS decrease heat resistance
26
Q

What does a larger D-value tell us about heat resistance of MOs?

A

If D-value is larger, it means more time taken to kill 90% of bacteria cells, showing that MO is more heat resistance

27
Q

Why are MO more heat resistant in the early lag phase and late exponential phase as opposed to the late lag / early exponential phase?

Note : late lag phase = early exponential phase

A

it is mainly due to levels of METABOLIC ACTIVITY

  • In the late lag/ early exponential phase, cells are most metabolically active. Respiration and growth of cells require specific optimal temperatures. Thus, a sudden rise in temperatures can denature proteins (enzymes) / affect cellular structure and causing cells to die. Thus they are the least heat resistant in this phase.
  • In the early lag phase, MO are dormant and metabolically inactive. - In late exponential phase, the metabolic activity of MO is also lower compared to late lag phase, due to depletion of nutrients
  • Thus, they are more heat resistant since they are not as metabolically active
28
Q

Why are MOs more heat resistant in the early stationary phase compared to the late stationary phase?

A

Microorganisms in the early stationary phase may have had better access to nutrients, allowing them to maintain better cellular integrity. (can synthesise lipids to maintain cell structure)

  • Thus with better conditions in early stationary phase, more heat resistant
  • In contrast, in the late stationary phase, MO have less access to nutrients and start to enter early death phase –> also show less heat resistance
29
Q

Other methods in controlling microbes : UV radiation

What is the idea behind UV radiation in killling microbes? What are the wavelengths of light involved?

A

UV radiation

  • Involves wavelengths of 240-280 nm
  • it involves damaging nucleic acids of MOs by cross-linking thymine dimers in DNA, which prevents repair and reproduction in MO cells
30
Q

Other methods in controlling microbes : UV radiation

What is the limitation of using UV radiation in killing MOs?

A
  1. Low penetration
  2. Difficulty in maintaining an exposure level over the food surface (not sure what this means but from online, you need to make sure that the object is directly blocking the UV light. “UV only works in its light path and can be blocked by objects. Make sure what you want sterilized is directly in line with the UV light.”)
31
Q

Other methods in controlling microbes : UV radiation

When is UV radiation typically used?

A
  • Disinfection of air and water during aseptic filling of liquid food
  • ripening rooms of cheese and dry sausages
32
Q

Other methods in controlling microbes : Pulsed light

What is the idea behind using pulsed light to kill microbes?

A
  • Using a xenon lamp light source to create several flashes of light per second
  • Each flash generates high heat in a mixture of ozone (shorter UV) and UV light
33
Q

Other methods in controlling microbes : Pulsed light

How high is the temperature of each flash of light ?

A

Up to approximately 160 degrees celcius

34
Q

Other methods in controlling microbes : Pulsed light

Although pulsed light generates heat, why is it considered a non-thermal process?

A

Each flash lasts for a very short duration (a few seconds) only.

35
Q

Other methods in controlling microbes : Pulsed light

What is the limitation of pulsed light?
Thus, what are the applications of using pulsed light? (When is it used)

A
  • Limitation : low penetration
  • Suitable to treat surface and packaging materials only
36
Q

Other methods in controlling microbes : Ionising radiation (irradiation)

What is the idea behind using ionising radiation to kill microbes?

A
  • using gamma-ray and X-ray electron beam radiations to break chemical bonds of substances in foods –> producing free radicals –> undergo radiolysis (changes) –> destroying microorganisms, insects and parasites
37
Q

Other methods in controlling microbes : Ionising radiation (irradiation)

What 2 factors affect the penetration of radiation into the food?

A
  1. Energy of rays
  2. Density of food
38
Q

Other methods in controlling microbes : Ionising radiation (irradiation)

What is the limitation of irradiation? What is done to overcome the limitations?

A
  • Limitation : poses safety concerns over products after radiation –> radioactive food increass risk of (thyroid cancer)
  • To overcome this, there are labels and regulations of how much dosage of radiation can be used for specific types of foods
39
Q

Other methods in controlling microbes : Ionising radiation (irradiation)

irradiation is used to decontaminate, eliminate insects and MOs in spices. Suggest why only irradiation, and not heat treatment , can get rid of MO in spices

A
  • Spices have low Aw. This means during heating, there is less water to conduct heat to bacteria, so they wont receive so much of heat and are more heat resistant.

MO in foods with low water activity also evolve to produce protective compounds to maintain celllular activity in low Aw envrionments. These compounds help stabilize the cells and counteract the effects of heat stress

40
Q

Other methods in controlling microbes : Higher Pressure Processing (HPP)

What is the idea behind HPP in killing microbes? How high is the pressure applied?

A

Food product is compressed by UNIFORM pressure from every direction

  • High pressure of around 400-600 MPa is applied AT CHILLED OR MILK PPROCESS TEMPERATURES (<45℃)
  • HPP : cold pasteurisation
  • this damages cellular membranes which causes leakage of cellular content and loss in cell structure ;; denatures proteins needed for growth and metabolism ;; destroys DNA and stops growth (mutation ,preventing replication)
41
Q

Other methods in controlling microbes : Higher Pressure Processing (HPP)

What is the advantage of HPP on foods?

A

HPP has MINIMAL effects on sensory properties (taste, texture, appearance) and nutritional value

42
Q

Other methods in controlling microbes : Higher Pressure Processing (HPP)

HPP is used in the production of what kinds of foods products ? [4]

A
  1. Jam
  2. Oyster
  3. Ham
  4. Sauce processing
43
Q

Other methods in controlling microbes : Pulsed Electric Field

What is the idea behind Pulsed Electric Field (PEF) in killing microbes? [include how high of a voltage is used]

A
  • Apply high voltage (20-80kV/cm) to induce rupture of cell membranes due to a certain electrical potential (1V)
  • affecting ALL processes related to permeabilisation of cell, leading to cell leakage and death
44
Q

Other methods in controlling microbes : Pulsed Electric Field

What kind of products does PEF work well on? [1]

A

Liquids with low electrical conductivity

  • Food with lower electrical conductivity are less able to conduct electrcity throughout the food –> meaning electric current will directly penetrate into MO and kill them

VS
- higher electrical conductivity –> electric current will flow through the food bc they can conduct electricity –> less penetration into MO

45
Q

Other methods in controlling microbes : Preservatives

What kind of food products are nitrites used in?

What kind of effects does nitrite cause in food?

A
  • Cured meats
  • Nitrietes maintains red colour of meat, contribute to flavours and aromas of food
46
Q

Other methods in controlling microbes : Preservatives

Why is excessive amount of nitrites undesirable in meats?

What is the regulation limit of nitrites in cured meat and bacon in SG?

A
  • Nitrites can react with amines to form nitrosamines, which are carcinogens (cancer-causing substances)
  • The regulation limit is 125ppm for cured meat and bacon in Singapore
47
Q

Other methods in controlling microbes : Biochemical preservatives

What is the idea behind biochemical preservation?

List some examples of biochemical preservatives [3]

A
  • Biochemical preservation involves the use of chemical compounds or natural substances to inhibit the growth of microorganisms and enzymes, by altering the biochemical reactions that occur in the food, thus preventing spoilage.

EXAMPLES
1. essential oils
2. polyphenols and proanthocyanins
3. Proteins : lysozome, Lactoferrin, Bacteriocins (antimicrobial constituents!!)

48
Q

Other methods in controlling microbes : Biochemical preservatives

What is the idea behind biopreservation?

List 2 examples of biopreservatives [3]

A

It involves the introduction of **beneficial live microorganisms **into the food matrix to compete with and inhibit harmful microorganisms.

EXAMPLES
1. Probiotics
2. Bacteriophage – viruses that infect bacteria in food

Y2S1_FST2110 - Microbiology and fermentation (12 decks)

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