Cross-Contamination, Time, and Temperature Control

Cross-contamination is the transfer of pathogens from one food, surface, or person to another. It is one of the five CDC risk factors, and it is the reason for most of the equipment and workflow rules in a kitchen.

The Four Ways It Happens

How to Prevent It

Separate. Keep raw meat, poultry, and seafood away from ready-to-eat food at every stage: in the cooler, on the prep table, and in the walk-in. Where an operation has color-coded boards, use them. Where it does not, prep ready-to-eat items first, before any raw protein touches the station.

Clean and sanitize. Any surface or tool that touched raw protein gets washed, rinsed, and sanitized before it touches anything else. Same for a surface that has been in continuous use for four hours.

Control yourself. Wash hands and change gloves between tasks. Do not wipe your hands on your apron. Do not use the same tongs for raw and cooked.

Watch the flow. Cooked food never goes back into the pan, tray, or container that held it raw. That is one of the most common real-world mistakes and one of the most commonly tested. Grill the chicken, plate it on a clean plate.

Store correctly. Storage order in the cooler is covered in Part 6, and it is entirely a cross-contamination control.

Exam Tip: A cook takes cooked burgers off the grill and puts them back on the tray that held the raw patties. That is cross-contamination, and the burgers must be thrown out. There is no cooking step left to fix it.

Wiping Cloths

A wiping cloth left on a counter between uses grows bacteria fast. Store wet wiping cloths in a bucket of sanitizer solution at the correct concentration, and change the solution when it gets dirty or weak. Use separate cloths for food-contact surfaces and for nonfood-contact surfaces such as floors and the outside of equipment. Do not carry a cloth over your shoulder or tucked into your apron.

---

This is the largest technical block on the exam, and every number in it is worth memorizing.

TCS Food: What It Is and Why It Matters

TCS food stands for Time/Temperature Control for Safety food. It is any food that needs time and temperature control to keep pathogens from growing or from producing toxins. TCS foods drive almost every rule in this section: cooking temperatures, holding, cooling, reheating, and date marking all apply to TCS food specifically.

The TCS food list:

The four that catch people out are cut melons, cut tomatoes, cut leafy greens, and garlic-in-oil. A whole watermelon on a shelf is not a TCS food. The moment you cut it, it is. A whole tomato is not. A sliced tomato is. The act of cutting exposes the moist interior and gives bacteria somewhere to grow.

Garlic in oil is TCS for a different reason. Oil seals out oxygen, and the bacterium Clostridium botulinum grows without oxygen and produces one of the deadliest toxins known. Untreated garlic-in-oil mixtures must be kept refrigerated and used within their date.

Exam Tip: Cut melons, cut tomatoes, cut leafy greens, sprouts, baked potatoes, cooked rice, and garlic-in-oil are all TCS foods. Whole uncut produce, dry goods, bread, and crackers are not.

The Temperature Danger Zone

The temperature danger zone is 41F to 135F (5C to 57C). Between those two temperatures, the bacteria that cause foodborne illness grow rapidly. Growth is fastest between 70F and 125F (21C and 52C).

The 4-hour rule. TCS food that spends a total of more than 4 hours in the danger zone must be thrown away. The time is cumulative. It adds up across prep, cooling, holding, and everything else. If chicken spent an hour on a prep table, then two and a half hours on a broken steam table, then another hour sitting out during a rush, that is 4.5 hours and the chicken goes in the bin.

Once past that point, no amount of reheating fixes it. Some bacteria produce toxins that are heat-stable, which means the toxin survives cooking even after the bacteria that made it are dead.

Exam Tip: The danger zone is 41F to 135F (5C to 57C). This is on almost every version of the exam. Memorize it exactly, and remember that freezing does not kill bacteria, it only stops them growing.

Time-temperature abuse is the term for letting food sit in the danger zone too long, or failing to cook, cool, hold, or reheat it correctly. It is the phrase the exam uses.

Thermometers

You cannot judge temperature by looking. Use a thermometer, and use it correctly.

• The most common type in a kitchen is a bimetallic stemmed thermometer or a digital thermocouple. A bimetallic stemmed thermometer must be inserted up to the dimple on the stem, because it senses across the whole length of that section.
• Insert the probe into the thickest part of the food, away from bone, fat, and gristle.
• For thin items such as burger patties, insert the probe through the side.
• Wait for the reading to hold steady before you take the probe out.
• Clean and sanitize the probe before and after every use. A dirty thermometer moves pathogens from one food to another.
• Check that the thermometer is accurate. A thermometer for measuring food temperatures must be accurate to within plus or minus 2F (1C).

Calibrating a thermometer. The easiest method is the ice-point method: fill a container with crushed ice, add water, stir, insert the probe without touching the sides, wait for the reading to settle, and adjust it to read 32F (0C). The other method is the boiling-point method, adjusting to 212F (100C) at sea level.

Calibrate a thermometer before each shift, after dropping it, after a big temperature change, and whenever you have reason to doubt it.

Exam Tip: Ice water calibrates to 32F (0C). Clean and sanitize the probe between foods. Insert into the thickest part and wait for the reading to stop moving.

Thawing Food

There are exactly four approved ways to thaw food. Nothing else is acceptable.

Never thaw at room temperature. Leaving a case of chicken on the counter overnight lets the outside sit in the danger zone for hours while the middle is still frozen. This is one of the most common violations in real kitchens and one of the most common wrong answers on the exam.

Exam Tip: The four methods are refrigerator, running water at 70F (21C) or lower, microwave followed by immediate cooking, and as part of cooking. Room temperature and standing water are never acceptable.

Minimum Internal Cooking Temperatures

Cooking to the right internal temperature is how pathogens get killed. Memorize this table. It is the most tested table in the entire guide.

How to remember it. The list runs from most dangerous to least. Poultry is highest because Salmonella is common in it and needs more heat. Ground meat is next because grinding mixes surface bacteria all the way through the product. Whole cuts are lowest because the bacteria are mostly on the outside, which hits a killing temperature quickly.

The details that catch people:

• Stuffed anything is 165F (74C). A stuffed pork chop is 165F, even though a plain pork chop is 145F. The stuffing is dense and heat gets in slowly.
• Ground means 155F (68C). A whole steak is 145F. Grind that same steak into a burger patty and it becomes 155F.
• Mechanically tenderized or injected meat is 155F (68C). Needles and blades push surface bacteria into the middle of the cut.
• Eggs depend on where they are going. A fried egg for a guest eating right now is 145F. Scrambled eggs going onto a breakfast buffet are 155F.
• Roasts are the odd one. 145F (63C) but held for 4 minutes, not 15 seconds. The longer time at temperature does the same job.
• Reheating for hot holding is always 165F (74C).

Exam Tip: The two most common cooking-temperature questions are poultry at 165F (74C) and ground meat at 155F (68C). If you learn nothing else, learn those two.

What Temperature Should Hot Food Be Held At?

Hot food must be held at 135F (57C) or above. That applies to every TCS food on a hot line: soups, sauces, gravies, rice, pasta, cooked vegetables, and every hot protein, for the entire time it is out for service.

The matching rule for the other end: cold food must be held at 41F (5C) or below.

How to hold hot food correctly:

• Use hot-holding equipment for holding, not for heating. Steam tables, heat lamps, and warming drawers keep food hot. They cannot bring cold food up through the danger zone fast enough. Reheat on a stove, in an oven, or in a microwave, then transfer.
• Stir the food at intervals so heat spreads evenly. The corners and the surface of a deep pan cool first.
• Keep the lids on whenever you can.
• Check with a thermometer, not with the dial on the equipment. The dial tells you about the water bath, not about the food.
• Never add fresh hot food on top of food already on the line. It drops the temperature of both. Swap the pan instead.

How to hold cold food correctly:

• Food must already be at 41F (5C) or below before it goes into cold holding. A cold well does not chill food down, it keeps it cold.
• If you are using ice, the ice has to come up to the level of the food inside the container, not just touch the bottom. A pan sitting on top of ice will be warm at the top.
• Do not overload the unit. Packing it tight blocks airflow and the temperature rises.
• Replace the ice as it melts. Melted ice water does not hold temperature.

How often to check. Check the temperature of held food at least every 4 hours. Best practice is every 2 hours, and here is why that matters practically: if you find a problem at the 2-hour check, you still have time to reheat or re-chill the food and save it. If you find it at the 4-hour check, your only option is to throw it out.

Exam Tip: Hot holding is 135F (57C) or above. Cold holding is 41F (5C) or below. Checking every 2 hours instead of every 4 is what gives you time to fix a problem rather than discard the food.

Holding without temperature control. Some operations are allowed to display or hold TCS food using time as the control rather than temperature, for example a pizza display or a catering line without power. This requires written procedures prepared in advance, kept on-site, and made available to the health department on request. The specific time limits differ for hot and cold food, and requirements vary by jurisdiction. This is your manager's responsibility, not yours, but you should know that it requires a written procedure and that the food must be labeled and discarded at the end of the allowed period.

Cooling Food

Cooling is where a huge number of real outbreaks begin, because hot food cools slowly and nobody watches it.

The two-stage cooling rule:

Stage 1 has the tighter limit because 135F down to 70F is the range where bacteria multiply fastest.

If food has not reached 70F (21C) within 2 hours, you have two options and only two:

1. Reheat it to 165F (74C) and start the whole cooling process again from the beginning, or
2. Throw it out.

You cannot extend the clock and you cannot skip to Stage 2.

Approved cooling methods:

Do not put a big pot of hot food straight into the walk-in. It cools far too slowly in the middle, and it raises the temperature of everything else in the cooler while it tries. Cool it down first using one of the methods above, then move it to cold storage.

Two more habits that help: leave the container uncovered or loosely covered while it is cooling so heat can escape, then cover it once it is at 41F (5C). And stir, because the middle of an unstirred pan of chili can still be at 100F long after the edges are cold.

Exam Tip: The three cooling numbers are 2 hours, 4 hours, and 6 hours total. Stage 1 is 135F to 70F in 2 hours. Stage 2 is 70F to 41F in 4 more hours.

Reheating Food

The main rule is 165F (74C) within 2 hours. If the food has not got there within 2 hours, throw it out, because slow reheating is just extra time in the danger zone.

Immediate service is the exception people forget. If you are warming a portion of yesterday's chili for one guest who is eating it right now, it can go to any temperature. The pathogen kill happened during the original cook, the food has been safely cooled and stored, and it is not about to sit on a steam table for four hours. A package of sous-vide food brought up for immediate plating works the same way. But the moment that same chili is going onto the hot line instead of onto a plate, the requirement becomes 165F (74C) within 2 hours.

Never reheat in hot-holding equipment. A steam table cannot heat food fast enough. Use an oven, a stove, a microwave, or a rethermalizer, get the food to temperature, then move it to the steam table.

Exam Tip: Reheating for hot holding means 165F (74C) within 2 hours. Reheating for immediate service has no temperature requirement, provided the food was cooked and cooled correctly.

Date Marking

Ready-to-eat TCS food that will be kept in the cooler for more than 24 hours must be date marked.

• The maximum is 7 days at 41F (5C) or below.
• The day you make it or open it is Day 1. Prepare chicken salad on Monday and it must be used or discarded by the end of Sunday.
• If you combine ingredients with different dates, the whole batch takes the oldest date.
• Freezing pauses the clock. Freeze on Day 3 and you have 4 days left when it thaws.
• If the manufacturer's use-by date comes first, use the manufacturer's date.

Exam Tip: 7 days at 41F (5C) or below, and the day of preparation counts as Day 1. A common wrong answer starts counting from the next day.

---